the contemporary carbon cycle overview marine carbon cycle marine sink for atmospheric co2 the...

The Contemporary Carbon Cycle

OverviewMarine Carbon cycle

Marine sink for atmospheric CO2The atmospheric imprintAtmospheric inversions

The Terrestrial carbon sinkPrecision oxygen measurements

Reading

Overviews and reviewsThe IPCC Third assessment report. http://www.grida.no/climate/ipcc_tar/Chapter 3, wg1 report “The scientific basis” Chapter 2-4, wg3 report “mitigation” Semi-popular overview of sinks for anthropogenic carbon:Sarmiento, J.L. and N. Gruber. Sinks for anthropogenic carbon, Physics Today, 55(8), 30-36, 2002. http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/sarmiento_pt_02.pdfOcean - atmosphere fluxes: Watson, A. J. and Orr, J. C. (in press). Carbon dioxide fluxes in the global ocean . Chapter 5 in “Ocean Biogeochemistry : a JGOFS synthesis” eds Fasham, M. Field, J. Platt, T. & B. Zeitzschel. Available at: http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/Watson_and_orr_in_fasham(ed)_2004.pdf Terrestrial net fluxes: Schimel DS, House JI, Hibbard KA, et al. Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems NATURE 414: 169-172 2001 Available at http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/schimel_2001.pdfAtmospheric CO2 and O2 measurements

Battle, M. et al. Global carbon sinks and their variability inferred from atmospheric O-2 and delta C-13. Science 287, 2467-2470 (2000). http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/battle_2000.pdf

More Reading

CO2 measurements in the atmosphere, and what you can do with them: Keeling, C.D., T.P. Whorf, M. Wahlen, and J. Vanderplicht, Interannual Extremes In the Rate Of Rise Of Atmospheric Carbon- Dioxide Since 1980, Nature, 375, 666-670, 1995.Keeling, C.D., J.F.S. Chin, and T.P. Whorf, Increased Activity Of Northern Vegetation Inferred From Atmospheric CO2 Measurements, Nature, 382, 146-149, 1996.Available at: http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/keeling_cd_1995.pdfhttp://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/keeling_cd_1996.pdf

Classic paper on Inverse atmospheric calculation, and the “missing sink”.Tans, P.P., I.Y. Fung, and T. Takahashi, Observational Constraints On the Global Atmospheric Co2 Budget, Science, 247 (4949), 1431-1438, 1990.http://lgmacweb.env.uea.ac.uk/ajw/Geochemical_cycling/tans_et_al_1990.pdf

Ocean uptake of CO2.Sarmiento, J.L., and E.T. Sundquist, Revised Budget For the Oceanic Uptake Of Anthropogenic Carbon-Dioxide, Nature, 356, 589-593, 1992. Watson, A.J., P.D. Nightingale, and D.J. Cooper, Modeling Atmosphere Ocean CO2 Transfer, Philosophical Transactions Of the Royal Society Of London Series B- Biological Sciences, 348, 125-132, 1995.

4 5 0 3 0 0 1 5 0 01 5 0

Age of Entrapped Air (kyr BP)

Vostok Re cord

IPCC IS92a Scenario

Current (2001)

Proje cted (2100)

Contemporary Re cord

Atmospheric CO2:Past, present and near future

The global carbon cycle

(Source, Sarmiento and Gruber, 2002)

The global carbon cycle

•Most of the “labile” carbon on Earth is in the deep sea.•The gross atmosphere-ocean and atmosphere-vegetation fluxes are of the same order.•The net atmosphere-ocean and atmosphere-vegetation fluxes are much smaller than the gross fluxes. •The flux through the marine biota (net productivity) is of the same order as that through the land vegetation.•The mass of the marine biota is 1000 times less than that of the land vegetation.

The (almost) unperturbed marine carbon cycle:Global mean air-sea flux, calculated from pCO2 measurements

Revision: Seawater Carbonate chemistry

• Inorganic carbon exists as several forms in sea water: – Hydrated dissolved CO2 gas.

