Proceedings of the Geologists’ Association 121 (2010) 334–341

Topical perspective

Quaternary climates: a perspective for global warming

James Rose a,b,*a Department of Geography, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UKb British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

A R T I C L E I N F O

Article history:

Available online 3 August 2010

Keywords:

Quaternary

Ice Age

Global warming

Milankovitch forcing

Sub-Milankovitch forcing

A B S T R A C T

This brief review provides an Earth Science perspective on present climate change (global warming)

using evidence from past ice ages with details from the Quaternary ice age. It places the present

(Quaternary) ice age in the context of Earth history and outlines possible causes of ice ages and the scale

and style of ice age climate. Milankovitch climate forcing is described and explained as the cause of

relatively predictable climatic variations within an ice age (and at other times), and this is followed by an

outline of the factors likely to be responsible for short and rapid sub-Milankovitch climate variations that

are superimposed on the predictable changes. Finally the anomalous, relatively constant climate of the

last 11.5 ka (Holocene) is highlighted and explained in terms of human input of greenhouse gasses into

the Earth’s atmosphere.

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1. Introduction

We are all made aware of climate change through mediaattention to global warming, but little time is given to placingpresent-day climate changes within the longer timescale of Earthhistory and indeed in the context of climate change over the periodthat humans have been on Earth. This brief review aims to providea perspective on these two issues. It will examine the maincharacteristics of the climates in which humans have lived, andwill consider what is special about the climate of the Earth atpresent – the climate that we understand as causing ‘globalwarming’.

2. Ice ages in Earth history

Currently we live in an ice age and humans have lived throughthis ice age. This is the Quaternary ice age and it covers a period ofabout 2.75 million years (the Quaternary extends from 2.58 Ma tothe present (Gibbard et al., 2010) but cooling began a little timebefore 2.58 Ma). What this means is that we live in a period of timewhen the climate is cold enough to cause glaciers and permafrostto be extensive on the globe, although the extent of glaciation hasvaried from ice reaching the latitudes of London and New York toice located primarily on Greenland and Antarctic, as at present.

Throughout much of the 4.56 billion years of Earth history,glaciers have not existed on the planet. Fig. 1 gives a summary of

* Correspondence address: Department of Geography, Royal Holloway, University

of London, Egham, Surrey TW20 0EX, UK.

E-mail addresses: j.rose@rhul.ac.uk, jrose@dircon.co.uk.

0016-7878/$ – see front matter � 2010 The Geologists’ Association. Published by Else

doi:10.1016/j.pgeola.2010.07.001

the occurrence of large continental ice sheets in relation to thelikely distribution of land areas (Ruddiman, 2001). There havebeen up to seven periods when the Earth was in this state andthere have been three intervals of glaciation in the last 500 millionyears, the approximate duration of the evolution of the mainanimal and plant phyla. The most recent period of global cooling –the ice age in which we now live, is first recorded in evidence forglaciation on Antarctica about 35 million years ago, but evidencefor glaciers or permafrost in temperate regions such as thoseinhabited by most people on the globe was first recorded about2.75 million years ago.

3. The causes of the present ice age

The factors that bring about global cooling and ice ages aresubject to much debate (Ruddiman, 2001). Essentially, for an iceage to occur we need a mechanism that can cause ice to develop atboth poles. As we know from current debate, greenhouse gassesare the most effective way of controlling the climate of the wholeEarth and variations in carbon dioxide (CO2), an abundantgreenhouse gas, are considered by many to be the most likelycause. However, we need a process that will control greenhousegasses on a long timescale (10s million years) and not like now,where variations can be significant on a centennial scale.Furthermore, we need a process that can take the CO2 out of theatmosphere for a long time, in order to bring about cooling.

There are many theories and much debate, but two explana-tions are worth consideration here. The first explanation is basedon the concept that increases in the rates of mountain buildingcause geochemical removal of CO2 from the atmosphere bychemical weathering reactions. This mechanism has much in its

vier Ltd. All rights reserved.

Fig. 1. Occurrence of glaciations during Earth history, along with the position of the

continents at particular periods. Note that the scales change at 1 billion years ago

and that the evidence for glacials and indeed the position of the continents before

about 500 million years ago are not well known (from Ruddiman, 2001).

