Anthropogenic climate change promotes increased release of the greenhouse gas CO2 from the terrestrial biosphere and the oceans -- this, in turn, increases global warming. For each degree Celsius of global warming, CO2 levels in the atmosphere increased by roughly three percent.

This conclusion represents the outcome of a study conducted by climatologists of the Swiss Federal Research Institute for Forest, Snow and Landscape (WSL) in collaboration with Bern and Mainz Universities. Their paper was published in the journal Nature on 28 January 2010.

Air inclusions in ice core samples from Antarctica and the annual rings of trees provide researchers with the most reliable data for comparing current climate changes with the global climate history of recent millennia and for validating climate models. The Swiss Federal Research Institute WSL, the University of Bern and Johannes Gutenberg University Mainz -- a group representing the leading international authorities with regard to the interpretation of tree rings and ice cores, and the modelling connections between climate and carbon cycle -- have analyzed the climate fluctuations of the last 1,000 years in detail.

The article published in Nature describes the comprehensive investigations conducted by David Frank and his research team, working under the umbrella of the Oeschger Center for Climate Change Research in Bern. As a result of these investigations, they are now in the position to estimate with considerably greater accuracy to what extent the atmospheric CO2 concentrations were influenced by climate change in the preindustrial past.

Global warming results in increased release of greenhouse gas
The burning of fossil fuels, such as coal, oil, and gas, is responsible for the current rise in CO2 levels, and is the main cause of on-going global warming. However, climatic change itself interferes with the natural equilibrium between the massive depots of carbon in the atmosphere, the oceans, and the biosphere: This results in additional release of CO2 from these long-term reservoirs. The additional release of CO2, by adding to the warming, acts as a feedback mechanism that amplifies climate change. However, the actual strength of feedback between carbon cycle and warming is still being extensively debated within the scientific community.

The results of this study now provide an answer. Nine different reconstructions of temperature fluctuations and three CO2 histories were tested for their reliability and used to constrain the carbon cycles response to climate change. The researchers calculated the links between temperature and CO2 levels for more than 200,000 possible data combinations.

They managed to produce a reliable mean value and a corresponding confidence interval for the extent of feedback between temperature and CO2. "By serving as a benchmark for the past, these new data may help the climate modelling community make more accurate predictions for the future," stated David Frank of the WSL, the lead author of the paper.

Initial testing of climate models
Just how accurately do current climate models reproduce the actual extent of feedback between carbon cycle and anthropogenic climate change? As a starting point, the researchers compared the results from ten climate models with the probabilistic estimates from their study. They found that the models were consistent with their results, but also that models that predicted a less pronounced feedback might be more realistic. Overall, the results suggest that current predictions for the near future are likely to be fairly precise. This means that in order to limit the extent of increases in atmospheric CO2 and anthropogenic warming, we will need to reduce our carbon emissions over the next decades.

Natural climate variation over the last 1,000 years already exceeded

The new analysis also makes it possible to reliably compare current rates of global warming with the rate of natural temperature variation. Global temperatures are currently more than 0.3 degrees Celsius higher than the warmest period of the last 1,000 years. This suggests that human activities have widened the range of natural temperature fluctuation in this millennium by 75 percent.

(ScienceDaily*, 27/01/2010)

*Adapted from materials provided by Mainz, Universitaet.