CO2 rise will accelerate in a warming world, research shows
Global warming forecasts may have to be scaled up significantly to account for higher emissions of CO2 from ecosystems in a warmer world, a team of European scientists has found.
Marten Scheffer of Wageningen University in Holland and colleagues Victor Brovkin and Peter Cox calculated that this effect would speed up global warming by 15-78% - a significant increase that has so far been largely ignored, they say.
"The essence of the problem stripped to the bare bones is that CO2 affects global temperature, while at the same time temperature affects the CO2 concentration," the scientists write in the Geophysical Research Letters journal published this Friday.
Accounting for this CO2 feedback effect sends the mid-range estimate of temperature rise made by the Intergovernmental Panel on Climate Change (IPCC) from around 3 degrees up to 4.5 by the end of this century.
"The IPCC estimate includes various feedbacks, but out of the range of models they used only one included the CO2 feedback we looked at, and even that took a very conservative estimate for its strength," Professor Marten Scheffer told edie.
The effects of temperature changes on the complex processes that send carbon cycling through terrestrial and oceanic ecosystems have long been the cause of uncertainties in climate forecasts. Their extent has proven difficult to quantify due to the complexity of the temperature-sensitive processes, such as CO2 being taken up by plants or dissolving in the oceans.
Prof Scheffer's team calculated the effect of rising temperatures on CO2 levels using a new method that bypasses the need to understand these processes in detail.
"There are very many mechanisms involved, simple things like higher temperatures promoting decomposition of vegetation, sending more carbon into the atmosphere, or the important effect of oceans taking up less CO2," Prof Sheffer explained. Some of these increase CO2 levels as temperatures rise, and some decrease them, but the overall on CO2 levels is positive, he said - that is climate change speeds up the CO2 rise.
"The interesting thing with this research is that we managed to go through the back door and bypass the uncertainties in estimating all these separate processes," he said.
Prof Sheffer's team looked back into the Little Ice Age, a period of prolonged cooling that the earth experienced between 1200 and 1700. During this period CO2 levels decreased as temperatures fell, and as humans did not yet influence atmospheric composition the fall in CO2 can be wholly attributed to the change in temperature.
The scientists used CO2 and temperature data from the Little Ice Age to estimate the effect of temperature on CO2 levels, and projected this onto the changes expected for the next 100 years.
The 15-78% acceleration of global warming that they found still contains large uncertainties, Marten Scheffer stressed. Nevertheless, it is yet another indication that our current estimates of global warming for the next century are underestimates.
In a report recently leaked to the media before official publication, the IPCC puts the warming estimate at 3 degrees for the next 50, rather than 100 years (see related story).
Many climate change forecasts also ignore other feedbacks, such as that of higher emissions of methane - another greenhouse gas - in a warmer world. The scientists said that their own estimate may be an underestimate for that reason:
"The estimated feedback effect might be conservative, as higher temperatures are also likely to promote concentrations of methane," the new research paper warns.