J. Rose / Proceedings of the Geologists’ Association 121 (2010) 334–341 335

favour as the last 50 million years have seen the collision of twocontinental plates (Indian and Eurasian Plates) (Fig. 2) whichhave, in turn, resulted in the formation of the exceptionally largeand high Tibetan Plateau and Himalayan Mountains. High ratesof geomorphological activity and erosion in these regions lead to,in terms of Earth history, relatively high rates of chemicalweathering because of the reaction of CO2 with eroded andtransported silicate rocks (CaSiO3). This reaction results in theproduction of lime (CaCO3) and silica (SiO2) which aretransported to the sea in solution, where they are used to formthe skeletons of plankton which eventually die, to form sea-beddeposits and other carbonate secreting marine organisms such ascorals. The carbon dioxide is therefore taken out of theatmosphere and locked in the sea-bed sediments. Greenhousegas is taken out of the atmosphere and the Earth cools. In thiscase, it is plate tectonics and mountain building that has broughtabout the present ice age.

A second explanation that needs consideration is the conceptthat the transfer of heat from the tropical oceans to the highlatitude oceans has become less effective, with consequentialcooling of the polar regions. Attention as been drawn to the factthat the land-bridge between the Antarctic continent and South

America (Drake’s Passage) was breached about 20 million yearsago and the landbridge between North and South America(Isthmus of Panama) (Fig. 3) as created between 10 and 4 millionyears ago. Clearly these changes in the geography of the Earthhave the potential to be very significant. The creation of DrakesPassage means that a strong circumpolar current can now isolateAntarctica from the warm waters of the rest of the ocean, and thecreation of the Isthmus of Panama means that warm, salty wateris diverted to the North Atlantic generating more moisturewithin polar regions leading to the formation of ice sheetsthrough increased precipitation of snow. However, modellingexercises do not support these scenarios, and it is interesting tonote that two diametrically opposing processes (opening andclosing of seaways) are used to bring about a single process:global cooling.

In these circumstances, mountain building is the favouredhypothesis for the initiation of the present ice age, although anycooling brought about by this mechanism would affect otherprocesses such as patterns of atmospheric and oceanic circulation.

4. Scale and styles of ice age climate

An evaluation of climate change over the present period ofglobal cooling (c. 35 million years) is not easy, as quantitativeestimates of climate are difficult to derive and those that do existare mainly from the oceans. However, patterns of change shown bythe various indicators do provide climatic trends and identifycritical changes. Fig. 4, which shows the pattern of climate changeover the last 70 million years, indicates the change fromgreenhouse conditions (warm climate with high atmosphericcarbon dioxide) to ice age and the variations that have occurredduring the ice age. The figure shows that cooling began about 55million years ago, but that the ice age switched on about 35 millionyears ago. Following that, there are periods of severe cooling atabout 15 million and about 2.75 million years ago. The last coolingis associated with hominin evolution and occupance of the Earthand takes us to the climate of the present day. The temperaturecurve is very generalised and does not indicate the small-scalechanges represented by periods of glacier expansion (glacials) andperiods of limited ice cover (interglacials).

When we look at the patterns of change over the last c. 6 millionyears as shown in Fig. 5 we see much more detail of change withthe variations between glacials and interglacials. It is clear that astyle of climate change began about 2.75 million years ago whenclimatic cycles increased their amplitude, and began to representlower temperatures. Both Figs. 4 and 5 use d18O values as a proxy ofdeep water temperature (see caption of Fig. 4), but they are also aproxy of ice volume. These figures therefore also indicate theexpansion of glacier ice, and Fig. 5 shows the periods when icesheets expanded over high and temperate latitudes. The figure alsoshows the very short intervals of time since 2.7 million years whenthe globe had temperatures similar to the present day. Wetherefore see an ice age world that has become progressivelycolder, but has experienced extreme swings of climate, fromtemperatures roughly similar to the present, to temperatures thatgenerated an ice-cover across polar and temperate latitudes of theglobe.

