articles

Sinks for Anthropogenic Carbon

Jorge L. Sarmiento and Nicolas Gruber

Figure 1

Organic carbon buried in sediments as coal, natural gas, and oil over literally hundreds of millions of years is being consumed as a result of human activities and returned to the atmosphere as carbon dioxide (CO₂) on a time scale of a few centuries. The energy harvested from this transformation of fossil fuels supplies us with electricity, heat, transportation, and industrial power. The clearing of forests for agricultural lands and the harvesting of wood, both of which remove carbon-bearing vegetation, have also added CO₂ to the atmosphere, in amounts equivalent to more than half of the fossil fuel source. The CO₂ added to the atmosphere because of man's activities, and the way it is currently distributed within the land, air, and sea, is depicted in the carbon cycle diagram shown in Figure 1.

Because of anthropogenic emissions, atmospheric CO₂ has climbed to levels that are presently more than 30% higher than before the industrial revolution,^1,2 as seen in Figure 2. Indeed, geochemical measurements made on ancient ocean sediments suggest that atmospheric CO₂ levels over the past 20 million years were never as high as they are today.³

Figure 2

The increase in atmospheric CO₂ has drawn a great deal of attention because of the impact it has on the trapping of long wavelength radiation emitted from Earth's surface. More than half of the increase in the direct trapping that has occurred since preindustrial times is attributed to CO₂, with the rest coming from other gases such as methane, nitrous oxide, and chlorofluorocarbons. The effect of this increased trapping on Earth's climate depends on a number of complex feedbacks. Nevertheless, the strong consensus of the scientific community is that the increased trapping will lead to global warming; it probably accounts for most of the 0.6 ± 0.2 °C warming that occurred during the last century.³ Humankind thus appears to be playing a significant role in altering Earth's climate.

Figure 3

Because CO₂ is nonreactive in the atmosphere, it has a relatively long residence time there. However, its growth rate is presently less than half of what would be expected if all the CO₂ released by fossil-fuel burning and land-use change remained in the atmosphere (Figure 3). The growth rate is lower because the terrestrial biosphere (plants and soils) and the ocean are taking up a significant amount of anthropogenic CO₂, that is, acting as "sinks." The scientific community has made much progress in establishing the relative role of these two major natural sinks on a global scale, and it appears that the missing carbon is about equally divided between them. However, scientists continue to debate aspects of the spatial distribution and mechanisms of these sinks. The future behavior of the sinks turns out to be highly sensitive to whatever mechanisms we assume. Thus, better understanding of their behaviors is key to predicting, and hopefully mitigating, the future impact of anthropogenic CO₂. An important starting point for forecasting the future behavior is to understand its past.

The most recent decade for which we have an estimate of all the carbon sources and sinks³ is the 1980s, during which the average fossil-fuel emissions were estimated at 5.4 ± .3 petagrams of carbon per year (1 Pg = 10¹⁵ g) and the atmospheric growth rate at 3.3 ± 0.1 Pg C/yr. The difference of 2.1 ± 0.3 Pg C/yr must be taken up by the ocean and terrestrial biosphere. New developments using observations of oxygen and carbon isotopes in the atmosphere allow us to partition the sink between these two reservoirs, as described in box 1. The partitioning gives an average net oceanic uptake of 1.9 ± 0.6 Pg C/yr and a net land carbon uptake of only 0.2 ± 0.7 Pg C/yr. However, there is an additional terrestrial source of CO₂ to the atmosphere, mainly due to deforestation in the tropics, estimated to be about 1.7 Pg C/yr. One can infer that the land must be taking up 1.9 Pg C/yr outside the areas affected by deforestation and hence appears to be as large a carbon sink as the oceans, as shown in Figure 1. Note, however, that the uncertainty in the estimated terrestrial source is very large, with a reported range of 0.6­2.5 Pg C/yr. The uncertainty in the land sink, which is estimated by differencing all the other components of the carbon budget, is correspondingly large.

