"Our results suggest that the decline of the world's grasslands may result in large part from positive feedback between warming-associated drought events and reductions in grasslands' potential production," said George Hendrey of City University of New York. "Anthropogenic warming of the climate leads to more frequent and severe drought events that weaken grasslands' carbon-sink function, possibly turning grasslands into carbon sources."

The amount of carbon stored by vegetation on Earth each year can vary by as much as 5 Pg, with grasslands tending to show the greatest range of values.

"Because grassland ecosystems are the most vulnerable to extreme climate events, we examined data collected by many other scientists to try and understand the relationship between rain events and drought on the potential productivity of grasslands," said Chuixiang Yi of City University of New York. "The questions we are interested in are: how does extreme weather alter the carbon-storage capacity of the terrestrial biosphere? Does it cause feedback to global warming by changing the amount of carbon that the biosphere can hold?"

The team examined data on carbon-dioxide flux, evapotranspiration, sensible heat, air temperature, net radiation and photosynthetic active radiation from five FLUXNET grassland sites in Canada, the US and Hungary, along with leaf-area index information derived from satellite data.

The team's novel technique – the "perfect-deficit" approach – assigns the best growth year at each site as "perfect" and compares other years to it. A growth curve of the ecosystem over the best year is called the perfect curve since it represents the greatest amount of plant growth that the ecosystem was capable of during the period of observation.

"We can identify the impacts of climate extremes on these grassland ecosystems by comparing ecosystem performance in each year to the perfect curve," said Yi. "The difference between the 'perfect curve' and the curve of any other year shows a deficit in plant growth and that deficit is highly correlated to weather, particularly to rainfall over the year."

The team found that large deficits in canopy photosynthetic capacity, evapotranspiration and leaf-area index occurred together, and were driven by the local-dryness index – a new measure of precipitation and net radiation – during the growing season.

According to Yi, the perfect-deficit approach is different to the widely used, traditional method of associating climate conditions with increased or decreased plant growth as anomalies relative to a multiple-year average of growth. It can also be applied to any continuous dataset of ecosystem–climate interactions. "Therefore, the same extreme events can be cross-verified by different independent datasets," he said.

Hendrey agrees: "We have shown this to be a useful analytical method that can assist other scientists in quantifying the impact of climate change and especially in evaluating the effect of drought on the flux of carbon dioxide into or out of grasslands."

Yi explained that since the atmosphere has warmed about one degree in his lifetime, it can now hold seven per cent more water than it did a few decades ago. "When warm, wet air meets cold air, the water vapour turns to rain and since there is more water vapour, we are getting these very heavy rain events and subsequent flooding," he said. "At the same time, since the air can hold more water, it may take more time for an air mass to become fully saturated with water, leading to droughts."

Now the researchers, who reported their study in Environmental Research Letters (ERL), would like to perform similar analyses for tropical and boreal forests, tundra and other ecosystems that store huge quantities of carbon.