REVIEWS OF GEOPHYSICS, SUPPLEMENT, PAGES 1249-1252, JULY 1995 U.S. NATIONAL REPORT TO INTERNATIONAL UNION OF GEODESY AND GEOPHYSICS 1991-1994

Anthropogenic C0 2 : The natural carbon cycle reclaims center stage

J. R. Toggweiler Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, New Jersey

Introduction Atmospheric CO2 concentrations are increasing by about

1.5 ppm every year. This 1.5 ppm increase integrated over the volume of the atmosphere accounts for about 50% of the CO2 released to the atmosphere from fossil fuel burning and deforestation (henceforth "anthropogenic" CO2). The rest of the anthropogenic release (the part that does not remain in the atmosphere) goes mainly into the oceans and the terrestrial biosphere and soils. Knowing how CO2 is partitioned between oceanic and terrestrial CO2 sinks is critical for making CO2 forecasts into the future. This paper provides a brief review concerning the oceanic uptake of anthropogenic COj. (A more detailed review is provided by Siengenthaler and Sarmiento, 1993.)

Thermodynamic considerations suggest that a 1.5 ppm per year rise in atmospheric CO2 levels should lead to a CO2 increase in ocean surface water of about 1 |imole/l. Until the late 1980s, dissolved inorganic carbon (DIC) measurements in seawater could not resolve CO2 differences less than 15-20 jimoles/1. Thus direct measurements of the anthropogenic CO2 increase were out of the question. Measurement capabilities have now improved to the point where DIC measurements can be made to an accuracy of 2-3 |Xmoles/l. Under the auspices of the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS), ocean chemists are accumulating an ocean-wide data set from which the ocean's current C 0 2 distribution can be mapped Unfortunately, it may still be 10 or 20 years before the ocean's C 0 2 levels will be mapped well enough a second time to document the CO2 increase by difference.

In the meantime, our primary methods for estimating oceanic C 0 2 uptake will have to be based on models which help us evaluate indirect or proxy constraints on CO2 uptake. Before 1990, most modelling efforts employed simple box models which attempted to predict the oceanic penetration using some kind of upper ocean mixing parameterization. In these models, ocean mixing rates were determined by adjusting or 'tuning' mixing parameters until the model could reproduce the transient penetration of bomb-produced tracers like tritium (%) and bomb 1 4 C . If one assumes that the mixing coefficients appropriate for bomb tracers are also appropriate for anthropogenic C 0 2 , the tuned model can be used to simulate the increase in oceanic CO2 using the observed atmospheric CO2 time history as a boundary condition.

Recently, two attempts have been made to calculate anthropogenic C 0 2 uptake using ocean general circulation

Copyright 1995 by the American Geophysical Union.

Paper number 95RG00181. 8755-1209/95/95RG-00181 $15.00

models (GCMs) in which CO2 uptake is driven by a fully resolved ocean circulation field (Maier-Reimer and Hasselmann, 1987; Sarmiento et al., 1992). The GCMs and the tracer-calibrated box models are in basic agreement that the oceanic uptake of anthropogenic CO2 during the 1980s was about 2 GtC/yr (2 x 1 0 " g C per year). This is about 30% of total anthropogenic emissions, or about 60% of the portion attributed to the ocean/terrestrial sink.

It is important to point out that ocean models generally treat the entry of anthropogenic C 0 2 into the ocean as if it is a simple perturbation of the ocean's natural carbon cycle. "Anthropogenic C 0 2 " is treated as a separate tracer from the background or natural DIC in the ocean. Of course, anthropogenic C 0 2 and natural C 0 2 are not separable. C 0 2

only goes into the ocean when the atmospheric partial pressure of C 0 2 (pCO^ exceeds the C 0 2 partial pressure in the ocean, Maps of ApC0 2 , the PCO2 difference between ocean and atmosphere, document large C 0 2 influxes and outfluxes which were presumably in balance in pre-industrial times. The anthropogenic perturbation skews the influx-outflux balance to favor influx overall.