The carbon balance is not static in time. As seen in Figure 3, the atmospheric growth rate varies by a large amount from year to year. Most of the interannual variability is correlated with the El Niño southern oscillation climate mode, with higher growth rates generally being associated with El Niño (warm climate) episodes. The climate cooling caused by the Mt. Pinatubo eruption in the early 1990s appears to have contributed to reduced atmospheric growth rates. The primary cause of the variability remains controversial, but is probably due mostly to the response of terrestrial vegetation to climate variability, with a smaller contribution due to the oceanic response.⁴

The total capacity of the oceans for taking up anthropogenic CO₂ is a function primarily of the solubility of CO₂ and the chemical buffering capacity of seawater. The chemical buffering stems from the reaction of CO₂ molecules with carbonate ions to produce bicarbonate ions. If a moderately sized pulse of CO₂ were added to the atmosphere today, about 85% of it would dissolve in the ocean, but the process would take more than 1000 years because of the sluggishness of vertical exchange between the surface and interior of the ocean. For example, the oldest ocean water in the world, which is found in the deep North Pacific, has been out of contact with the surface of the ocean for about 1000 years. Estimates of the oceanic concentration of anthropogenic carbon, shown in Figure 4, demonstrate that, except in a few locations, carbon from human activities generally has not yet penetrated in significant amounts below a depth of about 1000 m. Surface waters carrying anthropogenic carbon descend into the abyssal ocean primarily in the North Atlantic and Southern Oceans, as part of the ocean's thermohaline circulation, the so-called "conveyor belt." From there, those waters spread, over the next centuries, throughout the entire deep ocean. Currently one can see a modest signal of the anthropogenic carbon in deep waters only in the Atlantic north of the equator.

Figure 4

Model simulations suggest that the Southern Ocean around the Antarctic (south of 35 °S) accounts for nearly half of the net air­sea flux of anthropogenic carbon. Another sixth of the total uptake occurs in the tropics (13 °S to 13 °N). Those are both regions with large surface areas, where upwelling (in the tropics and Southern Ocean) and deep vertical exchange (in the Southern Ocean) bring to the surface older, relatively uncontaminated deep water that has a high capacity for uptake of CO₂. From those regions, a large part of the absorbed CO₂ is transported laterally toward the subtropics, producing a relatively uniform distribution of anthropogenic carbon, as shown in Figure 4. Different methods for estimating the uptake of anthropogenic CO₂ agree reasonably well with each other and with observational constraints in most regions except the Southern Ocean, where the relatively small-scale processes that destabilize the water column and lead to vertical overturning and formation of deep water are poorly understood and particularly difficult to model.

The land sink for carbon is the subject of considerable controversy at present, concerning not only its magnitude but also its cause. For many years, researchers have believed that the dominant sink mechanism is the fertilizing effects of increased CO₂ concentrations in the atmosphere and the addition to soils of fixed nitrogen from fossil-fuel burning and agricultural fertilizers. This fertilization mechanism has been incorporated into most existing models of the terrestrial biosphere that are used to predict future concentrations of atmospheric CO₂. However, a recent analysis of long-term observations of the change in biomass and growth rates, made by the US Forest Service, suggests that such fertilization effects are much too small to explain more than a small fraction of the observed sink in the US.⁵ In addition, long-term experiments in which small forest patches and other land ecosystems have been exposed to elevated CO₂ levels for extended periods show a rapid decrease of the fertilization effect after an initial enhancement.⁶

Figure 5

What other mechanisms might then explain the existence of a large land sink? One of the few land regions of the world where the observational coverage and model analysis are sufficient for putting together a carbon budget is the coterminous US. A recent estimate of carbon uptake in this region⁷ shows that the largest sink is due to regrowth in abandoned farmland and areas that had previously been logged.⁵ As shown in Figure 5, a large fraction of the forests in the eastern US were cut down before 1920. As the use of agricultural land shifted westward over time, land in the east that had been used for agriculture was abandoned and started to recover its forests. Moreover, as the logging pressure has decreased in the east over time, there has been a large increase in secondary vegetation.

After regrowth in farmlands, the second most important sink mechanism identified by the study of carbon uptake in the US is the encroachment of woody growth into areas where fires have been suppressed, especially in the southwest, as seen in Figure 5. The third largest mechanism is the growth of food and wood that is exported from the US before being consumed. But that is not a true sink because the CO₂ is returned to the atmosphere elsewhere in the world. Other sinks include storage of carbon in wood products and cropland soils, as well as in freshwater reservoirs, alluvium, and colluvium. Some carbon is taken up by weathering, such as the reaction of rock with CO₂ and water to form dissolved bicarbonate and calcium, but it is exported from the US by rivers.

