2 climate systems

8/4/2019 2 Climate Systems

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Unit Information

Unit Overview

Understanding climate change - and its impacts on development - depends critically on understanding the

scientific basis of the nature, causes and effects of climate change. That such understanding is vital is

acknowledged by the structure of the Intergovernmental Panel on Climate Change (IPCC), which has a Working

Group (WGI) devoted to the understanding of the scientific basis of climate change. The presentation of the

scientific evidence about climate change has, in the past, been highly contested, although greater consensus

has recently emerged. This unit explains the scientific basis of climate change, including the nature of the

global climate system and some key related concepts (such as the global energy budget, climate forcings, and

feedback mechanisms). It introduces the concepts of natural and anthropogenic (human-induced) climate

change - ideas that have provoked considerable debate and controversy - and it emphasises the complexity

and interconnection of those drivers. Finally, it focuses on anthropogenic climate change in more detail,

explaining its main causes.

Unit Aims

To provide an overview of the science of climate change, including key concepts such as the heatbudget of the Earth, the greenhouse effect, climate forcings, and feedback mechanisms.

To introduce and distinguish between natural and anthropogenic climate change, and to highlight thecomplexity and interconnection of the drivers of climate change.

Unit Learning Outcomes

By the end of this unit students should be able to:

define climate change and summarise the main components of the atmosphere and climate systemexplain the principal elements and physical processes involved in climate change, including influences

on the global energy budget, factors affecting radiative forcing, and different feedback mechanisms

distinguish between natural and anthropogenic drivers of climate change, and describe, withexamples, their complexity and interconnections between them

detail and review critically the main processes of anthropogenic climate changeUnit Interdependencies

This unit provides a basic understanding of climate science and systems and of climate change. It builds on the

overview of climate change and development in Unit 1, as it demonstrates the complexity and scale of climate


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the chapter by Stern (see below). This chapter explains the importance ofgreenhouse gases - especially of carbon dioxide - and their role in affecting radiativeforcing. The chapter demonstrates that the principal radiative forcing over the last twocenturies has been the increase in long-lived greenhouse gases, especially carbondioxide, although some of the effects of those gases have been partially offset by the

effects of other atmospheric pollutants (such as sulphates).As you read take note in Figure 3.1 of the relative amounts of carbon held in thedifferent components of the climate system (the atmosphere, oceans, and biosphere)and of the net flows between these - with atmospheric accumulation of 3.3 Gt/year asthe net balance between emissions from fossil fuels and cement of 6.3 Gt/year and netterrestrial and ocean uptake of 0.7 and 2.3 Gt/year, respectively (3.3 = 6.3 - 0.7 2.3).Note also (on page 37) the different timescales of carbon exchange and their effects onthe time taken for a change in carbon dioxide concentration in the atmosphere to workthrough all the other climate system components. You do not need to remember thedetailed data on carbon isotopes or on the biological pump in the oceans - take note ofthe general processes.The main lesson to be drawn from Figure 3.4 is that if all carbon dioxide released into

the atmosphere from fossil fuels between 1990 and 2000 were held in the atmospherethen the atmospheric concentration of CO2 would be 382 ppm (on the horizontal axis),but this is reduced to 367 ppm by ocean uptake and by land uptake. The oxygen (O2)concentrations on the vertical axis just show that fossil fuel burning removes oxygenfrom the atmosphere and emits carbon dioxide (the ratio between the two determiningthe slope of the 'fossil fuel burning' arrow) and ocean uptake removes carbon dioxidefrom the atmosphere without releasing any oxygen (the red arrow is horizontal). Landuptake, however, involves biological carbon sequestration with the release of oxygen byphotosynthesis, and hence an upward sloping arrow (this is not as steep as the fossil fuelarrow because land uptake is not all through photosynthesis).On pages 48 to 49 note the main feedbacks discussed. In Figure 3.5 the main point tonote is that comparison of (a) and (b) shows model predictions of the effects offeedbacks on land and ocean uptake of carbon dioxide and hence of changes inatmospheric carbon dioxide. Note that land uptake is plotted below the axis purely tomake it easier to separate it from ocean uptake (and the labelling below the axis shouldnot be 1000 but +1000). Land uptake is reduced when feedbacks are allowed for as a

result of increased respiration, forest (or boreal dieback), and forest fires. All thesepositive feedbacks override the negative feedback from carbon dioxide fertilisation.Ocean uptake is less affected by carbon dioxide emissions as there are fewer and smalleroverall positive feedbacks.On pages 50 to 57 take note of the major characteristics of methane, nitrous oxide,chlorofluorocarbons, and ozone as regards the major sources of each. Note that theirimportance as greenhouse gases depends upon (a) their concentration in theatmosphere, (b) the greenhouse effect of each molecule (as a result of their absorptionof radiation, a consequence of their chemical structure), and (c) the length of time that

the gases stay in the atmosphere. The latter two factors are used in estimates of theglobal warming potential of different gases as compared to the same quantity of carbondioxide (often described as carbon dioxide equivalents). These are discussed on page 63and detailed on page 296 of Houghton. On page 296 note the very high GWPs ofdifferent gases.Pages 57 to 63 contain important information on aerosols. Note their direct and indirectradiative forcing effects. Table 3.11 and (from page 63) the section on radiative forcingsummarise the overall effects and relative importance of the different greenhouse gasesand aerosols on radiative forcings. Note also the uncertainty regarding individualestimates and of the overall estimate of total net anthropogenic radiative forcing. Youmay find it useful to look back through this chapter and identify the various sources ofuncertainty in these estimates.


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Stern N (2007) The Economics of Climate Change: The Stern Review. Cambridge UniversityPress, Cambridge, pp. 3-24.

Pages 3 to 24 provide a succinct overview of the science of climate change, including

some of the insights of the main climate change projections made by theIntergovernmental Panel on Climate Change (IPCC).

Further Readings

Clare D (2009) Reducing black carbon. In: 2009 State of the World: Confronting ClimateChange. Worldwatch Institute, Earthscan, London, UK, pp. 56-58.Black carbon, a component of soot, is released as a result of incomplete combustion. Ithas a strong climate-forcing effect and some authors have argued that it is the secondmost important cause of global warming after carbon dioxide (CO2). However, blackcarbon has a very short atmospheric lifetime, with the implication that efforts to reduce

emissions of black carbon may have an almost immediate climate change mitigationeffect. In this reading, the author argues that, whilst reducing greenhouse gas emissionsshould remain the primary focus of climate policy, efforts to reduce emissions of blackcarbon could play an important role in mitigating climate change in the short-term.

Houghton J (2009) Global Warming: The Complete Briefing, 4th edn. CambridgeUniversity Press, Cambridge, pp. 69-92.This reading provides a historical context for current debates about climate change. Byconsidering evidence of climate change for three periods in the past, the author points tothe fact that climate change in the past has occurred relatively slowly, and that globalclimate has been relatively stable over the last 8 000 years, with the exception of thevery rapid warming that has occurred during the 20th and 21st centuries.

IPCC (Intergovernmental Panel on Climate Change) (2007) Climate Change 2007: ThePhysical Science Basis. Contribution of Working Group I to the Fourth Assessment Reportof the Intergovernmental Panel on Climate Change.Available from:http://www.ipcc.ch/publications_and_data/publications_ipcc_fourth_assessment_report_wg1_report_the_physical_science_basis.htm[Accessed 19 October 2009]This extensive, detailed document presents the authoritative view of theIntergovernmental Panel on Climate Change (IPCC) on the physical science basis ofclimate change. It contains a wealth of information about climate change and itrepresents the output of Working Group I of the IPCC and the contribution of that groupto the IPCC's Fourth Assessment Report. In its various chapters the document provides

detailed accounts of advances in scientific understanding of the human and naturaldrivers of climate change, of observed climate change, of climate processes andattribution, and of estimates of projected future climate change. This report alsodocuments the significant progress that has been made in climate science since the lastIPCC assessment; progress that is due especially to the analysis of new, morecomprehensive data, o more sophisticated data analysis, to improved understanding ofprocesses and model simulations, and to a greater focus on uncertainty ranges.

