They say that actual warming due to human fossil fuel emissions may be 15-to-78 percent 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.

This higher concentration leads to still higher temperatures, in what scientists call a positive feedback loop.

The researchers achieved their breakthrough by interpreting the high-resolution data from polar ice cores and temperature reconstructions based on geological proxy data in a new way. Although the effect of greenhouse gases on temperature is well known, the reverse effect is usually ignored. The latter has now been estimated through a correction of the past climate data, using a model of the greenhouse effect.

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.

As Marten Scheffer explains, "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 percent. 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."

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