The Global Carbon Cycle

OverviewThe atmospheric distributionSources and sinks of anthropogenic CO2

Sources and sinks of oxygen.

Reading Overviews and reviews: The 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. (available at http://www.atmos.ucla.edu/~gruber/publication/pdf_files/sarmiento_pt_02.pdf Ocean - 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/Watson_and_Orr_2002.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 Atmospheric 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). 

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.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.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.

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.

Atmospheric CO2 variations since 1000 AD

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.

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 

Dave Keeling

310

315

320

325

330

335

340

345

350

355

360

1955 1960 1965 1970 1975 1980 1985 1990 1995

year

Monthly m ean CO 2

Mauna Loa, H awaii

Concentrations

The Mauna Loa atmospheric record.

•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

2

4

6

8

Global(NOAA)

Cape Grim(CSIRO)

0

30

Fossil Fuel

Pinatubo

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

-30

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

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.

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.

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

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.

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 solved for.

•Good for interhemispheric distributions.

•Less good for latitudinal distributions.

•Poor for longitudinal distributions. – the atmosphere mixes rapidly in an east-west direction, so mean E-W concentration gradients are very small – difficult to measure given large variability in measurements.

Calculation of sinks by inversion

“Observational constraints on the Global Atmospheric CO2 Budget”

Tans, Fung and Takahashi, Science 247, 1431 (1990)

• Combined constraints from the observed inter-hemispheric gradients calculated by modelling transport in the atmosphere, with ocean surface data that place limits on ocean uptake.

• Their N. Hemispheric ocean data suggested N. H. Uptake was quite small, < 0.6 Pg C yr-1.

• But observed N-S atmospheric gradient was unable to transport all the remaining fossil fuel CO2 into the Southern Hemisphere.

• They deduced there was must be a large (2-3 Pg C yr-1 terrestrial sink in the Northern Hemisphere mid-latitudes.

• The total ocean sink they found was comparatively small -- < 1 Pg C yr-1

.

• Though they underestimated the ocean sink and overestimated the land sink, their key finding, that a large mid-latitude land sink exists, has been confirmed by many similar studies.

N. Hemisphere S. Hemisphere

NS atmospheric transportNH net land uptake

NH ocean uptake

SH ocean uptake

Tropical sources SH net land

uptake

SH fos. fuels

NH fossil fuel release

Atmospheric inverse models to quantify unknown CO2 sources and sinks.

Atmospheric inverse models to quantify unknown CO2 sources and sinks.

• At present models are being run that attempt to diagnose natural sinks over grids similar to this one – which has a total of 17 “boxes” in which the natural source/sink is to be diagnosed.

• However, it is difficult to distinguish between, for example, N. American and Eurasian land sinks. More observations may make this possible in the future.

-180 -120 -60 0 60 120 180

90

30

-30

-90

Longitude

La

titu

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

• A recent paper1 used the atmospheric inverse technique to try to locate the Northern hemisphere sink to a continent -- i.e. to resolve longitudinally the sources and sinks.

• Fan et al calculated that almost all of the land sink is on the North American continent, and that therefore the US and Canada are a net sink of CO2. They suggested that this is because of net regrowth of forest in these regions.

• This conclusion pushed the technique further than it can presently be trusted. More careful studies contradict it2.

• Nevertheless, the paper influenced the subsequent argument over the Kyoto protocol, and helped harden the US position against it.

1. Fan, S., M. Gloor, J. Mahlman, S. Pacala, J. Sarmento, T. Takahashi, and P. Tans, A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models, Science, 282, 442-446., 1999.

2. Gurney KR, et .al, Towards robust regional estimates of CO2 sources and sinks using atmospheric transport models. Nature 415 626-630 2002.

Inverse models and carbon politics.

• Most measurements of CO2 are near ground level, in the boundary layer.

• The thickness of the boundary layer varies seasonally and diurnally. It is deeper in summer than in winter, and by day than by night.

• Ground-based sources and sinks (e.g. vegetation) have a smaller effect on concentrations when the boundary layer is thicker.

• So changes in boundary layer thickness can be confused with changes in source/sink strength. Since it is difficult to measure boundary layer thickness, this is a substantial problem.

Problems with the inverse method: the rectifier effect

Diurnal Rectifier Forcing

Daily mean: Accumulation of CO2 near the ground, depletion aloft

Dilution of photosynthesis signal through deep mixing

Transport of low-CO2 air into upper troposphere

Mid-day

Deep PBLMixing

Low CO2

Concentration

Photosynthesis

StrongConvection

Accumulation of respiration signal near the surface

Elevated CO2 in lower troposphere

Midnight

Shallow PBLMixing

High CO2

Concentration

Decomposition

Weak Cumulus Convection

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.

• These measurements can also be added in to inversion calculations, to increase their accuracy.

Alert

Cold Bay Niwot Ridge

Mauna Loa

La Jolla

American Samoa

Cape Grim

South Pole

Locations of precision O2 measurements

Ralph Keeling

….Precision atmospheric oxygen measurements

• The concentrations are affected by:• fossil fuel burning• net land vegetation 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 buffering effect of ocean carbonate chemistry.

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.

0

-20

-40

0

-20

-40

350

370

350

370

CO

2(p

pm

v/v

)

O2(p

pm

v/v -- rela

tive to

stan

da

rd)

Alaska

Tasmania

Alaska

Tasmania

-34.3

-27.0

-10.1

-3.5

356.9

364.2

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 assumed 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.

L S

-8

-6

-4

-2

0

0 2 4 6 8

O2

mol

e fr

acti

on c

hang

e (p

pm)

CO2 mole fraction change (ppm)

initial

final

Global average concentrations of O2 vs CO2

• The Keeling oxygen technique is the method most recently used by the IPCC to partition the natural sink into “land” and “ocean” components.

• It disagrees with ocean modelling techniques for the period of the 1990s.

• It assumes that there is no net annual uptake or release of O2 by the oceans.

• However, if ocean stratification is changing, this would not be correct.

Problems with the Oxygen technique: is the ocean a net source of O2?

“Riverine” flux

Atmospheric inverse models to quantify unknown CO2 sources and sinks.

• If the distribution of sources and sinks were completely known, an atmospheric transport model could predict the concentration distribution of CO2 in the atmosphere (the forward calculation).

• In practice, the distribution of some of the sources and the concentration distribution in the atmosphere are known. A transport model can then be used to predict what distribution of unknown sinks and sources is consistent with these observations (the inverse calculation).

• The atmosphere mixes slowly across latitudinal bands – i.e. ~ 1 year mixing time between hemispheres, but more rapidly longitudinally.

• Therefore, relatively large concentration gradients are observed latitudinally and the separation of sinks into hemispheres and broad latitude bands is reliable. It is much more difficult to separate sinks in the same latitude bands by inverse modelling.