– This rapidly reacts with H2O to form undissociated carbonic acid:

CO2(g) + H2O H2CO3

– Which can dissociate by loss of H+ to form bicarbonate ion:H2CO3 H+ + HCO3

– which can dissociate by further loss of H+ to form carbonate ion:

HCO3- H+ + CO3

2-Typically, 90% of the carbon exists as bicarbonate, 9% as carbonate, 1% as dissolved CO2 and undissociated H2CO3 (usually lumped together).

Seawater Partial pressure of CO2

• The partial pressure of CO2 of the sea water (pCO2sw) determines whether there is flux from air to sea or sea to air: – Air-to-sea Flux is proportional to (pCO2air

* - pCO2sw)

• pCO2sw is proportional to dissolved CO2(g):

[CO2(g)] = x pCO2sw = where

is the solubility of CO2. The solubility decreases with increasing temperature.

*pCO2air is determined by the atmospheric mixing ratio, i.e. if the mixing ratio is 370ppm and atmospheric pressure is 1 atm, pCO2air is 370 atm.

What sets the net air-sea flux?

The flux is set by patterns of sea-surface pCO2sw, forced by:

• Ocean circulation; – Is surface water is cooling or heating?– Is water being mixed up from depth?

• Ocean biology;– Is biological activity strong or weak?– Is calcium carbonate being precipitated?

• The rising concentration of atmospheric CO2

– pCO2 of air is rising and this tends to favour a flux from atmosphere into the ocean.

Circulation influence on air-sea flux

• Warm currents, where water is cooling, are normally sink regions (NW Atlantic, Pacific).

• Source regions for subsurface water, where water is cooled sufficiently to sink are strong sinks (N. N. Atlantic, temperate Southern ocean)..

• Tropical upwelling zones, where subsurface water comes to the surface and is strongly heated, are strong sources (equatorial Pacific).

Water cools and sinks

Water warms and upwells?

•The Northern North Atlantic is a region of strong cooling, associated with the North Atlantic drift. Cooling water takes up CO2 and may subsequently sink.•The water upwells in other parts of the world ocean, particularly the equatorial Pacific. Upwelling regions are usually sources of CO2 to the atmosphere – deep water has high

CO2 and the water is being warmed. This circulation controls how rapidly old ocean water is brought to the surface, and therefore how quickly the ocean equilibrates to changes in atmospheric CO2 concentration.

The overturning thermohaline circulation

Biological influence on air-sea flux.

• Blooms of plankton fix carbon dioxide from the water and lower CO2, hence pCO2.

• Particularly marked in the North Atlantic which has the most intense bloom of any major ocean region.

• In the equatorial Pacific, plankton blooms are suppressed by lack of iron – part of the explanation for high pCO2there.

• In the equatorial Atlantic, upwelling is less intense and there is more iron from atmospheric dust.

Ocean carbon “pumps”

• Deep water has higher (10-20%) total carbon content and nutrient concentrations than surface water. There are several processes contributing to this:

• The "Solubility pump" tends to keep the deep sea higher in total inorganic carbon (CO2) compared to the warm surface ocean.

• The “Biological pump(s)" – the flux of biological detritus from the surface to deep, enriches deep water concentrations. There are two distinct phases of the carbon in this material:– The "soft tissue" pump enriches the deep sea in inorganic

carbon and nutrients by transport of organic carbon compounds.

– The calcium carbonate pump enriches the deep sea in inorganic carbon and calcium.

Ocean biological pumps

• Falling dead organisms, faecal pellets and detritus are "remineralised" at depth. Remineralization occurs– By bacterial activity.– By inorganic dissolution

of carbonate below the lysocline.

– The different phases have different depth profiles for remineralisation.

Soft tissue

Carbonate

Ocean Carbon: The Biological (soft tissue) Pump

• This mechanism acts continually to reduce the partial pressure of CO2 (pCO2) in the surface ocean, and increase it at depth.