Mate J. Davies, K, Kanter D (2009) The risks of other greenhouse gases. In: 2009 Stateof the World: Confronting Climate Change. Worldwatch Institute, Earthscan, London, UK,pp. 52-55.The effects of some of the main greenhouse gases are now well-known. However, other

powerful, yet little-known, greenhouse gases are also emitted into the atmosphere.Those gases include the fluorocarbons (or F-gases), which are extremely potent
http://www.ipcc.ch/publications_and_data/publications_ipcc_fourth_assessment_report_wg1_report_the_physical_science_basis.htmhttp://www.ipcc.ch/publications_and_data/publications_ipcc_fourth_assessment_report_wg1_report_the_physical_science_basis.htmhttp://www.ipcc.ch/publications_and_data/publications_ipcc_fourth_assessment_report_wg1_report_the_physical_science_basis.htmhttp://www.ipcc.ch/publications_and_data/publications_ipcc_fourth_assessment_report_wg1_report_the_physical_science_basis.htmhttp://www.ipcc.ch/publications_and_data/publications_ipcc_fourth_assessment_report_wg1_report_the_physical_science_basis.htm


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greenhouse gases. This reading explores the effects of, and the need to control,emissions of such greenhouse gases.

References

Adams WM (2009) Green Development: Environment and Sustainability in a DevelopingWorld, 3rd edn. Routledge, London.

Archer D (2005) Fate of fossil fuel CO2 in geologic time.Journal of Geophysical Research110 C09S05, doi:10.1029/2004JC002625.

Arrhenius S (1896) On the influence of carbonic acid in the air upon the temperature ofthe ground. Philosophical Magazine 4 237-276.

Barry RG, Chorley RJ (2003)Atmosphere, Weather and Climate, 8th edn. Routledge,

London, pp. 1-373.

Hansen J, Sato M, Kharecha P, Beerling D, Berner R, Masson-Delmotte V, Pagani M,Raymo M, Royer DL, Zachos JC (2008) Target atmospheric CO2: where should humanityaim? Open Atmospheric Science Journal.Available from:http://arxiv.org/ftp/arxiv/papers/0804/0804.1126.pdf[accessed 13November 2009]

Houghton J (2009) Global Warming: The Complete Briefing, 4th edn. CambridgeUniversity Press, Cambridge, pp. 20-111.

Hulme M (2009) Why we Disagree about Climate Change: Understanding Controversy,

Inaction and Opportunity. Cambridge University Press, Cambridge.

IPCC (2007) Climate Change 2007; Synthesis Report. Fourth Assessment Report of theIntergovernmental Panel on Climate Change, pp. 30-78.Available from:http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdf[Accessed 12 November 2009]

Lenton TM, Loutre M-F, Williamson MS, Warren R, Goodess CM, Swann M, Cameron DR,Hankin R, Marsh R, Shepherd JG (2006) Climate Change on the Millennial Timescale.Technical Report 41, Tyndall Centre for Climate Change Research, Norwich.

Mate J, Davies K, Kanter D (2009) The risks of other greenhouse gases. In: 2009 Stateof the World: Confronting Climate Change. Worldwatch Institute, Earthscan, London, UK,pp. 52-55.

Seinfeld JH, Pandis SN (2006)Atmospheric Chemistry and Physics: From Air Pollution toClimate Change, 2nd edn. Wiley, Hoboken, New Jersey, pp. ix-1040.

Smithson P, Addison K, Atkinson K (2008) Fundamentals of the Physical Environment,4th edn. Routledge, London, p. 45.

Stern N (2007) The Economics of Climate Change: The Stern Review. CambridgeUniversity Press, Cambridge, pp. 3-24, 194.

UNEP/GRID-Arendal (2000) World Ocean Thermohaline Circulation. UNEP/GRID-Arendal
http://arxiv.org/ftp/arxiv/papers/0804/0804.1126.pdfhttp://arxiv.org/ftp/arxiv/papers/0804/0804.1126.pdfhttp://arxiv.org/ftp/arxiv/papers/0804/0804.1126.pdfhttp://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdfhttp://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdfhttp://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdfhttp://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdfhttp://arxiv.org/ftp/arxiv/papers/0804/0804.1126.pdf


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Maps and Graphics Library.Available from:http://maps.grida.no/go/graphic/world-ocean-thermohaline-circulation[Accessed 21 September 2009]

UNEP/GRID-Arendal (2002) Greenhouse Effect. UNEP/GRID-Arendal Maps and Graphics

Library.Available from:http://maps.grida.no/go/graphic/greenhouse-effect[Accessed 21September 2009]

UNEP/GRID-Arendal (2007) Historical trends in carbon dioxide concentrations andtemperature, on a geological and recent timescale. UNEP/GRID-Arendal Maps andGraphics Library.Available from:http://maps.grida.no/go/graphic/historical-trends-in-carbon-dioxide-concentrations-and-temperature-on-a-geological-and-recent-time-scale[Accessed 22September 2009]

United Nations (1992) United Nations Framework Convention on Climate Change. United

Nations, New York.

United Nations (1998) Kyoto Protocol to the United Nations Framework Convention onClimate Change. United Nations, New York.

Interactive Features and Multimedia

Climate Change - Science. Videos of the Human Development Report 2007/2008.http://hdr.undp.org/en/reports/global/hdr2007-2008/videos/[Accessed 12 November2009]

Weblinks and Portals

IntergovernmentalPanel on Climate

Change (IPCC)

The Intergovernmental Panel on Climate Change (IPCC)produces authoritative reports on the science and impacts of,and human responses to, climate change.http://www.ipcc.ch/[Accessed 19 October 2009]

The Met Office The UK Met Office - and particularly its Hadley Centre -produces state-of-the-art research into climate change and itseffects.http://www.metoffice.gov.uk/climatechange/[Accessed 19October 2009]

1.0 The Global Climate System

Section Overview

This section provides an introduction to the global climate system. It describes the structure and composition

of the atmosphere, and it explains some of the main aspects of the transfer of energy in the atmosphere. This

section introduces the concepts of the heat budget of the Earth, the global heat engine, the natural greenhouse
http://maps.grida.no/go/graphic/world-ocean-thermohaline-circulationhttp://maps.grida.no/go/graphic/world-ocean-thermohaline-circulationhttp://maps.grida.no/go/graphic/world-ocean-thermohaline-circulationhttp://maps.grida.no/go/graphic/greenhouse-effecthttp://maps.grida.no/go/graphic/greenhouse-effecthttp://maps.grida.no/go/graphic/greenhouse-effecthttp://maps.grida.no/go/graphic/historical-trends-in-carbon-dioxide-concentrations-and-temperature-on-a-geological-and-recent-time-scalehttp://maps.grida.no/go/graphic/historical-trends-in-carbon-dioxide-concentrations-and-temperature-on-a-geological-and-recent-time-scalehttp://maps.grida.no/go/graphic/historical-trends-in-carbon-dioxide-concentrations-and-temperature-on-a-geological-and-recent-time-scalehttp://maps.grida.no/go/graphic/historical-trends-in-carbon-dioxide-concentrations-and-temperature-on-a-geological-and-recent-time-scalehttp://hdr.undp.org/en/reports/global/hdr2007-2008/videos/http://hdr.undp.org/en/reports/global/hdr2007-2008/videos/http://www.ipcc.ch/http://www.ipcc.ch/http://www.ipcc.ch/http://www.metoffice.gov.uk/climatechange/http://www.metoffice.gov.uk/climatechange/http://www.metoffice.gov.uk/climatechange/http://www.ipcc.ch/http://hdr.undp.org/en/reports/global/hdr2007-2008/videos/http://maps.grida.no/go/graphic/historical-trends-in-carbon-dioxide-concentrations-and-temperature-on-a-geological-and-recent-time-scalehttp://maps.grida.no/go/graphic/historical-trends-in-carbon-dioxide-concentrations-and-temperature-on-a-geological-and-recent-time-scalehttp://maps.grida.no/go/graphic/greenhouse-effecthttp://maps.grida.no/go/graphic/world-ocean-thermohaline-circulation


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effect, and the runaway greenhouse effect. It also provides some definitions of the terms 'climate', 'climate

change', and 'climate variability', and considers some of the main components of the global climate system.

Section Learning Outcomes

By the end of this section students should be able to:

outline the structure of the atmosphere, energy flows in the atmosphere, and the heat budget of theEarth

explain the greenhouse effect and its main determinantsdefine the terms 'climate', 'climate change', and 'climate variability'list the main components of the climate system and discuss their respective roles in climate system

processes

1.1 The global atmosphere

The composition of the atmosphere

The Earth's atmosphere is a thin layer of mixed gases extending to a height of around 80 kilometres (km)

above the surface; it envelops the Earth to a thickness of only 1% of the Earth's radius (Barry and Chorley

2003; Smithson et al2008). Despite its tenuous nature, the atmosphere is essential to life on Earth: it is a

source of oxygen for respiration, of carbon dioxide for photosynthesis and of nitrogen for nitrogen fixation; it

shields organisms from biologically harmful ultraviolet solar radiation; it stabilises the temperature of the

Earth's surface within certain limits (across the globe and between day and night and different seasons); and it

generates and transports precipitation as part of the hydrological cycle. The atmosphere has been the subject

of scientific interest for more than three centuries and the basic chemical composition of the atmosphere has

been known since the end of the 19th century. The main constituent gases of the dry atmosphere are nitrogen

(78.08%), oxygen (20.98%), argon (0.93%) and carbon dioxide (0.035%). Of course, in reality, the

atmosphere - at least in its lowest layer (the troposphere) - is not dry but contains a significant, and highly

variable, proportion of water vapour. In addition, an enormous range of other substances is present in the

atmosphere in very small concentrations; some of those substances (such as the noble gases) are chemically

inert whilst others (such as ozone) have effects that are disproportionate to their abundance.