5. Milankovitch forcing of climate – the yardstick for climatechange during human time

A closer look at the saw-tooth pattern of Fig. 6 shows threegroups. At the base, up to about 2.75 million years ago thevariations are (in most cases) of small amplitude and occur veryfrequently – at roughly 23,000-year intervals. Between about 2.75million years ago and about 900,000 years ago the variations have

Fig. 3. Location of Drake’s Passage and the Isthmus of Panama in relation to present day ocean currents. It can be seen how the creation of the waterway between Antarctica

and South America would enable cold water to encircle Antarctica and hence isolate this region from warm ocean water; and how the creation of the landbridge between

North and South America would isolate the Pacific from the Atlantic Oceans and how the warm, westward flowing tropical water in the Atlantic would be diverted northwards

to join the Gulf Stream.

Fig. 2. Pattern of plates across the globe and the location of the collision zone between the Indian and Eurasian continental plates. The area of high land that comprises the

exceptionally high Himalayan Mountains and the high and extensive Tibetan Plateau is shown.

J. Rose / Proceedings of the Geologists’ Association 121 (2010) 334–341336

Fig. 5. Climate change over the last 6 million years. The saw-tooth nature of the

curve represents climatic cycles. It is clear that the trend of the cycles is towards

cooling and that the amplitude of the cycles increases as the cooling becomes more

severe. It is in these high amplitude cycles, representing changes from interglacial

to glacial conditions that humans have lived and evolved. The change about 2.75

million years ago represents the onset of glacier expansion and the development of

more extreme climate.

Fig. 4. A representation of climate change over the last 70 million years from oxygen isotope measurements of benthic (deep water) foraminifera. The ratio of oxygen isotope

16–18 is taken as a proxy of deep water temperature and hence of global climate. The result shows that cooling began about 55 million years ago, but that the ice age switched

on about 35 million years ago. Following that, there are periods of severe cooling about 15 and about 2.75 million years ago. The last cooling is associated with human

occupance of the Earth and takes us to the climate of the present day. The curve is very generalised and does not indicate the small-scale changes represented by periods of

glacier expansion (glacials) and periods of limited ice cover (interglacials).

Fig. 6. This covers roughly the same period as Fig. 5, but highlights the nature of the Mil

million years ago that the climate was characterised by small amplitude cycles with a 23

its axis, known as the precession of the equinoxes, and this is precession forcing. Between

and has a frequency of 41,000 years. These are forced by the tilt or obliquity of the Eart

climate has large amplitude changes of climate and the cycles occur every 100,000 years.

eccentricity forcing.

J. Rose / Proceedings of the Geologists’ Association 121 (2010) 334–341 337

bigger amplitude and a lower frequency of occurrence – in this caseabout 41,000-year intervals. Finally, between 900,000 years agoand the present it is clear that the amplitude of the oscillations islarge and the frequency is about 100,000 years.

These frequencies are the same as the frequencies of the threeproperties that control the amount of insolation that reachesplanet Earth from the Sun. The 23,000-year interval is the durationof a wobble of the axis of the Earth as it rotates (precession), the41,000-year interval is the duration of a cycle determined by thetilt of the axis of the Earth (obliquity); and the 100,000-yearinterval is a product of the eccentricity of the Earth’s orbit aroundthe sun (eccentricity). These properties are illustrated in Fig. 7 andare well explained in Imbrie and Imbrie (1979) and Ruddiman(2006). These properties determine the amount of solar radiationthat reaches the Earth, and all occur in a predictable fashion. Thus itis possible to calculate the amount of insolation that will reach theEarth at any given time and predict when the Earth’s climate islikely to be cold and when it is likely to be warm (Fig. 8). This wasfirst recognised by James Croll (while janitor in the AndersonianCollege in Glasgow) in 1864, but elaborated more fully, withcalculations that are close to present-day predictions, by MilutinMilankovitch, a Serbian mathematician in 1920.

ankovitch cycles that dominate the Quaternary ice age. It is clear that prior to 2.75

,000 year frequency – these are forced by changes in the wobble of the Earth around

2.75 million years and 900,000 years ago the amplitude of climate change increases

h’s axis and this is obliquity forcing. From 900,000 years ago until the present the

This is forced by the eccentricity of the Earth’s orbit around the Sun and is known as

Fig. 7. Components of the Milankovitch astronomical theory of climate change. (A) Eccentricity of the orbit. (B) Obliquity or tilt of the axis. (C) Wobble of the tilt of the axis or

precession of the equinoxes. Taken from Lowe and Walker (1997).