• Over most of the ocean, upwelling water is depleted of inorganic carbon and nutrients (nitrate and phosphate) by plankton.

• In the process they remove about 10% of the inorganic CO2 in the water. Most of this goes to form organic matter via the reaction:

CO2 + H2O CH2O +O2.• Because the buffer factor ~10,

this has a large effect on surface pCO2, decreasing it by 2-3 times.

• The reverse reaction occurs by (mostly bacterial) respiration at depth, and increases CO2 concentration there.

C oncen tra tion

N itra te (o r phospha te )

Tota l C O 2

Surface pCO2, nutrient and surface temperature in the North Atlantic

te ( M

Mar Apr May Jun Jul Aug Sep Oct

Nitrate

The biological (calcium carbonate) pump.

• This mechanism also transfers carbon from the surface ocean to the deep sea.

• Some of the carbon taken up by the biota in surface waters goes to form calcium carbonate.

• The CaCO3 sinks to the deep sea, where some of it may re-dissolve and some become sedimented. The redissolution can only occur below the lysocline, which is shallower in the Pacific than the Atlantic.

• In contrast to the soft tissue pump, this mechanism tends to increase surface ocean pCO2 and therefore atmospheric CO2 . The net reaction is:

Ca++ + 2HCO3- H2O +CO2 + CaCO3

Photo: courtesy D. Purdie: See the Ehux home page: www.soc.soton.ac.uk/SUDO/tt/eh/index.html

Coccolithophores -- calcite precipitating plankton

The ocean sink for anthropogenic CO2

• The oceans are close to steady-state with respect to atmospheric CO2.

• Prior to the industrial revolution, the oceans were a net source of order 0.5 Pg C yr-1 CO2 to the atmosphere. Today they are net sink of order 2 Pg C yr-1.

• The main factor controlling ocean uptake is the slow overturning circulation, which limits the rate at which the ocean mixes vertically.

• Three methods are being used to calculate the size of the ocean sink. – Integration of the pCO2 map (difficult and inaccurate).– Measurements of atmospheric oxygen and CO2 (see later).– Models of ocean circulation. These are of two types:

• Relatively simple box-diffusion models “calibrated” so that they reproduce the uptake of tracers such as bomb-produced 14carbon.

• Ocean GCMs which attempt to diagnose the uptake from the circulation. (However, the overturning circulation is difficult to model correctly. In practice these models are also tested against ocean tracers.)

“Riverine” flux

The pre-industrial steady state cycle: to balance the flux of carbon coming down rivers, there must have been a CO2 flux of the order of 0.5 Pg C yr-1 from ocean to atmosphere and from atmosphere to land. Volcanic activity and sedimentation fluxes provide smaller net inputs and outputs to the system.

1960 19701950 1980

Tropospheric bomb radiocarbon

The atmospheric bomb tests of the 50s and 60s injected a “spike” of radiocarbon into the atmosphere which was subsequently tracked into the ocean. This signal provides a good proxy for anthropogenic CO2 over decadal time scales.

3-D model outputs for surface pCO2

• Capture the basic elements of the sources and sinks distribution.• Considerable discrepancies with one another and with the data

(Southern Ocean, North Atlantic).

How well is the global ocean sink known?

Estimates of the global ocean sink 1990-1999

Reference Sink (Pg C yr-1)

IPCC (2001) 1.7+/- 0.5Estimate(Keeling oxygen technique)

OCMIP-2 Model 2.5+/- 0.4Intercomparison(ten ocean carbon models).

Not very well!

Will ocean uptake change in the future?

• Yes: the models forecast that the sink will increase in the short term as increasing atmospheric CO2 forces more into the oceans.

• But, the buffering capacity of the ocean becomes less as CO2 increases, tending to decrease uptake.

• Also, if ocean overturning slows down, this would tend to decrease the uptake.

• Changes in ocean biology may also have an impact….