The oceanographic perspective on C 0 2 uptake was dealt a big blow at the outset of the 1991-1994 review period Keeling et al. (1989) and Tans et al. (1990) made estimates of the ocean's C 0 2 uptake by trying to fit the observed latitudinal variation of atmospheric C 0 2 concentrations using atmospheric transport models. The atmospheric models in these exercises are given known source distributions (anthropogenic C 0 2 enters the atmosphere mainly in the northern hemisphere) and different ocean C 0 2 sink scenarios. The atmospheric models then predict latitudinal C 0 2

distributions for each source-sink scenario. These can then be evaluated against the observed latitudinal distribution as derived from the network of C 0 2 sampling stations (Conway et al., 1988).

During the 1980s, atmospheric C 0 2 levels in the high latitudes of the northern hemisphere were only 3 ppm higher than C 0 2 levels in the high latitudes of the southern hemisphere. The atmospheric transport calculations of Keeling et al. and Tans et al. suggest that this gradient is too small to allow much transport between the hemispheres. Thus the main sink for atmospheric CO2 would appear to be in the northern hemisphere.

According to Tans et al., the terrestrial biosphere and soils must be the main C 0 2 sink because the ocean sink in the northern hemisphere is fairly small. Tans et al. conclude that there must be a substantial net increase in carbon storage in northern hemisphere terrestrial vegetation and soils every year. They partition roughly 80% of the global ocean/ terrestrial sink into the terrestrial biosphere and soils, and give the oceans only about 20%. They estimate that the oceans are currently taking up only 0.3-0.8 GtC/yr.

Tans et al.'s increased terrestrial carbon storage in

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temperate northern latitudes may be evidence for massive amounts of CO2 fertilization, the direct stimulation of vegetative growth by enhanced CO2 concentrations in the atmosphere. In their favored scenario, the annual carbon storage in northern terrestrial biomass and soils exceeds carbon losses due to tropical deforestation by about 1.0-1.5 GtCtyr. If the oceanic uptake is larger, the level of CO2 fertilization is still substantial, but northern terrestrial carbon storage is reduced to a level which more-or-less balances the release of CO2 by tropical deforestation.

The Tans et al. conclusion presents oceanographers with a major problem. Given the huge area of ocean in the southern hemisphere, the ocean's capacity to absorb CO2 should be skewed toward the south (Sarmiento et al., 1992). Bomb tracers are observed to penetrate vertically throughout much of the ocean's upper kilometer in both hemispheres. How could the southern hemisphere ocean be such a small sink for anthropogenic CO2?

The problem here, in one important aspect, is mainly a problem of definition. The atmosphere's north-south C 0 2

gradient is influenced by ocean-atmosphere CO2 fluxes which include both natural and anthropogenic components. The ocean models used to estimate oceanic uptake of anthropogenic CO2 explicitly ignore the natural cycle. Natural fluxes of CO2 between the ocean and atmosphere can be quite large with respect to the anthropogenic perturbation. The region between 5°N and 5°S in the equatorial Pacific by itself outgasses ~1 GtC/yr which must be balanced by CO2 uptake elsewhere.

Of particular interest here is the possibility that the ocean is naturally transporting carbon across latitude circles or between the hemispheres. Meridional transports within the ocean imply that there are natural CO2 fluxes between the ocean and atmosphere which influence atmospheric CO2 gradient The ocean and land also exchange carbon (via rivers). These aspects of the natural cycle are not taken into account in the Tans et al. analysis. The question then becomes: are there characteristics of the natural carbon cycle which can reconcile ~2 GtC/yr of ocean uptake with a small interhemispheric transport in the atmosphere? The answer, as will be reviewed below, would seem to be "yes", although our knowledge of the natural carbon cycle is still not adequate to close the carbon budget completely.

Interhemispheric C 0 2 Transport in the Ocean While Tans et al. (1990) concluded that the terrestrial

biosphere is a much bigger sink for atmospheric CO2 than the ocean, Keeling et al. (1989) drew a somewhat different conclusion. In their 1989 paper and in earlier work (Keeling and Heimann, 1986) Keeling and co-workers argued that anthropogenic CO2 is being added to a system in which the atmosphere naturally transports CO2 from the southern hemisphere to the north. The release of fossil fuel CO2 in the northern hemisphere currently overwhelms the natural interhemispheric cycle and reverses the atmospheric gradient