The composition of the atmosphere is not constant over time. In general terms, the atmosphere had evolved to

resemble its present form and composition by at least 400 million years ago, but it continues to change in

some critical respects (Barry and Chorley 2003). Recently, evidence has accumulated of increases in the


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abundance of some of the constituents of the atmosphere. In particular, substantial increases in the

concentration of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), tropospheric ozone (O3), sulphates,

and soot particles have been recorded, indicating that the basic chemical composition of the atmosphere is

changing more rapidly than had previously been thought. Furthermore, the constituents of the atmosphere are

not independent of each other; they interact in complex ways. Thus, a change in one component of the

atmosphere may lead to significant changes in others - and may also involve feedback mechanisms that

amplify or dampen the original perturbation (Seinfeld and Pandis 2006). Therefore, the atmosphere is

constantly changing, both in terms of its overall composition and in terms of the complex set of interactions of

its constituent substances.

The structure of the atmosphere

Pause and consider for a moment why an understanding of the structure of the atmosphere might be important

for understanding climate change.

We may expect conditions in the atmosphere to vary with increasing height above the Earth, and that these

conditions will affect the way that the atmosphere behaves. Understanding and communicating about these

varying conditions and the way that they are involved in and are affected by climate change requires a

knowledge and description of the atmosphere's structure.

The basic physical structure of the atmosphere is typically described in terms of the way temperature varies

with increasing height above the Earth's surface. These variations follow a well-recognised pattern and they

allow scientists to divide the atmosphere into several layers (for the purposes of description and analysis):

these layers are the troposphere, stratosphere, mesosphere, and thermosphere (see 1.1.1).

1.1.1 The temperature structure and main layers of the atmosphere


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Source: based on Smithson et al(2008) p. 45.

In the lowest layer (the troposphere) of the atmosphere, the air immediately above the surface of the Earth is

relatively warm. This is because the Earth's surface absorbs solar energy and then re-radiates that energy in

the form of heat, warming the adjacent air. With increasing height above the surface, that warming effect is

less pronounced, with the result that the temperature of the troposphere decreases with height above the

Earth's surface. However, at the level of the tropopause, that pattern is reversed and the temperature of the

atmosphere begins to increase with height; this effect is due to the influence of the 'ozone layer' in the

stratosphere, which absorbs incoming ultraviolet radiation from the Sun and warms that layer of the

atmosphere. Higher still, the warming effect of stratospheric ozone becomes negligible and in the mesosphere

the temperature again declines with height until the beginning of the thermosphere. The thermosphere is the

atmospheric layer in which ionic interactions produce a significant heating effect, with the result that

temperature once again increases with height.

It is important to emphasise that, so far, although we have been considering variations in temperature at

different heights within the atmosphere, we have not yet discussed anything that might be described as

'climate change'. Here, we are simply describing the natural temperature structure of the atmosphere, and we

are noting that the temperature of the atmosphere varies naturally at different levels. That pattern is purely a


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general, physical feature of the atmosphere, one that is a function of altitude above the Earth's surface rather

than of any changes in the composition of the atmosphere that occur over time.

Energy in the atmosphere

As the Sun's radiation passes through the atmosphere to the Earth's surface, some of that radiation is

absorbed or reflected by the atmosphere. There are several reasons why solar radiation may be absorbed or

reflected as it first passes through the atmosphere. Above, we have mentioned that some of the incoming

radiation (especially radiation in the ultraviolet part of the spectrum) is absorbed as a result of its interaction

with stratospheric ozone. Some solar radiation is simply reflected back into space by substances in the

atmosphere that are highly reflective, such as bright clouds and aerosol particles. The remaining incoming

radiation passes through the atmosphere to reach the Earth's surface, but even there it may simply be

reflected back into space by highly reflective surfaces such as ice and snow. The term used to describe the

reflectivity of the Earth's surface (or atmosphere) is 'albedo'; thus a bright, highly reflective surface such as a

frozen lake has much higher albedo than a darker surface such as exposed soil. If incoming solar radiation is

not absorbed by stratospheric ozone, and if it is not reflected back into space by surfaces with high albedo,

then it is absorbed by the Earth's surface (and to a lesser extent by the atmosphere just above it). That,

however, is not the end of the story, as the radiation absorbed by the Earth's surface is subsequently re-

emitted at a longer wavelength, in the form of heat.

The heat budget of the Earth

We have noted above that the Earth receives energy as radiation from the Sun (incoming solar radiation) and

some of this is reflected back into space but some is absorbed by the Earth's surface which then re-emits this

energy at a longer (infra-red) wavelength, as outgoing terrestrial radiation

What do you think will be the relationship between incoming solar radiation absorbed by the Earth's surface,

outgoing terrestrial radiation emitted by the Earth's surface, and the temperature of the Earth's surface?

We would expect incoming solar radiation absorbed by the Earth's surface to heat the Earth's surface, and

outgoing terrestrial radiation to cool the Earth's surface. Any difference between the heating effect of

absorption of energy from incoming solar radiation and the cooling effect of the loss of energy from outgoing

terrestrial radiation will lead to a change in the surface temperature of the Earth .


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This is the case, and according to well-understood physical principles, the temperature of the Earth and the

outgoing radiation should (in the absence of any interactions between the temperature and the composition of

the atmosphere, which we discuss below) adjust with each other so that the solar energy reaching the Earth is

balanced by the heat energy that is radiated from the planet (Seinfeld and Pandis 2006). This balance occurs

because incoming radiation greater (or lower) than outgoing radiation causes the temperature to rise (or fall),

and at the same time rises (or falls) in temperature lead to higher (or lower) rates of heat loss through

outgoing radiation. As a result, if the incoming and outgoing radiation are out of balance - with incoming

radiation higher (or lower) than outgoing radiation - then the imbalance causes increases (or falls) in

temperature that in turn increase (or reduce) the rate of outgoing radiation. This process continues until

outgoing radiation is equal to or balances incoming radiation. Put simply, changes in the Earth's temperature

and in the outgoing radiation interact until there is balance in the energy reaching and leaving the surface. This

balance is known as the 'heat budget of the Earth' (although other terms are sometimes used, such as the

'radiative budget for the Earth', the 'global energy budget', or the 'global radiation balance'). Again, it is

important to emphasise that we are not yet talking about 'climate change'.

The heat budget of the Earth is simply a physical property of the Earth and of the energy it receives from the

Sun. The Earth would have such a 'heat budget' even if it had no atmosphere whatsoever. Yet the concept of

the heat budget of the Earth is a useful one because it highlights three key points:

that energy is constantly dissipated from the Earth's surface into the atmosphere that the energy radiated from the Earth's surface is of a different (longer) wavelength from that

received from the Sun - a fact that has important implications for the climate, as we shall see later

that if for whatever reason there is a change in incoming radiation, in the temperature of the Earth, orin the outgoing radiation then this will upset the balance in the heat budget and there will be

consequent changes in the temperature and in outgoing radiation until the balance of the heat budget

is restored

Another important feature of the heat budget of the Earth is that, whilst it must balance for the Earth as a

whole, it does not balance at every individual point on the Earth's surface. Most of the solar energy that

reaches the Earth's surface is absorbed in the tropical regions, whilst very little is absorbed in the polar

regions, especially during winter (Seinfeld and Pandis 2006). This highly uneven distribution of solar energy

absorption across the surface of the globe sets up and drives vast movements within the atmosphere and


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oceans as heat is redistributed around the globe. The movement of energy through the Earth's surface,

atmosphere, and oceans may be thought of as a vast 'engine' that is constantly transferring heat from the

tropics to the poles. This 'global heat engine', in its ceaseless effort to equalise the temperature across the

Earth's surface, gives rise to the large-scale patterns of winds, ocean currents, weather systems and climatic

regimes that are familiar to people across the globe - and that collectively constitute the 'global circulation'

(Seinfeld and Pandis 2006).