Source: Manabe and Stouffer, Nature 364, 1993

North Atlantic pCO2

1994-1995

2002-2005

•Data 1994-1995•Near-continuous

data 2002-present•Sharp decrease in

ΔpCO2 relative to mid 90s

Possible Marine biological effects on Carbon uptake, next 100 years.

Iron fertilisation -- deliberate or

inadvertent

NO3 fertilisation

pH change mediates against calcite-

precipitating organisms

Reduction in THC offset by increased

efficiency of nutrient utilisation

Other unforeseen ecosystem changes

Process Effect on CO2 uptake

Marine carbon cycle summary:

• The ocean CO2 sink is affected both by circulation and biology. Changes in either would affect how much CO2 is taken up by the ocean. Climate change may cause both.

• Because different methods agree roughly on the size of the global ocean sink, it has generally been assumed that we know it reasonably well.

• However, there is an increasing discrepancy between the most accurate methods. Our present understanding allows us to specify the sink only to ~35%.

• We cannot at present specify how it changes from year to year or decade-to-decade.

• Acccurate knowledge of the ocean sink would enable us (via atmospheric inverse modelling) to be much more specific about the terrestrial sinks – useful for verification of Kyoto-type agreements.

The atmospheric imprint of anthropogenic carbon

Pre-industrial steady state.

•Fluxes into and out of the atmosphere were approximately at steady state before 1750.

•Small variations correlate with climate change (?) – i.e little ice age ~ 1600.

Atmospheric CO2 variations since 1000 AD

Fossil Fuel Emissions

• Well quantified from econometric data (Marland, Andres)

The budget for anthropogenic CO2

(1980s: numbers in Pg C yr-1.)

•Well-known numbers (<10% uncertainty): 1) Rate of fossil fuel release 5.42) Rate of build-up in the atmosphere: 3.3

•Poorly known number ( 0.8 Pg uncertainty?)3) uptake by ocean 1.9

Very poorly known number ( 1.3 Pg C yr-1).4) Rate of (mostly tropical) deforestation: 1.7

Extremely poorly known number calculated to balance budget (ie 1 +4 – 2 - 3).

5) Uptake by extra-tropical vegetation 1.9

Ocean uptake1.9 Pg C yr-1

Fossil fuel release5.4 Pg C yr-1

Accumulation in atmosphere3.3 Pg C yr-1

Land uptake?(1.9 by

difference)

Deforestation1.7 Pg C yr-1?

1980s budget of anthropogenic carbon dioxide.

The Mauna Loa atmospheric record.

Accurate measurements of CO2 mixing ratio in dried air have been made by C. Dave Keeling since 1958 at Mauna Loa observatory, Hawaii. From the 70s on, there have been an regular measurements at an increasing number of stations around the globe.

Late 1990s measurement network

C. Dave Keeling

1955 1960 1965 1970 1975 1980 1985 1990 1995

Monthly m ean CO 2

Mauna Loa, H awaii

Concentrations

•Overall increase in atmospheric CO2 of~4% per year.

•Inter-annual and inter-decadal changes in the rate of rise not due to changes in fossil fuel emissions -- indicate changes in the “natural” sinks. •An increasing amplitude of the northern hemisphere seasonal cycle correlating with increased global temperatures.

•Increasing length of the growing season.

Variation in the growth rate of atmospheric CO2, 1957-1999

Global(NOAA)

Cape Grim(CSIRO)

Fossil Fuel

Pinatubo

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

CO2 GROWTH RATE

El Nino

La Nina

Mauna Loa(Scripps/NOAA)

(R J Francey, pers. Com)

•Rate of growth is highly variable – not due to change in fossil fuel source.

•Variation correlates with Southern oscillation – El Ninos.

•Indicates the “Natural” sinks for atmospheric CO2 are highly variable.

•Though the land sink dominates variability, ocean is also important

The Mauna Loa atmospheric record…contd.