J. Rose / Proceedings of the Geologists’ Association 121 (2010) 334–341338

The details of these insolation forcing relationships, and thecauses of the changes of frequency and amplitude are complex(Ruddiman, 2001, 2006) but, for life on Earth they are simple – theymean that until about 2,700,000 years ago the climate of the Earthchanged little although glaciers did develop at high latitudes,especially at high elevations. After this, and until about 900,000years ago, the climate changed more dramatically with periods ofglaciation well developed at high latitudes and at high altitudes incool temperate latitudes. Paralleling these changes of ice cover,there were changes in sea-level and flora and fauna. However,these changes were relatively small compared to those in the final900,000 years and the time in which we live. This interval of timesaw massive expansion of glacier ice at about 100,000 intervals andconditions colder than now for nearly every 90,000 years of each100,000 cycle. Periods with temperatures like those of the Earthtoday only occurred at 100,000-year intervals and only occurredfor about 10,000 years on each occasion. For most of this period theEarth was a cold, dusty place with relatively little biomass and sea-levels much lower than now (up to about 120 m lower). This typeof climatic change is forced by solar radiation reaching the Earthand is called Milankovitch forcing.

6. Sub-Milankovitch forcing of climate – complicatingMilankovitch regularity

Milankovitch forcing provides a yardstick for the style ofclimate change while hominins have lived on the Earth, butunfortunately, this yardstick only provides a guide of the mainclimatic characteristics; it does not identify the smaller andirregular changes – changes that are still very important forhominin occupance.

The Milankovitch hypothesis was first tested and vindicated bythe records of climate change derived from ocean cores thatcovered long periods of time (Shackleton and Opdyke, 1973), andwere of relatively low resolution. They showed the Milankovitchvariations, but failed to identify the smaller and often very rapidchanges. With more recent work on marine cores from areas withrapid sedimentation and on ice cores from Greenland andAntarctica a much more detailed record of climate change hasbeen derived, showing climate changes at the scale of hundredsand thousands of years (Alley, 2000). These changes are shown inFig. 9, which is from an ice core in Greenland, but similar changescan be shown from detailed marine cores or detailed lake

Fig. 8. (A) Variations in eccentricity (c. 100,000 years), obliquity (c. 41,000 years)

and precession (c. 23,000 years) with an integrated composite curve for the time

period 800,000 years to the present. (B) A generalised climatic curve for the same

period of time derived from the oxygen isotope values from a number of marine

cores. Note the general similarity of the composite curve with the climatic record.

Taken from Lowe and Walker (1997).

Fig. 9. The pattern of rapid and extreme climate change shown by the ice core record

from Greenland over the last c. 120,000 years. This curve shows changes of

temperature on Greenland derived from the GRIP ice core. It can be seen that at the

scale shown on this figure, changes of around 10 8C are almost instantaneous, and

that such changes are frequent. This figure also shows the temperature change

coming out of the Last Glaciation about 11,500 years ago with a change in

temperature of about 15 8C. Taken from Jouzel et al. (2007).

J. Rose / Proceedings of the Geologists’ Association 121 (2010) 334–341 339

sediments from many regions of the world. The striking feature ofFig. 9 (Jouzel et al., 2007) is that changes of temperature inGreenland (but it could be anywhere with a fine resolution record)are rapid and frequent, and although these are most obvious attimes of relatively cold climate, records from lake and marinesediments elsewhere show that variations also occur in temperateperiods like now.

Fig. 10. Three figures to illustrate Ruddiman’s hypothesis to explain climatic

stability over the c. 11,000 years of Holocene time. (A) The pattern of solar radiation

and the natural methane (CH4) trend shown with a dashed line; the observed

methane trend shown with a solid red line. (B) The natural carbon dioxide (CO2)

trend and the observed CO2 trend. These two figures illustrate that methane and

carbon dioxide concentrations increased during the second part of the Holocene

despite the fact that solar radiation decreased and methane and carbon dioxide

values were expected to decrease over this period of time. (C) This figure

summarises the evidence that tests the Ruddiman hypothesis. The four blue lines at

the left of the figure show progressive decrease in methane values during ‘warm’

episodes, and the red line shows the same property during the Last Interglacial. The

green line refers to the Holocene and the dashed part gives the expected methane

value which should decrease like all the other curves, whereas the continuous green

line shows the actual observed trend. (A) and (B) are taken from Ruddiman (2003)

and (C) taken from Ruddiman and Thompson (2001).

