"There has been an increase in the uptake of carbon dioxide over time, and land ecosystems have together absorbed almost one third of all carbon dioxide emissions from human activity since the 1960s," said Anders Ahlström of Lund University, Sweden, and Stanford University, US. "What is perhaps even more surprising is that this trend is also dominated by the semi-arid lands."

The net carbon flux from vegetation depends on the amount of gross primary production, which absorbs carbon dioxide, and wildfires and respiration by the plants, both of which release carbon dioxide.

"For decades we thought that the major drivers of inter-annual variability of atmospheric carbon dioxide were related to changes in the carbon uptake and release in tropical regions, where the largest carbon and water vegetation-driven fluxes occur," Ahlström told environmentalresearchweb. "In this new analysis, we show the growing role of the world’s semi-arid regions in driving some of the increasing trend in the terrestrial carbon sink. More surprisingly, semi-arid regions dominate the inter-annual variability of atmospheric carbon dioxide given the timing of their flux dynamics compared to other large biomes in the world."

Ahlström and colleagues used LPJ-GUESS, a biogeochemical dynamic global vegetation model, as well as an ensemble of ecosystem and land-surface models from the TRENDY intercomparison project to assess the contributions of tropical forests, extratropical forest, grasslands and croplands, semi-arid ecosystems, tundra and arctic shrub lands, and sparsely vegetated lands.

"We analysed the climatic and vegetation controls over trends and variability of atmospheric carbon dioxide, the single most important greenhouse gas now being altered by human activities such as the burning of fossil fuels and land-use change," said Ahlström. "Understanding both the natural and human controls of atmospheric carbon dioxide is paramount for understanding the future of our climate."

Whilst the mean sink value was dominated by highly productive lands such as tropical forests, the trend and inter-annual variability in the size of the sink depended largely on semi-arid ecosystems, the researchers found. In turn, the carbon balance of these dryland ecosystems was linked to variations in precipitation and temperature.

Semi-arid ecosystems accounted for 39% of the inter-annual variability in net biome production, while tropical forest accounted for 19%, extratropical forest for 11%, and grasslands and croplands for 27%, the study showed. Warm and dry conditions in semi-arid ecosystems reduced gross primary production whilst cool, wet conditions boosted vegetation growth, according to the team’s model.

The findings are in line with the woody encroachment and increased vegetation greenness seen in semi-arid areas in recent decades. An earlier study attributed the bulk of 2011’s record land sink to an unusually wet period for semi-arid habitats in the southern hemisphere, particularly Australia.

"In order to preserve and try to enhance the terrestrial land sink, we not only need to look at conservation and management of tropical forests and temperate forests of the north, but southern hemisphere vast semi-arid regions are also critical," said Ahlström. "In fact, they dominate the short-term year-to-year variability, which raises questions about the future vulnerability of [the] land sink as these regions become more important in the future, partially also fuelled by increased climate variability."

It’s not clear how climate change will affect the terrestrial carbon sink. To get more of an idea, we need better understanding of the regions and processes governing the sink at the moment, the team believes.

"This study brings out clearly the importance of directing attention towards savannahs and other dry-climate ecosystems that have been largely neglected so far in climate policy discussions, and that moreover characterize the landscapes of some of the poorer countries of the Earth," said Benjamin Smith of Lund University.

Ahlström reckons that the next step is to understand what drives the increasing carbon dioxide uptake by semi-arid lands, which could reveal how "this important carbon dioxide sink may respond to a warmer world with associated changes in climate variability and extremes". The team reported the findings in Science.

Related links

• Science
• Anders Ahlström

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