Accurate measurements of CO2 mixing ratio in dried air have been made by C. D. Keeling since 1958 at the Mauna Loa Laboratory in Hawaii, and more recently at many other stations around the world.

The Mauna Loa record shows:

Keeling, C.D., et al., Nature, 382, 146-149, 1996

Distribution of CO2 in the atmosphere

•Seasonality is most pronounced at high latitudes Northern Hemisphere. Southern Hemisphere seasonality is small.

•The seasonality is mostly due to the land biota – almost all in the N. Hemisphere.

•The marine biological signal is buffered by carbonate chemistry and its seasonality is smoothed out – not apparent in the atmospheric signal.

Calculation of sinks by inversion

•Principle: Models of global atmospheric transport are used to deduce where the net source/sinks must be, in order to give rise to the observed (small) variations in atmospheric CO2 concentrations.

•If the locations of the (anthropogenic) sources are known, the (natural) sinks can be specified.

•Good for inter-hemispheric distributions.

•Less good for latitudinal distributions.

•Poor for longitudinal distributions.

Tans, Fung and Takahashi

•Combined constraints from observed interhemispheric gradient with ocean surface pCO2 data.

•N. Hemishere ocean data suggested N.H ocean uptake <= 0.6 Pg yr-1.

•They deduced:–Global net ocean sink <= 1 Pg C yr-1

–Large N. Hemisphere mid-latitidue terrestrial sink (2-3 Pg C yr-1)

•Subsequently it has been found that their ocean sink was too small, land sink too large, but the existence of a substantial NH land sink is now established.

Observational constraints on the global atmospheric CO2 budget, Sciecne 247, 1431 (1990).

Tans et al Fig 5: Observed mean annual CO2 concentrations (circles and solid curve) as a function of sine of latitude (-1 is S. Pole). These are compared with calculations from a model (squares and dashed curve), and expressed as deviations from a mean CO2 concentration.

Present distribution of Land sources and sinks

Firm conclusions:• A substantial sink in the Northern Hemisphere mid-latitudes.

– Unknown distribution among the continents• The tropical land areas are thought to be nearly neutral. • All sinks are variable from year to year and decade to decade.

-180 -120 -60 0 60 120 180

Longitude

de -0.5±0.6

0.1±0.6

-0.2±0.1

-0.8±0.2

-0.3±0.2

0.5±0.1

-0.7±0.3

-1.3±0.5

0.1±0.7

-0.3±0.5

0.8±0.4

-0.1±0.3

N. hemisphere

Tropics

S. hemisphere

Box inverse modelQuantity Symbol Value

PgC a-1

Fossil fuel flux FF 5.4

Accumulation in the atmosphere

AA 3.4

Interhemispheric flux*

IF 1.9

Northern hemisphere ocean sink

Southern hemisphere + equatorial ocean sink

Northern hemisphere land sink

Equatorial + Southern land sink

* We take the interhemispheric gradient to be g = 2 x 10-6 v/vThe residence time wrt interhemispheric exchange is τ = 1 yrThe Mass of the atmosphere M = 1.6 x 1020 molThe interhemispheric flux is then = 1.9 PgC

.(1) Total Mass balance: NO+SO+NL +EL = FF-AA = 2.0(2) N. hemisphere mass balance: 0.9FF-NO-NL-IF = 0.45AA= 1.5(3) N. hemisphere ocean sink by observation: NO= 0.60.15(4) Total ocean sink by model and observation: NO+SO = 1.40.5

This is a system four equations in four unknowns: Calculation:From (3) and (4): SO=0.8Sub values for FF, NO, IF in (2): NL=0.9FF-NO-IF-0.45AA = 0.9Sub (4) into (1): NL + EL = 0.6: Hence EL = -0.3

These calculations imply a modest NH land sink in mid latitudes, and a small net source in the land tropics (could be lots of deforestation + lots of re-growth). Note the sensitivity of the calculations to errors. This arises because the sinks are calculated as comparatively small differences between large numbers.