Broecker and Peng (1992) suggested that the atmosphere's pre-industrial south-to-north transport could be a straightforward consequence of the ocean's thermohaline circulation. They calculate that the ocean naturally takes up about 0.6 GtC/yr of atmospheric CO2 during the formation of North Atlantic Deep Water. This C 0 2 is transported across the equator at depth and is released again to the atmosphere in

the southern hemisphere. Inpre-mdustrial times a net north to south transport in the ocean would be balanced by a south to north C 0 2 transport in the atmosphere. If Broecker and Peng's conjecture is correct, the flux of CO2 into the southern hemisphere ocean predicted by ocean models may be consistent with atmospheric constraints after all: the flux of anthropogenic CO2 into the southern hemisphere ocean is simply opposed by the outgassing of CO2 as part of a natural cycle.

One of the most convincing pieces of evidence for this possibility is the fact that the interhemispheric CO2 gradient has been growing with time. During the early 1960s, the earliest time at which the interhemispheric CO2 gradient was measured, the atmospheric CO2 gradient was only 1 ppm. By 1980, the gradient had grown to some 3 ppm (Keeling and Heimann, 1986). This suggests that sometime shortly before 1960 a south-to-north transport driven by ocean circulation may have exactly balanced the north-to-south transport driven by anthropogenic releases.

The Ocean-Atmosphere p C 0 2 Gradient Broecker and Peng's conjecture about the ocean's role in

the natural C 0 2 cycle rests on one critical point The North Atlantic must currently be taking up an unusually large amount of C 0 2 in order to satisfy the C 0 2 uptake demanded by both the natural cycle and anthropogenic uptake. Since the North Atlantic is not especially large in terms of area, the partial pressures of C 0 2 in North Atlantic surface waters must be especially low in order to take up the required amount of carbon. This should be easily measurable by current techniques.

A global compilation of the partial pressure of CO2 (pCC^) in ocean surface waters by Taro Takahashi, one of the Tans et al. (1990) authors, limits the C 0 2 sink in the North Atlantic to 0.53 GtC/yr. Because the North Pacific has no deep water formation its capacity to take up carbon is small. Takahashi's estimate for the North Pacific is only 0.06 GtCftr. This leaves a total northern hemisphere ocean sink of about 0.6 GtC/yr. Mainlining an interhemispheric CO2 transport consistent with 2 GtC/yr of ocean uptake requires a northern hemisphere ocean sink more like 1.5 GtC/yr (Keeling et al., 1989). This is clearly not supported by the observations on hand. The limited ability of the North Atlantic to take up C 0 2 is a cornerstone of the Tans et al. argument

The North Atlantic is particularly famous for its large spring phytoplankton blooms which remove CO2 from the water and draw down the pC0 2 . The C 0 2 uptake during spring blooms is capable of pulling the oceanic p C 0 2 well below atmospheric levels and is strong enough to overcome the thermodynamic relationship between p C 0 2 and temperature (Watson et al., 1991; Takahashi et al., 1993). Watson et al. suggest that undersampling of North Atlantic spring blooms might significantly change the North Atlantic carbon balance, but it is doubtful that the effect of undersampling could be as large as 1 GtC/yr.

At the risk of confusing the reader, it is necessary to point out some of the uncertainty surrounding the estimation of ocean-atmosphere C 0 2 fluxes by ApC0 2 . Converting an ocean-atmosphere p C 0 2 difference into a C 0 2 flux requires knowledge of the rate at which gasses are exchanged across the air-sea interface. Ocean-atmosphere gas exchange rates are currently uncertain by a factor of two (Warminkhof,

TOGGWEILER: ANTHROPOGENIC C 0 2 1251

1992). Tans et al. (1990) combined gas exchange estimates with average ocean-atmosphere pC02 differences over 6 latitude bands to determine the ocean-atmosphere CO2 flux as a function of latitude. The Tans et al. compilation yields a total ocean uptake of 1.6 GtC/yr which is skewed toward the 15-50° latitude band in the southern hemisphere.