1.2 The greenhouse effect

The natural greenhouse effect

The atmosphere plays a critical role in the heat budget of the Earth. This arises because of an important point

made earlier about the energy reaching and leaving the Earth's surface: the outgoing terrestrial radiation (of

heat) is of a longer wavelength than the incoming solar radiation.

This difference is vitally important for climate - and for climate change - because many of the gases of the

atmosphere absorb energy selectively: in other words, they absorb energy of particular wavelengths whilst

allowing energy of other wavelengths to pass through the atmosphere unimpeded. In addition, the selective

absorption by different gases in the atmosphere occurs at different wavelengths, so that they absorb energy in

different parts of the energy spectrum, meaning that radiation of a given wavelength may be absorbed by the

molecules of one species (such as water vapour) but not by those of another (such as carbon dioxide). For the

atmosphere as a whole, it is then possible to construct an absorption spectrum illustrating the wavelengths

of energy that are absorbed by the various constituents of the atmosphere.

There are two important points here.

First, relatively little of the incoming solar radiation is absorbed by the gases and particles of theatmosphere but, in contrast, a much greater proportion of the outgoing terrestrial radiation - which is

of longer wavelength - is absorbed by the atmosphere, with the result that heat is retained in the

atmosphere.

Second, the overall absorption of outgoing terrestrial radiation by the atmosphere depends upon itscomposition (constituent gases), as some gases absorb (and retain heat from) particular wavelengths

more than other gases.

What are the implications of these two points for the heat budget of the Earth?


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The first point (that the atmosphere is relatively transparent to incoming solar energy but it is more opaque

to outgoing terrestrial energy) means that the atmosphere causes some heating of the Earth's surface and

atmosphere, as the atmosphere significantly reduces outgoing terrestrial radiation but does not significantly

affect income solar radiation. The second point (that the absorption of outgoing terrestrial radiation by the

atmosphere depends upon its gaseous composition) means that the extent of heating by the atmosphere

depends upon the composition of the atmosphere.

This phenomenon of heating of and by the atmosphere is known as the greenhouse effect. The greenhouse

effect is an important part of the Earth's heat budget, and thus the heat budget is affected by the composition

of the atmosphere (see 1.2.1).

1.2.1 The Earth's heat budget and the greenhouse effect

Source: UNEP/GRID-Arendal (2002)


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The metaphor of a greenhouse has been used to describe this phenomenon because it communicates the idea

that heat is 'trapped' close to the Earth's surface, much as a greenhouse retains heat beneath its glass panels.

This metaphor is illustrative rather than exact, however, because the heating effect of a greenhouse has much

more to do with the way that the glass panels prevent heat losses from turbulent convection of air as

compared with the way that localised heating occurs due to the selective absorption of energy by gases in the

air. Nevertheless, the metaphor of the 'greenhouse effect' has become a popular and vivid way of representing

the idea that heat is retained close to the Earth's surface by a 'blanket' of atmospheric gases. Hence the gases

that are involved in absorbing some of the energy radiated by the Earth's surface are known as 'greenhouse

gases'.

Greenhouse gases are well-known because they are a prominent theme in debates about climate change; we

will have much more to say about greenhouse gases later, when we consider the central role that they play in

anthropogenic (human-induced) climate change. However, it is important to emphasise that, at this stage,

we have still not yet reached the stage of discussing climate change! The greenhouse effect described above is

an entirely natural phenomenon and some authors underline this point by insisting that it should be described

as the 'natural greenhouse effect'.

A vital phenomenon

The natural greenhouse effect occurs directly as a result of the composition of the atmosphere and the

radiative properties of its constituents, especially of water vapour. The natural greenhouse effect is not a result

of human activities (whose effects are discussed separately below); it is simply a consequence of the fact that

the Earth has an atmosphere that contains gases (such as water vapour) that absorb energy. Thus, even in the

absence of any human impact on the atmosphere, the natural greenhouse effect ensures that the temperature

of the Earth is more than 20 C warmer than would otherwise be the case in the absence of the gases and

particles of the atmosphere (Barry and Chorley 2003; Houghton 2009). The natural greenhouse effect has

been a critical element in allowing the evolution of life on Earth, since it is unlikely that water would have

existed in liquid form - or that even primitive organisms could have evolved - in its absence.

The 'runaway' greenhouse effect

Above, we have noted that the Earth displays a natural greenhouse effect due to the fact that certain gases in

its atmosphere - notably water vapour - absorb heat energy and retain that energy close to the Earth's


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surface. Another term is sometimes used in relation to the idea of the greenhouse effect: the 'runaway'

greenhouse effect. The concept of the runaway greenhouse effect describes the situation that has developed on

our neighbouring planet, Venus, due to the very high abundance of carbon dioxide in its atmosphere. Venus

has a large amount of carbon dioxide in its atmosphere and carbon dioxide selectively absorbs more outgoing

radiation wavelengths than incoming radiation wavelengths. This means that on Venus a very strong

greenhouse effect allows very little of the heat radiated from the planet's surface to escape its atmosphere; the

greenhouse effect on Venus has increased the planet's temperature by around 500 C (Houghton 2009). The

result of that strong greenhouse effect has been to boil and evaporate all of the water that must, at one stage,

have existed in liquid form on the planet. As water vapour accumulated in the atmosphere of Venus, it in turn

acted as a strong greenhouse gas (since it too selectively absorbs more outgoing than incoming radiation) and

this then led to further atmospheric warming. The concept of the runaway greenhouse effect has entered

popular debates because of environmentalist concerns that the Earth's climate - as a result of human activities

- could emulate that of Venus and become utterly inhospitable to life. However, as Houghton (2009) explains,

such a scenario is unlikely to occur on Earth.

1.3 The climate system

Climate and climate change definitions

The subject of climate and the ways in which it changes has been the focus of scientific enquiry for a

considerable period of time. Hence climate change is not a new environmental issue; in 1896, Svante Arrhenius

suggested that the carbon dioxide (CO2) released by human activities could increase global temperature

(Arrhenius 1896; Houghton 2009; Seinfeld and Pandis 2006; Stern 2007). However, climate change - a term

that encompasses increasing global temperature and its associated impacts - has gained much greater

prominence in recent scientific and policy debates (Adams 2009; Houghton 2009; IPCC 2007; Stern 2007). Yet

terms such as 'climate' and 'climate change' require careful definition, for climate varies naturally over all

temporal and spatial scales (Seinfeld and Pandis 2006 p. 1027).

How would you define the term 'climate'?

'Climate' may be defined as the condition of the atmosphere over many years: 'the mean behaviour of the

weather over some appropriate averaging time', which is conventionally taken to be 30 years (Seinfeld and

Pandis 2006 pp. 4, 1026). Note that 'mean behaviour' may also include the amount of variability - for

example the number of extreme events.


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'Climate change' may be defined in various ways.

The United Nations Framework Convention on Climate Change (UNFCCC), in its Article 1, has adopted a narrow

view of climate change, defining it as 'a change of climate which is attributed directly or indirectly to human

activity that alters the composition of the global atmosphere and which is in addition to natural climate

variability observed over comparable time periods' (United Nations 1992 p. 3). The UNFCCC therefore makes a

distinction between 'climate change', which is regarded as a purely anthropogenic phenomenon caused by

human activities altering the composition of the atmosphere, and 'climate variability', which is attributed to

natural causes (cf IPCC 2007; United Nations 1992). Given that its primary purpose is the task of curbing

greenhouse gas emissions from human activities, it is not surprising that the UNFCCC is concerned only with

anthropogenic effects on climate.

In contrast, the Intergovernmental Panel on Climate Change (IPCC 2007 p. 30) has defined climate change

more broadly as 'a change in the state of the climate that can be identified (eg using statistical tests) by

changes in the mean and/or the variability of its properties, and that persists for an extended period, typically

decades or longer. It refers to any change in climate over time, whether due to natural variability or as a result

of human activity'. In this definition, the IPCC acknowledges that climate change has both natural and human

causes; thus climate change may result from natural processes, such as variations in the Earth's orbit, as well

as from anthropogenic changes, such as alterations in the composition of the atmosphere or of land-use

patterns (IPCC 2007; Seinfeld and Pandis 2006).

IPCC also differs in its use of the term 'climate variability', which it takes to refer to variations in the means,

standard deviations, and ranges of climate parameters on all spatial and temporal scales beyond those of

individual weather events, although those variations do not necessarily amount to a trend over an extended

period. In the IPCC's (2007) definition, climate variability, like climate change, may have both natural and

anthropogenic causes. Overall, the IPCC's (2007) definition of climate change implies the existence of more

consistent trends in climate variables beyond the periodic fluctuations that constitute climate variability. Such

distinctions and nuances suggest that considerable care is required in defining and debating climate change.