In the 1980s, the total “natural” sink (fossil fuel input - accumulation in atmosphere) was on average 2.0. In the early 1990s, (period 1991-1994) natural sinks were nearly double this. Today they are in the range 3 – 4 Pg C yr-1

Box model calculations for the period 1980-1989

Possible causes of the NH mid-latitiude sink

• Land use Change

• Anthropogenic fertilization, chiefly nitrogen deposition

• CO2 fertilization

Land-Use change

• “REVERSE PIONEER” REGROWTH OF FOREST

– In the last century, large areas of forest near population centres in N. America were cleared for crops.

– With the coming of the railways, the centres of crop production moved to the mid-western prairies. Farmland was abandoned and new-growth forest re-established.

– The process is continuing today.

– Similar, less dramatic trend in Europe and Russia.

• FOREST CONSERVATION:

– Suppression of fire

– Suppression of insect infestation

• INCREASED ORGANIC SEDIMENTATION IN RESERVOIRS?

Land use change and the US carbon budget:

estimates from “carbon accounting”

Houghton RA, Hackler JL, Lawrence KTThe US carbon budget: Contributions from land-use changeSCIENCE 285 (5427): 574-578 JUL 23 1999

Sources of anthropogenic nitrogen

• Agricultural fertilizer

• Animal husbandry:– Runoff from farms

– Ammonia emissions

• NOy emissions from transport, other fossil fuels

Current deposition of atmospheric NOy

(mmol N m-2 yr-1)

Cross-section of trunk of Picea abies from the fertilised and irrigated (IL) treatment at the Flakaliden study site -- Boreal forest, Northern Sweden.

Effect of fertilization on tree growth

Effect of beta-factor

0.5 1 1.5 2 2.5

C / C0

CO2 Fertilization effect.

CO2 is a limiting factor on growth of plants. Higher CO2 may therefore stimulate net growth. CO2 fertilization is usually quantified by the "beta factor";

)/(1 00 CCLnPP

where is usually in the range 0-0.3 P,P0 are the carbon assimilation rates at CO2 concentrations C,C0

Free-air CO2 Enrichment (FACE) experiments

• Designed to enrich the CO2 in air over a circle of vegetation, with minimal other disturbance.

• A ring of towers able to release CO2, sensors to detect wind speed and direction and measure CO2 concentration.

• Continuous rapid monitoring of the CO2 concentrations. Control system to decide which towers to release from and adjust release rates to keep concentration constant.

Uncertainties about CO2 Fertilization

Easily measurable in many plants in “greenhouse” situations, but it is difficult to extrapolate this to the natural world. Questions include:

• How big is the effect in natural ecosystems?

• How is it modified by other limiting nutrient availabilities?

• Does it result in continuous storage of carbon in plants and soils, or is a new equilibrium state rapidly reached?

Sink saturation?

• Assume that the sink is mostly due to CO2 fertilization.

• Rising CO2 has an immediate effect on photosynthesis

– Leading to net ecosystem uptake of CO2.

• Rising CO2 has a delayed effect on global temperatures.

• Rising temperatures will enhance respiration in the future

– Leading to net ecosystem release of CO2

• Therefore presently observed uptake of CO2 may be a transitory phenomenon only, and the sink will “saturate”.

• The sink may be even more transitory if it is due in whole or in part to land use change, or nitrogen fertilization.

Courtesy John Grace, U. Edinburgh

Sink saturation?

• FACE experiments suggest uptake of CO2 due to CO2 fertilization is itself transitory.

• But: soil warming experiments suggest that the temperature effect on soil respiration may also be transient.

Carbon cycle:change of carbon in vegetation and soils according to the Hadley Centre coupled carbon-climate model.

Precision atmospheric oxygen measurements

Since 1990, direct measurements of oxygen concentrations, at ppm accuracy, have been made at certain sites throughout the world, by R. F. Keeling and others.