The Tans et al. ApC0 2 compilation puts the main CO2 sink in the 15-50° latitude band during southern winter, a time of year for which very few observations were available before 1990. Tans et al. ultimately chose to downplay the poorly constrained ApC0 2s from the southern hemisphere and to focus on the better constrained ApC0 2s from the North Atlantic. Subsequent work has shown that this was a good strategy: poorly sampled areas of the South Pacific and South Indian Oceans do not yield the large CO2 sink implied by the original A0CO2 compilation. Murphy et al. (1991) showed that the western South Pacific is a C 0 2 sink, but the eastern South Pacific is a CO2 source. Similarly, work in the southern Indian Ocean (Metzl et al., 1991) has shown that the areas of C 0 2 uptake are also negated by other areas of CO2 source.

The Role of River Fluxes and Ocean Skin Temperature

An interhemispheric C 0 2 transport in the deep ocean which might reconcile Tans et al. with the ocean models would seem to be ruled out by the lack of sufficiently large p C 0 2 deficits in the North Atlantic. If the story ended here we would clearly be at an impasse. However, Sarmiento and Sundquist (1992) reminded us that the oceanic C 0 2 sink determined by ApC0 2 actually underestimates the oceanic sink because the ocean naturally outgasses CO2 to the atmosphere to balance terrestrial sinks.

CO2 is taken up on land by chemical reactions when carbonate and silicate rocks are weathered Cations released by the weathering reactions and bicarbonate ions from atmospheric C 0 2 flow into the ocean via rivers. Riverine bicarbonate is released back to the atmosphere as C 0 2 when calcium is taken up by marine organisms to form CaC03. If rock weathering on land is in balance with the burial of CaC03 on the sea floor and decarbonation reactions during sediment metamorphism, then an amount of CO2 is outgassed from the ocean which balances the C 0 2 uptake on land (Sarmiento and Sundquist, 1992). This outgassing reduces the ocean-wide ApC0 2 and reduces the amount of anthropogenic C 0 2 uptake that one would infer from ApC0 2

measurements. Rivers also carry particulate and dissolved organic carbon

from the land to the sea. Some of this organic matter is buried in sediments, but much of it is remineralized in the ocean. The remineralized carbon must also be outgassed from the ocean and cycled back through the atmosphere in order to maintain a steady cycle. Thus river fluxes of carbon, both as bicarbonate and as organic matter, tend to mask the flux of anthropogenic C 0 2 going into the ocean. Sarmiento and Sundquist estimate that as much as half of the 1.0-1.5 GtC/yr discrepancy between the oceanic uptake in Tans et al. (1990) and in ocean models can be explained by aspects of the natural carbon cycle involving rivers.

River fluxes are also important because of the location within the ocean where CaC03 is formed and where riverine organic matter is oxidized The North Atlantic and Arctic Oceans receive the drainage from a disproportionate share of

the world's land area. This means that the oxidation of riverine organic matter should be especially large in the North Atlantic and Arctic on a per unit area basis. The polar seas of the North Atlantic are also dominated by organisms which produce much greater sinking fluxes of CaC0 3 than organisms in polar seas elsewhere (Honjo, 1990). Both of these effects tend to focus the C 0 2 outgassing due to river fluxes in the North Atlantic and to mask the flux of anthropogenic CO2 that one infers from ApCC^.

Sarmiento and Sundquist (1992) and Robertson and Watson (1992) raise the issue of ocean "skin" temperature. The temperature of ocean water within 1 mm of the ocean-atmosphere interface is known to be about 0.3°C cooler than water in the bulk mixed layer due to thermal radiation and evaporation. This cooling while small has a rather large effect on the ApC0 2 that governs ocean-atmosphere C 0 2 exchange. A skin effect of 0.3°C lowers the PCO2 at the interface by 4 ppm. Since the average ocean-wide ApC0 2 needed to move 2 GtC/yr into the ocean is only about 8 ppm, it is easy to see how important the skin effect can be. Robertson and Watson estimate that the skin effect may account for an additional 0.7 GtC/yr of ocean uptake.

River fluxes and the skin temperature effect are large enough to bring the global system close to balance and still allow substantial CO2 uptake by the ocean (Sarmiento and Sundquist, 1992). However, it remains to be seen whether these effects can produce a big enough effect in the North Atlantic to satisfy the atmospheric transport models.