J. Rose / Proceedings of the Geologists’ Association 121 (2010) 334–341340

The factors that bring about these changes are called sub-Milankovitch forcing, and include for instance, variations inatmospheric transparency caused by volcanic activity, changes inthe position of ocean currents caused by massive freshwater-pulses entering the ocean, or indeed, human activity –contemporary global warming is a sub-Milankovitch change ofclimate.

7. Climate change during the Holocene (the last 11,500 years)

Reference to the left hand side of Fig. 9 (the present interglacial,labelled Holocene) shows a contrast to most of the curve. TheHolocene is a period with relatively small change whereas theremainder of the curve records rapid and extreme climaticchanges.

The question must therefore be asked: what caused this relativelack of climate change during the Holocene, especially when thepattern of insolation forcing should have cooled the Earth’s climate(Fig. 10A)? Why is this the only time in the last 2.75 million years(as far as we know) that climate change has been very small for aperiod of around 10,000 years? As with most big questions, this is amatter of much debate. However, an explanation has been putforward by Ruddiman and Thompson (2001) and Ruddiman(2005a,b) that has gained considerable support. Ruddiman’shypothesis suggests that cooling has not occurred because humanagriculture, starting about 5000 years ago began to pumpgreenhouse gasses, in the form of methane and carbon dioxide(Fig. 10A and B), into the atmosphere, and this at first held theclimate deterioration and subsequently maintained it at arelatively constant level with a balance between insolation

Fig. 11. The record of carbon dioxide (CO2) and methane (CH4) changes over the last

four glacial-interglacial cycles as derived from the Vostok (Antarctica) ice core

records. Added to the end of these curves (right hand side) are the IPCC 2000

predicted values and it can be seen that they are at a level that has no analogue from

any previous interglacial. This is very worrying as this means that it is difficult to

learn lessons from previous interglacials and we must go back to the Greenhouse

Earth that existed before the present ice age – a totally different world with no ice

on the Earth and sea-levels many tens of metres higher than now. Taken from

Oldfield and Alverson (2002).

driven-cooling and human-forced warming. This hypothesis canbe tested against past interglacials/interstadials which werewithout any human induced warming, and these are shown tohave experienced a deterioration of climate throughout thewarmer episode (Fig. 10C).

8. Present-day human-forced climate warming – abandoningthe balance!

From the above review, it is clear that climate change is anatural phenomena and a normal and persistent feature of Earthhistory, albeit with greater detail known about the variation overthe present ice age, the last 2.75 million years. Changes of climatehave become more extreme over this period (Figs. 5 and 6), andthese changes have been towards increased cooling during thecolder episodes. Present climate warming is totally different fromwhat has happened over this time period. This can be shown inFig. 11. As is evident in this figure, global temperature changes,over the period for which we have adequate measurements, haveparalleled greenhouse gas values. Since the human activity of theindustrial revolution has begin to pump greenhouse gasses(mainly carbon dioxide and methane) into the atmosphere fromthe burning of coal and hydrocarbons, the greenhouse gas valueshave risen beyond any previous levels (Zalasiewicz et al., 2008).With regard to climate change we are now in a non-analoguesituation for the present ice age and we must go back to aGreenhouse Earth before the present ice age to see any comparablevalues (Fig. 4). For human presence on the Earth, we have cause tobe worried.

9. Comment and acknowledgments

I wish to stress that this is a topical perspective on a scientificissue of great societal relevance and not a research paper. It ispublished in order to bring to a wider readership aspects of thescientific basis to global warming and I hope it will be consideredin this context.

I would like to thank Jenny Kynaston for drawing the figures, allof which are based on other publications which are identified asappropriate and I would like to thank all the sources forcontributing the important knowledge contained therein. I wouldalso like to thank John Powell, Richard Howarth and DanielleSchreve for reading the manuscript, providing advice andencouraging publication.

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