A team of European scientists reports that climate change estimates for the next century may have substantially underestimated the potential magnitude of global warming. They conclude that actual warming due to human fossil fuel emissions may be 15% to 78% higher than warming estimates that do not take into account the feedback mechanism involving carbon dioxide and Earth’s temperature.
In a paper to be published on 26 May in Geophysical Research Letters, Marten Scheffer of Wageningen University in the Netherlands and colleagues at the Potsdam Institute for Climate Impact Research in Germany and the Centre for Ecology and Hydrology in the United Kingdom use newly acquired ancient climate data to quantify the two-way phenomenon by which greenhouse gases not only contribute to higher temperatures, but are themselves increased by the higher temperatures—a positive feedback loop.
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. To analyze the feedback our model should include both effects.
The effect of CO2 and other greenhouse gases on global temperature is relatively straightforward...The effect of temperature on greenhouse gases is the more difficult aspect to model.—from the paper
Examples of positive feedback loops in warming include increased release of CO2, methane and N2O from terrestrial ecosystems; increased oceanic denitrification and stratification, resulting in nutrient limitation of algal growth reducing the CO2 sink to the ocean; and the reduction of CaCO3 neutralization at higher temperatures.
The researchers interpreted high-resolution data from polar ice cores and temperature reconstructions based on geological proxy data in a new way.
One complicating factor was that some of the processes that play a role in the feedback loop are quite fast, taking place over a period of years, while others take centuries or even millennia. This implies that the strength of the feedback effect depends on the time scale being analyzed. Another factor was that the modern world looks quite different than it did tens of thousands of year ago, when the ice in the cores was formed.
Therefore, the authors focused especially on relatively recent climatic anomaly known as the “Little Ice Age.” During this period (about 1550-1850), immortalized in many paintings of frozen landscapes in Northern Europe, Earth was substantially colder than it is now.
This, scientists have concluded, was due largely to reduced solar activity, and just as during true ice ages, the atmospheric carbon level dropped during the Little Ice Age. 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.
Although there are still significant uncertainties, our simple data-based approach is consistent with the latest climate-carbon cycle models, which suggest that global warming will be accelerated by the effects of climate change on the rate of carbon dioxide increase.
In view of our findings, estimates of future warming that ignore these effects may have to be raised by about 50%. We have, in fact, been conservative on several points. For instance, we do not account for the greenhouse effect of methane, which is also known to increase in warm periods.—Marten Scheffer
The research was funded by Wageningen University and the United Kingdom Natural Environment Research Council.
“Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change”; Marten Scheffer, Victor Brovkin, and Peter Cox; Geophys. Res. Lett, 33, L10702, doi:10.1029/2005GL025044