Feedback between temperature and CO2 may enhance global warming beyond previous estimates
22 May 2006
A European team of scientists reports that climate change projections for this century may have underestimated the potential magnitude of global warming. Actual warming due to human fossil fuel emissions can be 15 to 78 percent higher than the warming estimates which do not take into account the intricate feedback between carbon dioxide and Earth's temperature.
In a paper to be published on 26 May in Geophysical Research Letters, Marten Scheffer (Wageningen University in the Netherlands), Victor Brovkin (Potsdam Institute for Climate Impact Research in Germany), and Peter Cox (Centre for Ecology and Hydrology in the United Kingdom) use recently retrieved ancient climate data to quantify the two-way phenomenon by which greenhouse gases not only contribute to higher temperatures, but are themselves enhanced by the warming. The CO 2 rise leads, in turn, to still higher temperatures in what scientists call a positive feedback loop.
The researchers interpret the high-resolution data from polar ice cores and temperature reconstructions based on geological proxy data in a novel way, focusing on a relatively recent climatic anomaly known as the "Little Ice Age." During this period (about 1550-1850), the Northern Hemisphere temperatures were lower than they are now, and - just as during true ice ages - the atmospheric CO 2 level dropped as well. The authors used this information to estimate how sensitive the carbon dioxide concentration is to temperature, which allowed them to calculate how much the climate-carbon dioxide feedbacks will affect future global warming.
Victor Brovkin adds: "Climate feedbacks are very important to quantify because they are often hidden drivers of abrupt climate changes. We are routinely using our models to estimate the feedback strength, but the outcome is strongly model-dependent. Therefore, we need some independent data to evaluate the model results. An emerging approach is to use geological archives of past climate and CO 2 dynamics. At the moment, the uncertainty in our results is high, but data precision is improving, and this gives us hope that we can constrain the feedbacks even better. And, in spite of uncertainty, our new method already tends to exclude those climate projections that are at the low end."
Scheffer, M., Brovkin, V., and Cox, P. Positive feedback between global warming and atmospheric CO 2 concentration inferred from past climate change.Geophysical Research Letters, vol. 33, doi:10.1029/2005gl025044, 2006.
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