Climate change predictions are inaccurate, says report

The range of possible climate futures predicted for the 21st century is almost certainly larger than commonly presented because climate change assessments overlook critical vegetation and land-use factors, researchers at Colorado State University have claimed.


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Roger Pielke, professor of atmospheric science, and colleagues claim that the effect of vegetation growth and land-use changes can have a profound effect on climate variability and change. This work calls into question the realism of the climate predictions used in the US regional, national and international assessments because these factors have not been included in the model.

“If land-use change is as important on the climate system as our results suggest, there is a large uncertainty in the future climate, since there is no evidence that we can accurately predict the future landscape,” Pielke said.

He maintains that the General Circulation Models used by US researchers and the UN-sponsored Intergovernmental Panel on Climate Change (IPCC) only investigate a subset of the effects of greenhouse gases and aerosols. They do not, he said, incorporate other important effects on climate, such as land-use change and the biological effect of increased carbon dioxide. As a result, the range of possible climate futures that have been predicted for the 21st century is almost certainly larger than commonly presented.

“This does not mean that we should not worry about future changes in climate,” Pielke said. “Rather, it should raise serious questions as to our ability to reliably predict such changes.”

Addressing the the American Meteorological Society’s 11th Symposium on Global Change, Pielke discussed how vegetation influences the earth’s energy budget, directly and otherwise, through a variety of effects.

For example, as carbon dioxide concentrations increase, a corresponding increase in plant coverage could increase the release of water vapour into the atmosphere as a result of plant metabolism. While this water could cool the region’s atmosphere directly or through cloud formation, it also could increase the amount of water vapour, which acts as a greenhouse gas. “This is an example of a complex feedback between vegetation and the atmosphere that we do not completely understand,” Pielke said.

Pielke’s believes the global energy budget is also affected by land-use change. Cutting tropical forests, increasing urban sprawl and converting forest or grasslands to agriculture can alter the amount of sunlight absorbed or reflected and the amount of water vapour released into the atmosphere by transpiration, he claims.

“Since landscape and other atmosphere-surface interactions involve complex, non-linear feedbacks, it becomes impossible to predict accurately future climate,” Pielke said. “This suggests that the scientific community might be overstating the certainty in the predictive information that is currently being provided to other researchers and to policy makers.”

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