1 3 C Constraints and Changes in Ocean 1 3 C C 0 2 derived from the burning of fossil fuels and

deforestation is depleted in the isotope 1 3 C due to isotope fractionation during photosynthesis. This has caused the 1 3 C / 1 2 C ratio of atmospheric carbon to decline at the same time that COj levels have gone up. The change in atmospheric 1 3 C / 1 2 C has penetrated into the ocean. In principle, the 1 3 C signal in the ocean should be easier to detect that the C 0 2 signal itself (Tans et al., 1993). The ratio 1 3 C / 1 2 C is usually expressed in terms of a quantity 8 1 3 C which is referenced to an arbitrary standard, i.e. 8 1 3 C = ( 1 3 c / 1 2 c sample) / ( 1 3 c/ 1 2 cstandard) "

Quay et al. (1992) compiled a set of 8 U C profiles at eight locations in the Pacific Ocean which had been measured about 20 years apart They found that levels had indeed declined over the 20 year period They used the depth-integrated change in 5 1 3 C to estimate how much anthropogenic CO2 had gone into the ocean. They found 2.1 GtC/yr, a result which agrees fairly well with ocean models.

Tans et al. (1993) and Broecker and Peng (1993) pointed out a major problem with Quay et aL's 1 3 C observations. The observed decrease in atmospheric 8 1 3 C over the 20-year period was about 0.4 per mil. The average 8 1 3 C decrease in ocean surface waters was also about 0.4 per mil. If changes in atmospheric 8 1 3 C are indeed driving the changes in the ocean one would expect the 8^C change in the surface ocean to be substantially smaller than the change in the atmosphere. This is because the isotope signal coming into the ocean should be diluted as it is stirred into the ocean's upper few hundred meters. If changes in atmospheric 8 1 3 C really are driving 8 1 3 C changes in the ocean one would expect the ocean's uptake of lighter atmospheric carbon to be analogous to the ocean's uptake of bomb^C.

1252 TOGGWEILER: ANTHROPOGENIC C 0 2

There are a number of factors which might contribute to this discrepancy. One possibility is that eight station pairs do not constitute an appropriate average for monitoring 8 1 3 C changes in the ocean. There could also be temporal changes in oceanic 8 1 3 C brought about by changes in photosynthetic isotope fractionation (due to higher C02(g) concentrations in the ocean) or temporal changes brought about by growth or shrinkage in the ocean's large dissolved organic carbon reservoir. Either of these factors could alter the temporal evolution of 8 1 3 C in the ocean without any effect on bomb 1 4 C uptake.

Ideally one would want to set the oceanic 8 1 3 C changes in the context of a complete isotopic budget for the atmosphere. A major problem here is knowing the isotope exchange between die atmosphere and terrestrial biosphere. As atmospheric 8 1 3 C values fall over time terrestrial ecosystems should respire older or heavier carbon back to the atmosphere. The atmosphere's isotope budget is quite sensitive to the 8 1 3 C difference between old carbon being respired and new carbon being fixed by photosynthesis. No one really knows how much the isotope exchange is currently out of balance.

Summary The atmospheric transport studies of Keeling et al. (1989)

and Tans et al. (1990) raise important issues about the carbon cycle. They force us to look at the carbon cycle as a global system, which may include large terrestrial carbon sinks due to CO2 fertilization Tans et al. have challenged the idea that the ocean is an important CO2 sink. However, research over the last four years supports the idea that the ocean is probably taking up - 2 GtC/yr of anthropogenic CO2. It suggests that this level of uptake can be reconciled with atmospheric CO2 gradients. A complete reconciliation involves a renewed emphasis on the natural carbon cycle as it pertains to interhemispheric transport, nutrient cycles, and river fluxes.

The next four years of carbon research will be focused on a better characterization of the ocean's DIC and ApC0 2 fields and a better characterization of the 8 1 3 C system, including atmospheric measurements of 8 1 3 C, which should allow use of atmospheric 8 1 3 C gradients to constrain aspects of the carbon cycle. We should also expect to see a renewed emphasis on the natural carbon cycle in ocean general circulation models (GCMs). Ocean GCMs have not, to this point, been used to very effectively to study the natural carbon cycle and its effect on the perturbed modern carbon system.

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J. R. Toggweiler, GFDL/NOAA, Princeton University, P.O. Box 308, Princeton, NJ 08542

(Received June 24,1994; accepted November 22,1994.)