Nonetheless, it is worth acknowledging that, in popular usage and in many contemporary scientific debates,

the term 'climate change' tends to refer only to anthropogenic climate change.


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Components of the climate system

Terms such as 'climate', 'climate change', and 'climate variability' tend to evoke images that are strongly

related to atmosphericphenomena: hot, cloudless days; droughts; intense rainfall; melting glaciers; and

tropical storms (although images of rising sea levels are also prominent). Whilst atmospheric phenomena are

indeed a critical aspect of climate (and of climate change), is important to remember that the global climate

system is much larger than the atmosphere alone.

What do you think are the main components of the global climate system?

Broadly, the global climate system consists of several distinct, yet interrelated, systems: the atmosphere;

the oceans; the cryosphere (ice); the lithosphere (geology); and the biosphere (biota).

It is worth pointing out that some authors classify the component parts of the global climate system in

different ways (in order to emphasise particular aspects of climate, or in order to focus on the behaviour of

particular subsystems). For instance, some people use the term 'hydrosphere' (water) - which includes both

terrestrial (freshwater) and marine (saltwater) components. In addition, all of these systems may be

subdivided further into smaller systems, such as soil, vegetation, permafrost, or glaciers.

Climate scientists devote considerable effort to understanding the behaviour of each of the large-scale systems

atmosphere ocean cryosphere lithosphere biosphere

that constitute the global climate system - and the linkages between each of them. The significance of some of

these systems for global climate may not be immediately apparent, yet they may far outweigh the atmosphere

in terms of their importance.

All of these interrelated systems are involved in the processes of absorbing, transferring, and redistributing

energy at the global scale; hence they all form interconnected parts of the global heat engine. They are also


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important in their interactions with the atmosphere as regards greenhouse gases - as they variously store and

release these gases. We briefly consider each of these in turn.

The atmosphere has been the focus of much of this section, in terms of its composition, structure, and the

importance of greenhouse gases in the global heat budget. Brief mention was also made of the role of winds

which, with ocean currents, redistribute energy from the tropical to the Polar Regions. Another important

element of the atmosphere in the climate system are clouds, which both reflect some incoming solar radiation

away from the Earth's surface and reflect some outgoing terrestrial radiation back to the Earth. The balance

between these effects differs between different types of cloud at different altitudes (and of course between day

and night). The presence and absence of cloud also affects land and sea surface temperatures - in different

ways in different areas at different times. Winds and atmospheric temperatures also affect the water vapour

content of the atmosphere, and since water vapour is an important greenhouse gas, this also affects the

Earth's heat budget. Winds, temperatures and clouds are also important determinants of precipitation

(principally rain and snow) and winds are also important in driving ocean currents. Finally, it is important to

mention the importance of aerosols in the atmosphere, small particles which reflect incoming solar radiation -

with their reflectivity varying according to their size and composition, and their altitude in the atmosphere.

The oceans play a number of major roles in the climate system. First, they are a major but slow absorber of

heat energy. The water contained in the oceans has a much greater specific heat capacity than air, meaning

that it can absorb much more heat than air for a given temperature change - thus the top few metres of the

water in the oceans contains as much heat as does the entire atmosphere (Houghton 2009). As the oceans are

so deep, they absorb energy and heat up slowly, as it takes very long periods of time (hundreds of years) for

them to adjust completely to temperature changes in the atmosphere. Second, ocean currents play a major

role in transferring heat from the tropical to the Polar Regions, principally through the global thermohaline

circulation also known as the ocean 'conveyor belt'. This is illustrated in the figure in 1.3.1. This shows how

warm water moves east to west along the surface of the ocean (across the Indian Ocean) and up the Atlantic.

As it nears Greenland it cools and sinks. It then proceeds south back down the Atlantic and then back from

west to east across the Antarctic, before heading North across the Pacific, warming as it goes until it completes

the loop (with a similar loop in the Indian Ocean). The convection current is driven by temperature differences

in polar and tropical regions affecting the density of water, but water density is also affected by the

concentration of salt. Thus, for example, the evaporation of water from the warm shallow current in the Indian


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Ocean and southern Atlantic makes it more dense, and this later strengthens the down-current in the North

Atlantic. Winds are also important in driving currents.

1.3.1 The world ocean thermohaline circulation


Third, the oceans are a major heat source for the atmosphere, through evaporation and the rising of warm

water vapour from the oceans into the atmosphere. This is of course also very important for the formation of

clouds and for precipitation.

The fourth role of the oceans in the climate is their storage of greenhouse gases. Apart from the evaporation of

water vapour affecting the greenhouse gas concentrations discussed above, the oceans also trap large

amounts of carbon dioxide in two ways. First, carbon dioxide dissolves in water and second, marine organisms

contain large amounts of carbon which was originally absorbed from the atmosphere (or from the ocean) by

plants. When these organisms die, many of them sink down to the ocean floor. This is the process by which

some fossil fuels were formed in the past and some carbonate rocks were also formed through a similar

process, trapping carbonates in the shells of marine animals.


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Ice exists in the cryosphere in glaciers, sea ice (for example, over the North Pole and around Antarctica), ice

caps (over Greenland and Antarctica), and permafrost, including lakes (in northern Russia). Snow and ice have

important albedo effects, increasing the reflection of solar radiation from the Earth's surface. Large amounts

of fresh water held in glaciers and ice caps would, if melted, affect the salinity of the oceans and this,

particularly its local effects, could affect the thermohaline circulation discussed above. Snow and ice also affect

seasonal temperature patterns and winds. Finally, permafrost affects soils containing large amounts of organic

matter (peat) and methane, is therefore an import store of greenhouse gases.

The major climate impacts of the lithosphere are relatively constant and unaffected by other elements of the

climate system (as they are determined by tectonic plate movements and the global pattern of continental land

masses), but these are important when considering long-term climate changes and influences in geological

history. Short-term impacts on climate (or weather) arise from large volcanic eruptions which may release

large amounts of gases or of ash and other aerosol particles into the atmosphere. The lithosphere also

contributes relatively small amounts of carbon dioxide to the atmosphere through weathering of rocks.

The biosphere plays a major role in the climate system through its sequestration and storage of carbon

dioxide. Living organisms and detritus hold over three times as much carbon dioxide as the atmosphere, and

2.5 times as much CO2 as the surface ocean, but only 6% of the CO2 stored in the deep ocean (IPCC, AR4,

WG1, Figure 7.3). Carbon dioxide sequestered in the past, in geological history, is held in fossil fuels.

Sequestration and release of carbon dioxide by the biosphere is strongly affected by different aspects of the

climate system discussed above - surface temperature, precipitation - and hence most immediately by the

atmosphere and oceans. Anaerobic decomposition of organic matter (for example, in paddy fields and ruminant

digestion and landfill sites) also contributes to methane emissions into the atmosphere.

Anthropogenic activities - agriculture, deforestation and other land-use changes - make a major

contribution to the release of greenhouse gases from the biosphere. Burning of fossil fuels also contributes

carbon dioxide to the atmosphere. These activities may also release aerosols into the atmosphere.

This brief description of the global climate system demonstrates some of the intimate linkages that exist

between its components. In fact, the linkages between those different systems are so tightly integrated that

the global climate system is sometimes termed the 'Earth-atmosphere-ocean system' (Barry and Chorley 2003

p. 2).


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How does the fact that the global climate system encompasses much more than the atmosphere alone affect

the task of understanding and responding to climate change?

The atmosphere, ocean, cryosphere, lithosphere and biosphere are each very large and complex systems

that behave in different ways over different timescales. Their complex and varied interactions in the climate

system make it very difficult to predict with certainty the range, scale and timing of possible multiple impacts

of changes in the climate system.

Understanding climate change is further complicated by the range of different factors that can and do cause

climate change

2.0 Climate Change

Section Overview

This section provides an introduction to some of the key concepts associated with the subject of climate

change. In recent debates about (anthropogenic) climate change, it is often pointed out that global climate has

changed dramatically over geological timescales. It is now well-understood that climate is not static but highly

dynamic: it changes constantly over all spatial and temporal scales. This section covers the topics of natural

climate change, the reconstruction of past climates, climate forcings, feedback mechanisms, radiative forcing,

and climate sensitivity. It therefore provides a foundation and context for understanding recent debates about

anthropogenic climate change.



outline major climate changes in the pastdefine and distinguish between and list the major types of internal, external and radiative forcing, and

discuss the relationships between them

2.1 The climatic record

Climate is dynamic

The 'climatic record' provides a means of understanding how climate has changed in the past. Reconstructions

of climates in the distant past (palaeoclimates) have transformed scientific understanding of the nature and


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behaviour of the global climate system. At one time, it was believed that the global pattern of climate was a

static, constant, and predictable one (Hulme 2009). By the 1960s, however, overwhelming evidence had

accumulated to suggest that the widely-held view of climate as unchanging was no longer tenable (Barry and

Chorley 2003).

