The recovery lag could cause a rethink about the ability of grasslands and soil to act as a sponge, also known as a "sink," that removes carbon dioxide (CO2) from the air, its authors said.

In an unusual experiment, scientists in Nevada carved out plots of grassland in central Oklahoma, each measuring more than 2.4 metres (7.8 feet) long, 1.2 metres (3.9 feet) wide and 1.8 metres (5.85 feet) deep, with their plant communities and soil left intact.

These miniature ecosystems were then installed in four container-sized labs in which light, darkness, temperature and rainfall could be carefully replicated and levels of CO2 monitored.

Over the next four years, two of the four chambers were programmed to reproduce the weather conditions of the original site, previously determined by a seven-year monitoring of that location.

The other two chambers, though, were exposed to a sudden rise in temperature during the second year of the experiment -- a hike of four degrees Celsius (7.2 degrees Fahrenheit).

"We wanted to create an extreme year and look at how the ecosystem recovered from it," said Jay Arnone, a professor at the Desert Research Institute (DRI) in Reno, Nevada, told AFP.

"Basically, we dosed it with heat one year, then we removed the heat and asked, 'how long do the effects that occurred in that year persist, and which processes are affected?'"

Arnone's team found that during this anomalously warm year and the year that followed, the two plots sucked up two-thirds less carbon than the plots that had been exposed to normal temperatures.

Carbon capture comes from plant growth, through synthesis, and from micro-organisms that break down carbon in the soil.

The hike of 4 C (7.2 C) is at the far end of core estimates made last year by the UN expert group, the Intergovernmental Panel on Climate Change (IPCC), for the average worldwide warming this century.

But such an increase is most likely to be gradual or in smaller hikes, not the dramatic year-on-year rise increase replicated by Arnone's team.

Arnone said the 4 C increase was based on between eight and 11 exceptionally warm years in the weather records, spanning from 1873 to 1999, where the plots had been dug up. These warm years were between 1.0 and 3.8 C (1.8 and 6.9 F) higher than the region's long-term average, but were not accompanied by drought.

The findings are important because they point to the impacts of extensive heatwaves on grasslands and forests that together account for 20 percent of Earth's land surface, said Arnone.

They also throw in a new factor for calculating the value of natural sinks, which play a critical role in offsetting man-made greenhouse gases.

Hot years or prolonged heatwaves, according to the IPCC, are likelier to become more frequent under climate change.

In such a scenario, sinks will be less able to soak up CO2. In turn, the CO2 that is not absorbed will remain in the atmosphere rather than be sequestered in the ground, thus accelerating the greenhouse effect.

In scientists' parlance, this is a "positive feedback," or a vicious circle likely to amplify climate damage.

The study is published by Nature, the British-based science journal.

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