Cold Bay Niwot Ridge

Mauna Loa

La Jolla

American Samoa

Cape Grim

South Pole

Locations of precision O2 measurements

….Precision atmospheric oxygen measurements

• The concentrations are affected by:• fossil fuel burning• net land vegetation net uptake• Seasonal uptake/release of oxygen from both

the land and the ocean biota -- unlike the case of CO2 which is little affected by ocean seasonal cycle, because of the long air/sea exchange time for CO2.

Oxygen and CO2 Comparison

• O2 decrease year-on year of the same order as the CO2 increase.

• Seasonal cycles of O2 in antiphase with those of CO2.

• The Southern Hemisphere O2 seasonal signal is much larger than is the case for CO2.

v/v -- rela

tive to

Alaska

Tasmania

Alaska

Tasmania

352.8 362.9

Oxygen

Carbon dioxide

Deductions from Oxygen

1) Net land and ocean sinks of carbon:The molar ratio of oxygen utilisation relative to carbon dioxide

release during the following three processes are all known;a) fossil fuel burning, Rff = (DO2/DCO2)~-1.3b) photosynthesis/ respiration, Rpr=(DO2/DCO2)~-1.1c) ocean uptake of CO2 (DO2/DCO2)=0 They can be plotted on a vector diagram of mean annual O2

change versus CO2 change. From a knowledge of how much fossil fuel has been burned, the size of the net ocean and land sinks can be determined.

0 2 4 6 8

CO2 mole fraction change (ppm)

initial

Complications with the O2 signal..

Ocean release: The oceans are not in general “neutral” w.r.t atmospheric oxygen.– As the ocean warms, O2 becomes less soluble and some dissolved

oxygen outgasses from ocean to atmosphere.– Seasonally mixed ocean regions such as the N. Atlantic tend to be

oversaturated with O2 in summer and undersaturated in winter, when deep mixing brings up “old” water. If mixing patterns change this can mean a net source or sink.

– From these causes, there is evidence that the oceans have outgassed O2 in recent decades.

Uncertainty in C:O ratios: – Different fossil fuels (e.g. coal, gas) have markedly different ratios of

carbon released to oxygen used.– Different vegetation types may have different C:O ratios: ratios may

vary between soil and vegetation, and may change seasonally even for the same vegetation type.

Conclusions

• The carbon cycle was closely in steady state prior to the industrial revolution. It is now substantially perturbed.

• Natural CO2 sinks both on land and in the ocean are very important for slowing the rate of global warming due to greenhouse gases.

• These sinks change substantially from year to year and decade to decade. Changes are synchronous with climate oscillations such as El Niño/ La Niña cycles.

• The cause of the land sink for anthropogenic CO2is poorly understood. It may turn into a source because of global warming, in the next 100 years.

• The ocean CO2 sink is affected both by circulation and biology. Changes in either would affect how much CO2 is taken up by the ocean. Climate change may cause both.

Conclusions

• We know 3 or 4 possible reasons for the global vegetation sink, but presently we cannot be sure which of these are most important.

• We cannot be sure how long the sink will continue, and whether it will increase or decrease. Many lines of evidence point to a decrease.

Questions

• Should land sequestration of carbon be considered as a serious option for climate change mitigation, given

– our poor understanding of current land sinks

– their possibly transitory nature

– their vulnerability to climate change

• The precautionary principle: if near-catastrophic outcomes of present practices cannot be ruled out, should we be putting maximum effort into emissions reductions?

Distribution of CO2 in the atmosphere

•The space and time-averaged Northern hemisphere concentration is about 2ppm higher than that in the Southern hemisphere. This is because nearly all fossil fuel emissions occur in the North.•Combined with atmospheric transport model, we can estimate the latidudinal distribution of natural sinks.

Two views of global CO2 versus time and latitude between 1986 and 1993

Box inverse model

NH SH + equatorial region

Accumulation0.45AA

Accumulation055AA

Fossil fuel (90%)0.9FF

Fossil fuel (10%)0.1FF---Unknown sinks ----

NO NL SO EL

Interhemispheric fluxIF

Pre-industrial steady state.