Evidence of dramatic changes in climate included the insights - obtained from geology and palaeography - that

the Earth had undergone a succession of prolonged 'ice ages' (glacials) and warmer intervening periods

(interglacials), as well as many, shorter cold periods (stadials) and warm periods (interstadials) superimposed

on those longer period variations. Advances in geological and palaeographic dating methods have allowed

scientists to reconstruct previous climates with increasing precision. In addition, the development of the

'astronomical theory of climatic changes' - initially by Croll and subsequently by Milankovitch - provided a

convincing explanation of how dramatic changes in climate could be linked to variations in the Earth's orbit

around the Sun (Barry and Chorley 2003).

Other palaeographic and palaeoclimatological studies have provided reconstructions of various changes in

climate due to natural alterations in the composition of the atmosphere and to global-scale geological and

tectonic processes. Those natural climate changes have occurred both on evolutionary timescales (such as the

changes in atmospheric circulation that have occurred in response to the changing distribution of the Earth's

tectonic plates) and on much shorter timescales (for example, as the result of the release of gases and

particles from volcanic eruptions). Scientific knowledge of palaeoclimates has developed rapidly and

reconstructions of past climates may now be made with remarkable precision; such reconstructions reveal that

the history of Earth's climate has been characterised by large-scale, and sometimes abrupt, changes. Indeed,

global climate is now understood to be a naturally complex and dynamic system: it displays changes at all

spatial and temporal scales, even without taking into consideration any of the effects of human activities.

Ice ages and climate forcings

Over geological timescales, global climate has alternated between generally warm conditions and 'ice ages'

characterised by lower global mean surface temperatures and major continental ice sheets (Barry and Chorley

2003). At least seven major ice ages have occurred during Earth's history. Although the term 'ice age' refers to

a general, prolonged cold period, it should be noted that considerable variability in temperature has occurred

both within and between those ice ages. During the ice ages, vast ice sheets formed over the continental land-


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masses. The more recent ice ages involved the formation of ice sheets over what is now North America and

northern Europe; during earlier glacial periods, the configuration of the continental land masses was different

owing to the fact that continents have moved over geological time due to the process of continental drift.

The fact that the glacial events - as well as the smaller-period stadials and interstadials within those glacials -

have occurred with considerable regularity has led scientists to search for, and to identify, various forcing

factors (or 'climate forcings'): in other words, driving mechanisms of climate change. Those climate forcings

include both external and internal forcings; external forcings refer to processes originating outside of the

atmosphere (such as variations in solar output, astronomical effects, tectonic processes, and volcanic

eruptions), whilst internal forcings refer to processes originating within the atmosphere (such as changes in

atmospheric composition and cloud cover; Barry and Chorley 2003). The major ice ages of the climatic record

have occurred due to a combination of and interaction between external and internal forcings.

The last glaciation reached its maximum around 20 000 years ago and then ended abruptly approximately 14

700 to 13 000 years ago (although a short, cold interlude occurred between 12 900 and 11 600 years ago). By

10 000 years ago, at the beginning of the most recent period of geological time (known as the Holocene), the

climate warmed rapidly and the ice sheets of continental Europe and North America were in retreat. A thermal

maximum occurred around 5000 years ago, which was followed by another temperature decline and by wetter

conditions in Europe and North America. Fluctuations in temperature have continued to occur since that time,

and more recent temperature variations have been relatively well-documented: they include a warm phase in

around the year 1300 CE (common era) and a cold phase between 1450 and 1700 CE (known as the 'Little Ice

Age'). A variety of smaller period climate fluctuations have also been identified, such as the El Nio-Southern

Oscillation (ENSO).

Why is it important to study climate change in the past?

Variations in climate demonstrate that natural climate change is the normal state of affairs, at all spatial and

temporal scales. This forms the context in which more recent, anthropogenic climate changes should be

interpreted. The study of climate change in the past also allows investigation of the natural causes of climate

changes, of the ways that different parts of the climate system behave and interact in responding to these

causes, and of the multiple impacts of these various changes.


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Reconstructing past climates

The climatic record is reconstructed by scientists using a wide variety of techniques. A major source of

information about palaeoclimates is the record stored in the vast ice sheets of Greenland and Antarctica that

extend to heights of several kilometres. By drilling and removing ice cores from these ice sheets for analysis,

scientists have access to a record of atmospheric conditions that extends back for over 500 000 years

(Houghton 2009). Within the ice cores, small bubbles of air are trapped; analysis of those air bubbles reveals

the composition of the atmosphere at the time at which various strata (layers) of the ice sheet were formed.

Analysis of the isotopes of oxygen present in the ice cores can also provide valuable information about the

variation in temperature in polar regions at various times in the past, since a heavier isotope of oxygen (18O) is

present in the ice caps in varying concentrations depending on the average temperature at the surface (since

18O levels depend on the rate of evaporation of sea water and the rate of snowfall accumulation).

Further palaeoclimatic information can be obtained from corals and from sediment cores drilled from the ocean

floor; those methods adopt a similar approach to the analysis of ice cores, since the 18O content of ocean water

varies with temperature and can be used to reconstruct the total volume and extent of the polar ice caps at

different times in the past. In addition, analysis of radioactive isotopes (such as 14C) can yield information that

may be used to date evidence from coral and sediment cores (Houghton 2009). By using a variety of different

palaeoclimatic techniques, scientists have reconstructed the climatic record over many hundreds of millennia.

The figure in 2.1.1 shows variation in CO2 concentration in the atmosphere and in temperature over the past

400 000 years as estimated from the ice cores and in the last 500 years. Note the correlation between these

two variables. It should also be noted that variation in polar average temperature is approximately twice the

variation in global average temperature (Houghton 2009)

2.1.1 Historical trends in carbon dioxide concentrations and temperature, on a geological and

recent timescale


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What can the correlation between historical atmospheric carbon concentrations and temperature in the figure

in 2.1.1 tell us about the causes of climate change?

The existence of a correlation between these two variables by itself provides no information on whether high

temperatures cause high carbon dioxide concentrations, or high carbon dioxide concentrations cause high

temperatures, or indeed if both are caused by some third factor. The knowledge that carbon dioxide is a

greenhouse gas might suggest that high carbon dioxide concentrations are a contributor to the higher

temperatures that they are correlated with. However, we still need to determine the causes of historical

variation in carbon dioxide concentrations and the processes and extent of possible positive feedbacks

whereby high temperatures might increase carbon dioxide concentrations.

2.2 Climate forcings and feedbacks


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External and internal forcings

Global climate is determined by a combination of various climate forcings (in other words, driving mechanisms

of climate change) and feedback effects. As mentioned above, climate forcings include both external and

internal forcings.

External forcings originate outside of the atmosphere; they include solar variability, astronomical effects,

tectonic processes, and volcanic eruptions, and are briefly described below.

Solar variability occurs because the Sun is a variable star whose irradiance varies over an 11-yearsolar cycle (the 'Sunspot cycle') and whose magnetic field varies over a 22-year period. Periods during

which the activity of Sunspots and solar flares is reduced correspond with reduced temperatures on

Earth.

Astronomical variations include three principal cyclical changes in the Earth's orbit around the Sun:(a) variations in the eccentricity of the orbit; (b) variations in the obliquity (tilt) of the Earth's axis;

and (c) a 'wobble' in the direction of the Earth's axis of rotation, which leads to a phenomenon known

as the precession of the equinox.

Tectonic processes may influence global climate by redistributing the continental land masses of theglobe through the process of continental drift; the concentration of land masses at high latitudes

causes the more rapid accumulation of snow and ice sheets which, in turn, alter the albedo of the

Earth and climatic patterns.

Volcanic eruptions may inject large quantities of dust and sulphate aerosols into the atmosphere -especially in previous periods of Earth's history when volcanic activity was much greater than at

present - and may cause global climatic cooling, although with pronounced regional variations.

