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Plants absorb less carbon after hot years

AFP

Climate change

Flowers

Credit: iStockphoto

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PARIS: Plant and soil can take up to two years to recover from an exceptionally hot year, says a new study which has implications for the battle against global warming.

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

Micro-fields

In an unusual experiment, scientists in Nevada, USA, carved out plots of grassland in central Oklahoma, each measuring more than 2.4 m long, 1.2 m wide and 1.8 m 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 were exposed to a sudden rise in temperature during the second year of the experiment – a hike of 4 ºC.

"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. "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?'"

Heat hikes

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 photosynthesis, and from micro-organisms that break down carbon in the soil. The study is published by the British journal Nature.

The hike of 4 ºC is at the far end of estimates made last year by the U.N. 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 ºC 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 ºC higher than the region's long-term average, but were not accompanied by drought.

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