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Climate Feedback Effect Reduced

Dr David Whitehouse

Writing in the journal Nature Dr David Frank of the Swiss Federal Research Institute in Birmensdorf and colleagues have produced the most accurate estimate yet about how rising temperatures will trigger the release of more CO2 from the ocean and land, thus amplifying the greenhouse effect. In the recent past this effect has been said to “significantly accelerate climatic change in the 21st century.”

It is an interesting study showing that the most alarming forecasts of this so-called carbon feedback may be too high. However, its predictive power as to what will happen in the future may be somewhat limited.

One impact of an increase in the atmospheric concentration of CO2 and other greenhouse gases is global warming which in turn influences the carbon cycle and thus, in turn, the concentration of CO2 in the atmosphere in a positive feedback loop. Until now there has been a large uncertainty in the magnitude of this effect. The IPCC’s fourth assessment report had a broad range of estimates as to how far natural systems would contribute to a spiral of warming. The Nature paper narrows that range to the lower end of previous estimates.

Frank et al have looked at temperature data and atmospheric CO2 data for the period 1050–1800 because mankind’s influence on the climate was negligible during this time. This period constitutes a unique natural laboratory for studying climate variation. CO2 levels in the atmosphere have already risen to about 390 ppm from about 280 ppm before the Industrial Revolution.

Their conclusion suggests that a rise of one degree C would increase CO2 concentrations by about 7.7 parts per million in the atmosphere. This is far below other recent estimates of 40 ppm that would have given a much stronger boost to climatic changes such as floods, desertification and rising sea levels to give a few examples.

The conclusions, if correct, are comforting and some have said it indicates that there will be fewer unwelcome surprises in the decades to come. But it would be unwise to draw too much from this good news. Variations in CO2 and temperature over the last millennium have been small and Frank et al are assuming that the relationship they derive from these very small variations can be applied to the larger variations in temperature some predict later this century. In the future different aspects of the carbon feedback cycle may come into effect.