Internal forcings originate within the atmosphere (although they may be stimulated by external forcing,

through a variety of mechanisms and feedback effects, which we discuss later); they include changes in

atmospheric composition and cloud cover (Barry and Chorley 2003). Whilst changes in the atmospheric

concentrations of greenhouse gases (such as carbon dioxide) are now well-documented and are prominent in

discussions about anthropogenic climate change, the composition of the atmosphere has varied considerably

through geological time for natural reasons. These include

Growth in the biosphere (both on land an in the oceans) with increasing carbon dioxide sequestrationby plants and 'locking up' of organic carbon in living matter (for example, in trees), in detritus on the

surface and in soils, and in sediments (coal, oil and gas).

Major changes in the Earth's thermohaline circulation - the transfer of cold, salty water in the oceans -have, in the past, also caused dramatic changes in vegetation patterns, and thus in large changes in

atmospheric carbon dioxide and methane concentrations.


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Changes in cloud cover have accompanied changes in the distribution of continental land masses andocean basins (as a result of the process of continental drift), since the occurrence of large, shallow

seas in low latitudes leads to increased rates of evaporation and, in turn, to increased cloud formation.

Changes in greenhouse gas concentrations in the atmosphere may arise as a result of changes in thebiosphere (sequestering carbon dioxide as described above) or in global temperatures (changing

evaporation from the oceans and the water vapour content of the atmosphere).

Another form of internal forcing involves changes in the extent of the cryosphere affecting the extent of snow

and ice and hence the Earth's albedo.

Feedback mechanisms

Global climate change cannot be understood purely in terms of the effects of single or simple climate forcings,

whether those forcings are external or internal. This is because the global climate system involves the complex

interaction of various constituent systems, including the ocean, the cryosphere, the lithosphere and the

biosphere; these systems exhibit multiple interactions that occur over an enormous range of spatial and

temporal scales. The behaviour of these systems and their interactions is not fully understood and, in addition,

attempts to understand their functioning are hindered by the way that human activities now exert impacts that

tend to obscure the magnitude and rate of underlying natural processes.

It is clear, however, that climate change involves the operation of a multitude of feedback mechanisms:

processes that have the effect of modifying the original perturbation. Feedback mechanisms may be positive (if

they have the effect of amplifying the original perturbation) or negative (if they have the effect of dampening

the original perturbation). In addition to (external and internal) climate forcings, climate change involves a

vast array of positive and negative feedback mechanisms whose effects add up to an overall 'climate response'.

Unsurprisingly, the task of understanding - or even identifying - the behaviour of all the significant feedback

mechanisms that influence global climate is a daunting task for climate scientists.

Positive feedback mechanisms are common in the global climate system; in particular, they tend to amplify

temperature rises. An example of a positive feedback mechanism is the increase in evaporation rates that

accompany a rise in global average temperature: greater evaporation leads to an accumulation of water

vapour in the troposphere and, since water vapour is a greenhouse gas, its rising concentration will tend to

exacerbate the original temperature rise. Another positive feedback mechanism associated with rising

temperature is the reduced capacity of land and ocean sinks to absorb carbon dioxide, with the result that


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anthropogenic carbon dioxide tends to accumulate in the atmosphere more rapidly and to compound the

original temperature rise (IPCC 2007). Many other examples of positive feedback mechanisms could be cited:

for instance, a rise in global average surface temperature leads to the melting of glaciers, ice sheets, and

permafrost (reducing albedo and, in the last case, releasing methane and carbon dioxide), and to the increased

occurrence and severity of forest fires (also releasing carbon dioxide), all of which tend to exacerbate the

original temperature rise.

Negative feedback mechanisms are less numerous and they actually tend not to cause global cooling, although

they may moderate the rate of temperature increase. Greater uncertainty also surrounds the subject of

negative feedback mechanisms, although some authors have argued that increased global average surface

temperature may lead to increased evaporation and thereby to increased cloud cover - and that this effect may

partially offset the original temperature rise (Barry and Chorley 2003). Another example of (weak) negative

feedback is the way that increasing atmospheric concentrations of carbon dioxide can lead to increased carbon

dioxide uptake by oceans and by plants.

From the point of view of climate modellers, feedback mechanisms - both positive and negative - represent a

profound challenge to the task of understanding climate change because, even if the sign (positive or negative)

of a feedback mechanism is known, the magnitude of the effect may be very difficult to quantify. It is not

difficult to appreciate that the existence of even a relatively small number of feedback mechanisms can

complicate the task of understanding the climate response considerably - yet a vast multitude of feedback

mechanisms exists, operating at all spatial and temporal scales. A further complicating factor is that feedback

mechanisms - and climate processes more generally - may behave in a non-linear fashion, meaning that the

magnitude of their effects may be determined by more than one variable and may be extremely difficult to

predict. The existence of non-linear positive feedbacks leads to the possibility of 'tipping points', where

temperature increases reach a point where positive feedbacks may reinforce each other, leading to sustained

natural processes that reduce albedo and increase the atmospheric concentrations of greenhouse gases and

hence global temperatures. The term 'tipping points' may also refer to non-reversible changes in parts of the

global climate system.

By consulting the Key Readings for this unit and other sources (web resources, textbooks etc),

identify major possible 'tipping points' and/or Earth/climate system changes, noting down the

positive feedback processes which are involved, and their possible effects.

2.3 Radiative forcing, climate sensitivity


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Radiative forcing

We have noted that the various driving mechanisms that cause global climate to change - whether for natural

or anthropogenic reasons - are collectively known as 'climate forcings' (Barry and Chorley 2003). As suggested

above, natural climate changes have occurred - and continue to occur - as a result of external climate forcings

such as the astronomical variations in the Earth's orbit, tectonic activity, and volcanic eruptions, and internal

forcings such as changes in the composition of the atmosphere. All external and internal forces lead to changes

in the balance between the incoming solar radiation and the outgoing terrestrial radiation. Any imbalances that

result are immediate drivers of global temperature change and therefore are a direct climate forcing, and is

therefore described as a 'radiative forcing'. Radiative forcing may be positive or negative; thus a positive

radiative forcing implies that the atmosphere becomes warmer, whilst a negative radiative forcing implies a

cooling effect.

The relationships between external, internal and radiative forcing are illustrated in the figure in 2.3.1. You may

like to refer back to the earlier description of external and internal forcings and feedback effects as you look at

this figure. It shows how a range of different external climate forcings may cause changes in net solar radiation

while a range of internal forcings may cause changes in terrestrial radiation - and some of the external forcing

may stimulate internal forcings, as explained earlier. There are also multiple feedback effects and interactions

between the different internal forcings in the climate system. Changes in net solar radiation or in terrestrial

radiation caused by external or internal forcings lead to changes in radiative forcing, and this, in turn, leads to

changes in terrestrial radiation (a negative feedback effect) and to other positive and negative feedback effects

on the climate system and on internal forcings.

2.3.1 External, internal, and radiative forcings, and global temperature change


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Source: unit author

Since radiative forcing refers to the balance of energy in the Earth's atmosphere, it is expressed in units of

watts per square metre (W m-2). For reasons of consistency and standardisation, radiative forcing is typically

measured at the top of the troposphere (the lowest layer of the atmosphere). Therefore, formally, radiative

forcing is defined as the change in the average net radiation at the top of the troposphere. The concept of

radiative forcing is an extremely useful one in understanding climate change because it offers a consistent,

standardised means of expressing the effects that a wide range of factors - whether natural or anthropogenic -

may have on global climate. Furthermore, the concept of radiative forcing permits us to define climate change

in yet another way: as the response of the climate system to a radiative forcing, regardless of whether that

forcing is positive or negative.

Climate sensitivity

Human and natural factors influence global climate by altering the radiative properties of the atmosphere by

which energy is scattered, absorbed, and re-emitted. We have noted that changes in those factors are known


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as radiative forcing, which provides a measure of the significance of particular climate change mechanisms.

Furthermore, the concept of radiative forcing describes a perturbation of (or change in) the energy balance of

the Earth's atmosphere in units of W m-2, with positive values indicating atmospheric warming whilst negative

values imply cooling (Houghton 2009). This allows the convenient quantification of the radiative forcing effects

of various natural events and human activities. However, measuring 'radiative forcing' is not the same thing as

measuring 'climate change' directly. How good a measure of climate change is radiative forcing? If we are to

use the concept of radiative forcing as a 'surrogate' for climate change, it is helpful to know how accurate such

an approximation is.

Fortunately, there is a very close relationship between radiative forcing and a standard measure of climate

change - global mean surface temperature. Indeed, global mean radiative forcing displays an approximately

linear relationship with global mean surface temperature change (Seinfeld and Pandis 2006). Therefore, it is

fairly safe to assume that the same radiative forcing - from whatever source - will result in the same climate

response. This allows us to introduce another important term in relation to climate change: that of 'climate

sensitivity'. Climate sensitivity is a measure of how much the climate is expected to change in response to a

given perturbation. This concept is useful as it provides a way to compare the different projected climate

responses that accompany various scenarios of future levels of economic activity and types of social and

political organisation.