What do we know about the land sink in the rest of the world? For the past decade, "inverse modeling"⁸ has been one of the primary tools to improve our understanding of the large-scale behavior of both the terrestrial and the oceanic sinks. Inverse models use information about how CO₂ is currently distributed in the atmosphere and knowledge of how the atmosphere transports CO₂ to infer the spatial distribution of carbon sources and sinks at Earth's surface. From such models, as discussed in box 2, we have learned that more than half of the sink for anthropogenic carbon must be in the Northern Hemisphere. Furthermore, a majority of the Northern Hemisphere sink appears to be in the terrestrial biosphere.

While many attempts have been made to determine the location of this Northern Hemisphere land sink more precisely, the results show considerable variation among the various inversion models. Unfortunately, in situ verification of those estimates has proven to be extremely challenging. Despite the development of innovative new methods to measure in situ fluxes (as seen on the cover and as described at http://www.as.harvard.edu/chemistry/hf) and detailed distributions of atmospheric CO₂ from aircraft, it continues to be difficult to close the gap between lower in situ estimates and the large sinks inferred from the models. The estimate of the land sink in the coterminous US discussed previously gives a total carbon sink that is on the low side but within the uncertainty of what is implied by atmospheric inversions. Although the US data suggest that the land sink there is driven primarily by land-use history and not fertilization by atmospheric CO₂ or atmospheric nitrogen deposition, it is not clear if the same findings hold true for other land regions. If they do, however, they have major implications about the future land sink: The global CO₂ uptake capacity from changes in land-use such as forest regrowth is much more modest than that predicted for fertilization.⁹

The usual approach for evaluating the future trajectory of atmospheric CO₂ is to predict potential concentrations under each of several possible emissions scenarios. The emissions scenarios are beset with uncertainties: How will population changes, economic growth, technology developments, and so forth affect the amount of CO₂ emitted? And how likely is it that nations will agree to curb those emissions? Models for determining the resulting atmospheric concentrations are further beset by uncertainties about the land and ocean sinks.

A common starting point for climate predictions is a scenario for anthropogenic emissions of CO₂ and other greenhouse gases known as IS92a; it was developed by the Intergovernmental Panel on Climate Change (IPCC) to represent "business-as-usual," with no intentional efforts to mitigate the greenhouse gas. This scenario estimates total fossil-fuel emissions between 2000 and 2100 of 1450 Pg C--a substantial fraction of the estimated remaining resource base of 3500 Pg C.¹⁰

To estimate how much of this CO₂ stays in the atmosphere, one must use models of the global carbon cycle. Until about the mid-1990s, most such models assumed that the climate remains stationary and that the land sink is primarily driven by CO₂ fertilization. Many such models project that about half of the CO₂ emissions will have accumulated in the atmosphere by the year 2100, and will lead to atmospheric CO₂ concentrations of around 700 parts per million (ppm). How the rest of the carbon is partitioned between land and ocean varies widely with the model applied:³ In one particular simulation by the British Met Office's Hadley Centre,¹¹ 60% of the remaining CO₂, or 450 Pg C, ends up on land and the remaining 300 Pg C in the ocean.

Such predictions can change dramatically if one considers feedbacks due to global warming. In another run of their climate model, Hadley Centre researchers calculated that an increase of atmospheric CO₂ concentrations to 713 ppm by 2100 would cause global temperatures to rise by 4.0 °C, (5.5 °C over land) with an associated intensification of the hydrological cycle. In that simulation, the climate change is allowed to influence the terrestrial and oceanic carbon sinks but not the atmospheric concentrations, which remain fixed. As the climate warms, the land no longer absorbs carbon, but emits it; in other words, the land sink turns into a source, adding 60 Pg C to the atmosphere by 2100. At the same time, the oceanic sink becomes less effective, soaking up 250 Pg C, rather than 300 Pg C. The changed nature of the land sink in this model results first from the disappearance of the Amazonian rainforest due to increased dryness of soils and second from an increased rate of bacterial breakdown of organic carbon in soils due to increased soil temperatures. The oceans lose some solubility due to higher water temperatures, slowed vertical exchange, and changes in the way that biological processes redistribute carbon within the ocean. The combined terrestrial and oceanic carbon sinks predicted by the Hadley model for 2100 are thus 190 Pg C rather than 750 Pg C, and this simulation is out of balance with the IS92a emissions by about 560 Pg C.

