Journal Article

Contribution of semi-arid ecosystems to interannual variability of the global carbon cycle

Canadell, Josep G., Chevallier, Frederic, Sitch, Stephen, van der Werf, Guido R., Broquet, Gregoire, Myneni, Ranga B., Poulter, Benjamin, Bi, Jian, Running, Steven W., Frank, David, Liu, Yi Y., Ciais, Philippe, Andela, Niels

2014

Overview

The unusually large land carbon sink reported in 2011 can mostly be attributed to semi-arid vegetation growth in the Southern Hemisphere following increased rainfall and long-term greening trends. Australia driving carbon uptake Land and ocean take up around half of the annual anthropogenic carbon emissions, and a thorough understanding of this process is important for predicting future greenhouse gas concentrations and thus climate change. This study investigates the largest uptake of land carbon since atmospheric CO 2 measurements began in 1958. Three independent methods of global carbon budget determination point to an exceptionally large land carbon sink in response to extraordinary La Niña rainfall in semi-arid regions in the Southern Hemisphere, with almost 60% of carbon uptake attributed to the Australian ecosystem and an increase in the sensitivity of continental net carbon uptake to precipitation. Tropical rainforests have been thought to dominate the terrestrial processes driving global carbon cycle interannual variability, but this work suggests that semi-arid biomes might become the dominant drivers in future. The land and ocean act as a sink for fossil-fuel emissions, thereby slowing the rise of atmospheric carbon dioxide concentrations 1 . Although the uptake of carbon by oceanic and terrestrial processes has kept pace with accelerating carbon dioxide emissions until now, atmospheric carbon dioxide concentrations exhibit a large variability on interannual timescales 2 , considered to be driven primarily by terrestrial ecosystem processes dominated by tropical rainforests 3 . We use a terrestrial biogeochemical model, atmospheric carbon dioxide inversion and global carbon budget accounting methods to investigate the evolution of the terrestrial carbon sink over the past 30 years, with a focus on the underlying mechanisms responsible for the exceptionally large land carbon sink reported in 2011 (ref. 2 ). Here we show that our three terrestrial carbon sink estimates are in good agreement and support the finding of a 2011 record land carbon sink. Surprisingly, we find that the global carbon sink anomaly was driven by growth of semi-arid vegetation in the Southern Hemisphere, with almost 60 per cent of carbon uptake attributed to Australian ecosystems, where prevalent La Niña conditions caused up to six consecutive seasons of increased precipitation. In addition, since 1981, a six per cent expansion of vegetation cover over Australia was associated with a fourfold increase in the sensitivity of continental net carbon uptake to precipitation. Our findings suggest that the higher turnover rates of carbon pools in semi-arid biomes are an increasingly important driver of global carbon cycle inter-annual variability and that tropical rainforests may become less relevant drivers in the future. More research is needed to identify to what extent the carbon stocks accumulated during wet years are vulnerable to rapid decomposition or loss through fire in subsequent years.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 704/158/47 / Annual variations / Atmosphere / Atmosphere - chemistry / Atmospheric carbon dioxide / Australia / Carbon / Carbon cycle / Carbon cycle (Biogeochemistry) / Carbon Dioxide - analysis / Carbon dioxide emissions / Carbon Sequestration / Carbon sinks / Climate change / Continental interfaces, environment / Desert Climate / Ecosystem / Ecosystems / El Nino-Southern Oscillation / Fires / Growth rate / Humanities and Social Sciences / La Nina / letter / Methods / Models, Theoretical / multidisciplinary / Ocean, Atmosphere / Rain / Rainforests / Science / Sciences of the Universe / Seasons / Terrestrial environments / Uncertainty / Vegetation / Vegetation cover