Another, related concept that is useful as a means of expressing and communicating the effects of different

future scenarios is that of the 'equilibrium climate sensitivity'; this is defined as the equilibrium global average

surface warming following a doubling of the atmospheric carbon dioxide concentration. Thus the equilibrium

climate sensitivity expresses the climate response to a sustained radiative forcing of known magnitude, the

projected doubling of global carbon dioxide concentrations in the atmosphere (IPCC 2007). The climate

sensitivity is estimated by the IPCC (2007) from an assessment of various climate modelling results to be in

the range of 2 C to 4.5 C, with a best estimate of 3 C and the same estimate is reached by Hansen (2008)

from analysis of historical data ice-core data.

3.0 Anthropogenic climate change processes

Section Overview


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We conclude this unit with a very brief consideration of how anthropogenic activities may influence the climate

and cause climate change. We do not in this unit consider in any depth the evidence for anthropogenic

activities actually driving climate change. Two principal processes are considered: the accumulation of

greenhouse gases, and the accumulation of aerosols and associated changes in albedo.



list the main 'anthropogenic greenhouse gases' and describe their radiative forcing characteristicsdiscuss ways in which human activities lead to the accumulation of these greenhouse gases in the

atmosphere

describe the radiative forcing effects of different aerosolsexplain the importance and causes of uncertainty in climate change predictions

3.1 Anthropogenic greenhouse gases

Overview of greenhouse gases

In previous sections we have seen that the atmosphere contains a diverse mixture of gases and particles,

some of which play an important role in absorbing some of the energy radiated by the surface of the Earth -

and thus giving rise to the greenhouse effect, whether that greenhouse effect is natural or anthropogenic

('enhanced'). In particular, the so-called 'greenhouse gases' of the atmosphere selectively absorb outgoing

radiation at various wavelengths and thereby help to retain heat in the atmosphere.

What do you think are the major greenhouse gases?

This question is not as easy to answer as one might think as it depends upon what we mean by 'major

greenhouse gases'. The two gases that are currently responsible for the greatest radiative forcing in the

atmosphere are water vapour and carbon dioxide. Other significant greenhouse gases that are found

naturally in the atmosphere are methane (CH4), nitrous oxide (N2O) and ozone (O3). The major

anthropogenic greenhouse gases are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O),

hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6). These are listed in

declining importance as regards volume of anthropogenic emissions but in (broadly) increasing potency as

regards the global warming potential of each gram of gas in the atmosphere.


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Although the term 'greenhouse gas' has acquired a strongly negative connotation as a result of concerns about

the significance of a major waste product of industrialised societies (carbon dioxide) it is important to

remember that, in themselves and in the right quantities, greenhouse gases are not necessarily harmful to

species, ecosystems or habitats. Indeed, the presence of greenhouse gases in the atmosphere has been vital in

the evolution of life, both in ensuring that global temperatures are supportive of life as it has evolved, and in

other ways that they are used in photosynthesis, respiration, and other life processes.

Many of the gases present in the atmosphere act as greenhouse gases to some extent, and the assortment of

greenhouse gases includes such substances as benign and ubiquitous as water vapour. Although water vapour

is a major greenhouse gas (in terms of its importance in 'trapping' heat in the atmosphere), it is an important

component of other environmental systems and processes (such as the hydrological cycle) and its

concentration in the atmosphere is largely determined by temperature and other features of the climate

system. Increasingly acute concerns about anthropogenic climate change have focused attention on six main

greenhouse gases (or groups of gases) in international climate change negotiations: carbon dioxide (CO2),

methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur

hexafluoride (SF6) (United Nations 1998).

The importance of different gases in promoting global warming depends upon

1. their concentration in the atmosphere2. the greenhouse effect of each molecule (as a result of their absorption of radiation, a consequence of

their chemical structure)

3. the length of time that the gases stay in the atmosphere

The latter two factors are used in estimates of the global warming potential (GWP) of different gases as

compared to the same quantity of carbon dioxide. This allows quantities of greenhouse gases other than

carbon dioxide to be converted to, and expressed in terms of, their 'carbon dioxide equivalent' (CO2-equivalent

or CO2e) values (Houghton 2009; Stern 2007).

It must be noted, however, that global warming potential and hence the calculation of carbon dioxide

equivalent for different gases varies with the time period being considered, as different gases differ with regard

to the time that they stay in the atmosphere - gases that are more long-lived than carbon dioxide therefore

have higher 200 year GWP and CO2e values than 100 year GWP and CO2e values (100 year values being those


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normally used). Conversely the most widely used HFC has a relatively short atmospheric lifetime as compared

with CO2 and hence its 20 year GWP (3830) is over 2.5 times its 100 year GWP (1400) (Mate et al2009).

The most significant anthropogenic greenhouse gas is carbon dioxide, due to both its relative abundance and

its long atmospheric lifetime and, consequently, it has become highly prominent in contemporary debates

about anthropogenic climate change. The accumulation of anthropogenic carbon dioxide in the atmosphere is

expected to result in climate change on timescales ranging from decadal to millennial, and carbon dioxide may

potentially affect the climate system for hundreds of thousands of years (Archer 2005; Houghton 2009; IPCC

2007; Lenton et al2006). Carbon dioxide is used as the basis for calculating global warming potential (GWP, as

discussed above) because it is the anthropogenic greenhouse gas of principal concern.

Two important naturally occurring greenhouse gas emissions are omitted from the list of 'anthropogenic

greenhouse gases': water vapour (H2O), and ozone (O3). Water vapour is omitted for reasons outlined earlier.

Human impacts on concentrations of ozone in the atmosphere are similarly affected not so much by direct

anthropogenic emissions as by the effects of chlorofluorocarbons (CFCs) in the atmosphere, as these break

down ozone. This process is well known in relation to another, different global environmental issue: that of the

depletion of the polar stratospheric ozone layer. The issue of stratospheric ozone depletion should not be

confused with that of climate change, and it is the latter issue which concerns us here. It should be noted

however, that

ozone is greenhouse gas, but its radiative forcing effects are complex because although ozone in thetroposphere has positive (ie greenhouse gas) radiative forcing effects in the troposphere its presence

in the stratosphere has weaker negative radiative forcing effects which undermine its overall strength

as a greenhouse gas;

the international agreement for controlling chlorofluorocarbons in the atmosphere (the Montrealprotocol) is often discussed as a model for climate change negotiation and agreement processes;

hydrofluorocarbon (HFC) gases promoted for use under the Montreal protocol to reduce ozonedepletion have the potential to become serious greenhouse gases if they accumulate in the

atmosphere, as discussed below.

Anthropogenic greenhouse gas accumulation processes

So long as the atmospheric concentrations of greenhouse gases remain constant, there is no imbalance in the

radiative budget for the Earth, and hence no radiative forcing and no climate change. However, any change in

the abundance of greenhouse gases is likely to cause radiative forcing and climate change. The most important


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cause of anthropogenic climate change is the accumulation of greenhouse gases in the atmosphere, especially

the accumulation of carbon dioxide.

Can you suggest two broad ways in which human activities are promoting an accumulation of greenhouse

gases in the atmosphere?

Human activities may directly release greenhouse gases into the atmosphere, and human activities may lead

to a reduction in the rate at which greenhouse gases are absorbed from the

atmosphere.

An accumulation of greenhouse gases is now occurring for several reasons. First, and most significantly,

greenhouse gases are emitted directly as a result of various human activities - especially the combustion of

fossil fuels (such as coal, gas, and oil). Fossil fuels represent concentrated stores of organic (carbon-

containing) material that, when burned, is oxidised to release carbon dioxide and water vapour. Over many

centuries in some cases, industrialised societies have burned coal, gas, and oil in order to supply energy for the

processes of production, heating, cooling, and transport. The emission of carbon dioxide as a result of fossil

fuel combustion is compounded by the direct emission of other greenhouse gases: methane is released from

the anaerobic decomposition of organic matter (in the digestive systems of cows and other ruminants, in

flooded rice cultivation, and in the decomposition of waste in landfill sites, for example); nitrous oxides are

released when nitrogen fertilisers break down in soils; and various artificial compounds such as

chlorofluorocarbons (CFCs) and perfluorocarbons (PFCs) are released in different manufacturing processes and

when pieces of equipment such as refrigerators are scrapped.

Another reason why greenhouse gases are now accumulating in the atmosphere is the fact that the 'sinks' that

2 climate systems

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