In a third simulation, Hadley Centre researchers put the excess 560 Pg C back into the atmosphere using a new fully coupled simulation of the carbon cycle and climate. Concentrations of atmospheric CO₂ climb to 980 ppm instead of 713 ppm, and the surface air temperature warming is 5.5 °C (8.0 °C on land). The greater warming induces the land to release even more carbon, but that release is counterbalanced by an increased oceanic absorption caused by the faster growth rate of atmospheric CO₂ , as illustrated in the top panel of Figure 6. All the additional CO₂ remains in the atmosphere.

Figure 6

The results are as uncertain as they are disconcerting. The bottom panel of Figure 6 shows the results of another simulation, done by the Institut Pierre Simon Laplace (IPSL), in France, which also predicts the impact of changing climate on the atmospheric carbon concentrations.¹² The IPSL model uses a newer emissions scenario, similar to the IS92a. The model's oceanic uptake is almost two times more effective than that of the Hadley model ocean, with most of the difference being in the difficult-to-simulate Southern Ocean. More important, however, the land uptake in the IPSL model is less than half as sensitive to temperature as the Hadley model, and does not allow the distribution of terrestrial vegetation to change. The IPSL model with global warming thus predicts an atmospheric CO₂ level in 2100 that only reaches 780 ppm, as contrasted with 980 ppm for the Hadley model, as shown in the lower panel of Figure 6.

We scientists cannot hope to narrow the range between such simulations until we have a better understanding of the fundamental mechanisms that control the carbon sinks. The uncertainties that must be addressed include the magnitude of CO₂ uptake in the Southern Ocean and the worldwide impact of fertilization and land-use changes.

Although we don't yet fully understand the global carbon cycle, it's safe to say there are no magic bullets in the carbon sinks to rescue the world from high atmospheric CO₂ levels any time in the next few centuries. Quite to the contrary, most of the feedbacks between the global carbon cycle and global warming seem to be positive--that is, global warming reduces the sink strengths. Furthermore, the land carbon sink, if it is due primarily to land-use changes, will likely saturate much earlier than if it were due largely to fertilization. Thus, the emission reduction and sequestration measures required to stabilize future atmospheric CO₂ at levels that will minimize impacts are likely to be more stringent than estimated with models that did not include carbon­climate feedbacks and that did include fertilization as the main mechanism for the terrestrial carbon sink.

Some researchers have proposed enhancing the natural carbon sinks by, for example, reforestation, fertilizing with missing nutrients such as iron to stimulate the oceanic biological pump, and injecting CO₂ directly into the deep ocean. But the projected business-as-usual emissions of order 1450 Pg C over this century are far too large for the first two measures to alter significantly the final fate of CO₂. Deep ocean injection might reduce the future peak value of CO₂ in the atmosphere, but would not affect the final atmosphere­ocean equilibration. Moreover, it would require extremely large injections, with unknown technological challenges and environmental consequences. Geological sequestration is also being explored. Currently, the only method that can be guaranteed to mitigate the expected increases in atmospheric CO₂ is reduced fossil-energy utilization.

The past two decades have seen an improvement in our understanding of the global carbon cycle. We now know the history of atmospheric CO₂ for many hundreds of thousands of years. We have a good grasp of the partitioning of the present carbon sink between land and ocean, and strong constraints on the latitudinal distribution of those sinks. However, there is much need for improvement. On land, new findings call into question the fundamental mechanisms to which the historic terrestrial carbon sink had been ascribed. If the sink is primarily due to land-use changes, it will likely saturate much earlier than if due to CO₂ fertilization.

Regarding the oceans, we understand their chemistry and most of the fundamental mechanisms driving the ocean circulation and can develop sophisticated models of them. However, we have to settle for only a simplified representation of such important related processes as convective overturning and the formation and melting of sea ice, although progress is being made in these areas. We also face limitations in computer capabilities. Consequently, simulations of both the present uptake and response of the ocean to future warming using different models disagree substantially. The problem is particularly acute in the Southern Ocean around Antarctica, which we believe may account for 40­50% of present uptake of anthropogenic CO₂ and will continue to dominate the uptake in the future. In addition, we have only begun to understand how the cycling of carbon by organisms in the ocean will respond to climate and chemical changes.

The international scientific community is well aware of the problems we have identified and has developed long-term observational and modeling strategies for tackling them. Those strategies include proposals for improved observational capabilities on land and in the ocean, with a particular focus on monitoring the changes that the carbon system will undergo in response to warming. Improved models of the coupled carbon­climate system are being developed at a number of locations around the world. Considerable additional progress can be expected over the next decade.