“This result was surprising considering the low productivity of semi-arid biomes,” Ben Poulter of Montana State University, US, told environmentalresearchweb. “But we discovered that our findings were explained by a prolonged La Niña event that led to high rainfall, and that greening trends in dryland systems contributed to greater carbon uptake.”

Drylands cover roughly 45% of the Earth’s surface. Around 60% of the additional land carbon sink in 2011 was due to plant growth in semi-arid regions of Australia, the scientists discovered. La Niña conditions have brought six consecutive seasons of extra rainfall to such areas. Temperate South America and southern Africa were also significant contributors.

“Because dryland systems have low productivity and store low amounts of carbon in vegetation and soils compared to forests in tropical or boreal systems, they have been overlooked in terms of their role in the global carbon cycle,” said Poulter. “Our study is the first to point out that the atmospheric carbon dioxide growth rate is becoming more and more influenced by vegetation activity in dryland systems.”

Previously, tropical rainforests were thought to be the main cause of the variability in the land carbon sink from year to year. Now it seems that semi-arid ecosystems will play an increasing role. Since 1981, vegetation cover in Australia has expanded by 6%, perhaps due to altered rainfall patterns, increased atmospheric carbon dioxide affecting leaf pores and water use efficiency, or woody encroachment following land-use changes. At the same time, the sensitivity of the continent’s net carbon uptake to rainfall has increased by a factor of four.

“There has been an increase of scientific publications highlighting greening trends of vegetation in dryland ecosystems, where various metrics of vegetation activity have increased since the early 1980s,” said Poulter. “We build on these findings to demonstrate that the greening trends are altering the biogeochemistry of dryland systems during extreme climate years.”

Poulter and colleagues estimated the land carbon sink over the last 30 years using three different techniques – a terrestrial biogeochemical model, atmospheric carbon dioxide inversion and global carbon budget accounting. All three methods showed a record land carbon sink in 2011. The researchers also used satellite data to look at vegetation cover.

“We first observed an anomalously large 2011 land carbon sink during the annual global carbon budget assessment coordinated by the Global Carbon Project,” said Poulter. “This annual activity estimates the land carbon sink as the 'residual' of a carbon balance equation that includes the better known source terms, fossil fuels and net land use change, and sink terms, the atmosphere and ocean.”

Now the team would like to understand what’s causing the dryland greening. Current candidates for the explanation include land use and grazing, fire management, climate change, and increasing atmospheric carbon dioxide. “In addition, the carbon stored in dryland systems is vulnerable to wildfires and tends to be returned to the atmosphere rather quickly,” said Poulter. “A better understanding of the residence time of dryland carbon stocks is important to interpret global interannual carbon dioxide variability.”

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