Climate forecasts are blowing hot and cold again. US researchers have shown that plants and bacteria in prairie soil cut their production of greenhouse gas as they adapt to rising heat1. So current predictions of global warming could be over-estimates.

Climate-change models assume that soil microbes accelerate global warming. Rising temperatures are expected to speed the bacteria's respiration, which burns food and releases the greenhouse gas carbon dioxide (CO2).

In fact, grassland soil acclimatizes to heat. In climes such as those on the prairie, CO2 release "is not going to be as much as feared", says Linda Wallace of the University of Oklahoma in Norman. This could slow climate change.

Soil respiration contributes up to 100 billion metric tonnes of carbon a year to the global carbon cycle. "If we tweak it by 10-15% it's equivalent to all our emissions combined," says Lindsey Rustad, who studies soil respiration with the US Department of Agriculture's Forest Service in Durham, New Hampshire. Man-made emissions total a mere 7 billion tonnes.

The idea that CO2 release would escalate came from earlier, short-term studies of heated soil. "We have to be very careful about making long-term predictions on the basis of short-term experiments," cautions Rustad.

The heat is on

Wallace's team warmed Oklahoma grassland plots using infrared heaters. Respiration rose, but not as fast as climate models assume. "This assumption is critical," says climate modeller Peter Cox of the Meteorological Office in London.

The Earth's average temperature has risen by 0.6 ºC over the past century. Cox recently predicted a 5.5 ºC hike in global temperatures by 2100. Taking soil respiration feedback out of the equation cuts the rise to 4.5 ºC, he works out.

This more conservative estimate would only hold true if respiration continues to drop off at higher temperatures, Cox warns. That depends on what is causing it in the soil, where plant roots, bacteria and fungi all play a part.

One possibility is that when the heat is on, microbes burn up their supply of sugars quickly; then running out of fuel reigns in their respiration. In this case, the respiration slowdown would not sustain its initial pace. Longer studies are needed: "We might not get the answer until after climate change has happened," says Cox.