•Fluxes into and out of the atmosphere were approximately at steady state before 1750.

•Small variations correlate with climate change (?) – i.e little ice age ~ 1600.

Atmospheric CO2 variations since 1000 AD

Ocean uptake1.9 Pg yr-1?

Fossil fuel release5.4 Pg yr-1

Accumulation in atmosphere3.3 Pg yr-1

Land uptake?(1.9 by

difference)

Deforestation1.7 Pg yr-1?

1980s budget of anthropogenic carbon dioxide.

1955 1960 1965 1970 1975 1980 1985 1990 1995

Monthly m ean CO 2

Mauna Loa, H awaii

Concentrations

“Riverine” flux

The pre-industrial steady state cycle: to balance the flux of carbon coming down rivers, there must have been a CO2 flux of the order of 0.5 Pg C yr-1 from ocean to atmosphere and from atmosphere to land. Volcanic activity and sedimentation fluxes provide smaller net inputs and outputs to the system.

Calculation of sinks by inversion

•Principle: Models of global atmospheric transport are used to deduce where the net source/sinks must be, in order to give rise to the observed (small) variations in atmospheric CO2 concentrations.

•If the locations of the (anthropogenic) sources are known, the (natural) sinks can be specified.

•Good for inter-hemispheric distributions.

•Less good for latitudinal distributions.

•Poor for longitudinal distributions.

Tans et al Fig 5: Observed mean annual CO2 concentrations (circles and solid curve) as a function of sine of latitude (-1 is S. Pole). These are compared with calculations from a model (squares and dashed curve), and expressed as deviations from a mean CO2 concentration.

….Precision atmospheric oxygen measurements

• The concentrations are affected by:• fossil fuel burning• net land vegetation net uptake• Seasonal uptake/release of oxygen from both

the land and the ocean biota -- unlike the case of CO2 which is little affected by ocean seasonal cycle, because of the long air/sea exchange time for CO2.

Deductions from Oxygen

1) Net land and ocean sinks of carbon:

The molar ratio of oxygen utilisation relative to carbon dioxide release during the following three processes are all known;

a) fossil fuel burning, Rff = (DO2/DCO2)~-1.3

b) photosynthesis/ respiration, Rpr=(DO2/DCO2)~-1.1

c) ocean uptake of CO2 (DO2/DCO2)=0

They can be plotted on a

vector diagram of mean annual O2 change versus CO2 change. From a knowledge of how much fossil fuel has been burned, the size of the net ocean and land sinks can be determined.

0 2 4 6 8

CO2 mole fraction change (ppm)

initial

Deductions from O2 continued….

2) Gas exchange rate and primary productivity of the oceans• To examine the contribution of the oceans to the oxygen

signal,– 1) The year-to-year decrease is removed leaving only a

seasonal signal.

– 2) The CO2 seasonal signal is multiplied by the ratio Rpr of O2/CO2 changes due to land vegetation, and subtracted from the O2 signal; this leaves only the signal due to seasonal release/uptake by the oceans.

– 3) The magnitude of this signal at different stations is a function of the amount of oxygen released by the marine biota, and the rate at which it escapes into the atmosphere. Using additional information on the sea-surface concentrations of oxygen, estimates of both of these can be made.

Precision atmospheric oxygen measurements

Since 1990, direct measurements of oxygen concentrations, at ppm accuracy, have been made at certain sites throughout the world, by R. F. Keeling and others.

Cold Bay Niwot Ridge

Mauna Loa

La Jolla

American Samoa

Cape Grim

South Pole

Locations of precision O2 measurements

Oxygen and CO2 Comparison

• O2 decrease year-on year of the same order as the CO2 increase.

• Seasonal cycles of O2 in antiphase with those of CO2.

• The Southern Hemisphere O2 seasonal signal is much larger than is the case for CO2.

Alaska

Tasmania

Alaska

Tasmania

352.8 362.9

Oxygen

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