Summary and Conclusions

Our analysis of the vulnerability of the carbon pool on land and in the ocean reveals that a substantial amount of carbon is at risk of becoming mobilized and being released into the atmosphere (Figure 3.2, Figure 3.3, and Table 3.1). Recognizing that our estimates are quite uncertain, we find that on the order of several tens of Pg could be lost from the land and ocean carbon pool over the next 20 years and several hundred Pg over the course of this century. On land, we estimate that the maximum potential losses are substantially larger than the maximum potential sinks, suggesting that the overall sign of the feedbacks arising from carbon-climate-human interactions is positive—that is, accelerating climate change. For the ocean, we must differentiate between the ocean uptake that exists in the absence of climate change and the ocean sink that is due to climate feedbacks. The ocean uptake in the absence of any feedback is very large, on the order of several hundred Pg C. This uptake could be substantially offset by the feedbacks, since their net sign is likely positive as well.

Our results imply that the fraction of the anthropogenic CO2 emissions that will remain in the atmosphere and force a climate change will increase in the future. As a consequence, the permissible anthropogenic emissions in a stabilization pathway are smaller in the presence of interactions between the carbon-climate-human systems. The

circulation feedback (ant + natural)

200 to 700 <400

DIC,

DICsolub [2700-400]

Units are Pg C for reservoirs and integrated fluxes from 2000-2100

DIC,

DICsolub [2700-400]

Units are Pg C for reservoirs and integrated fluxes from 2000-2100

Figure 3.3. Global carbon cycle, its sink potential, and its vulnerability in the 21st century. (a) Preindustrial carbon cycle, (b) major ocean carbon pools and their vulnerability, and (c) major land pools and their vulnerability. The italic numbers shown in (b) and (c) represent our maximum estimates of the vulnerability. See also Table 3.1 and text. The oceanic changes in the pool sizes are larger than the induced air-sea CO2 fluxes because of the buffering effect of seawater.

Figure 3.3. Global carbon cycle, its sink potential, and its vulnerability in the 21st century. (a) Preindustrial carbon cycle, (b) major ocean carbon pools and their vulnerability, and (c) major land pools and their vulnerability. The italic numbers shown in (b) and (c) represent our maximum estimates of the vulnerability. See also Table 3.1 and text. The oceanic changes in the pool sizes are larger than the induced air-sea CO2 fluxes because of the buffering effect of seawater.

magnitudes of the feedbacks depend to a large degree on the state of the climate system. The feedbacks tend to get stronger with larger climate forcing, implying that the magnitude of the offsetting effect for the permissible emissions increases with the concentration of the atmospheric CO2 stabilization target. A final observation that arises from the inspection of Table 3.1 is that the vulnerability of the global carbon cycle stems from the vulnerability of many different pools and processes. This finding highlights the need for a highly integrative and interdisciplinary approach for studying the global carbon cycle and the need to view Earth as a coupled system rather than as an entity that can be studied in parts.

Although we view our analysis as a first step toward identifying and quantifying these feedbacks, our estimates are preliminary at best. We hope, however, to encourage subsequent research that will improve our initial assessment.

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