Positive feedbacks from the carbon cycle

Two papers appeared in Geophysical Research Letters today claiming that the warming forecast for the coming century may be underestimated, because of positive feedbacks in the carbon cycle. One comes from Torn and Harte, and the other from Scheffer, Brovkin, and Cox. Both papers conclude that warming in the coming century could be increased by carbon cycle feedbacks, by 25-75% or so. Do we think it’s time to push the big red Stop the Press button down at IPCC?

The approaches of both papers are similar. The covariation of temperature versus CO2 (and methane in Torn and Harte) is tabulated for a record in the past. For the Torn and Harte paper, the time frame chosen is the last 360,000 years, while Scheffer et al. focus on the Little Ice Age, from 1500-1600 A.D. In both cases it is assumed that the climate shift is driven by some external thermal driver. As the temperature warms (in the case of the deglaciation) or cools (the LIA), the CO2 concentration of the atmosphere changes in the sense of a positive feedback, rising associated with warming or falling in response to cooling. The changing CO2 drives a further change in temperature.

In general, it is clear that eventually the sense of these articles could be correct. The response of the terrestrial biosphere to rising CO2 could go either way; toward an increase in uptake because of CO2 fertilization or a longer growing season (as we see today) versus an increase in soil carbon respiration in warmer conditions (the reason why tropical soils contain so little carbon). Uncertainties in the response of the terrestrial biosphere to rising CO2 is a major source of uncertainty for the climate change forecast (Cox et al., 2000).

The oceans are presently taking up about 2 Gton C per year, a significant dent in our emissions of 7 Gton C per year. This could slow in the future, as overturning becomes inhibited by stratification, as the buffer loses its capacity due to acidification. Eventually, the fluxes could reverse as with a decrease in CO2 solubility due to ocean warming.

The biggest question, however, before pushing the Stop the Press button at IPCC, is timing. The CO2 transition through the deglaciation took 10,000 years. (Actually this helps to constrain the cause of the CO2 transition, because the air/sea equilibration time scale for CO2 would be considerably shorter than that.) The timescale that seems intrinsic to IPCC is a century or so, during which we should be able to reap only a small fraction of any harvest that takes 10,000 years to grow. The Scheffer et al paper avoids this issue by restricting its attention to a time period of just a century.

Scheffer et al illustrate the potential feedback for the coming century in a figure which looks something like Figure A.

Temperature depends on CO2 concentration via radiative equilibrium in the blue curves, and CO2 concentration in the air is affected by temperature according to the red lines. A rise in CO2 concentration from an external source changes the equilibrium CO2 as a function of T relation toward higher CO2, to the right, labeled “forcing”. The stable final equilibrium is where the two relations cross, with further CO2 degassing from the land or the ocean, so that more CO2 ends up in the atmosphere than would have if there were no feedback (a vertical red line). A climate sensitivity calculated from the coupled system is higher than one that ignores any carbon cycle feedbacks.

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