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Nature. 2014 May 29;509(7502):600-3. doi: 10.1038/nature13376. Epub 2014 May 21.

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

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1] Montana State University, Institute on Ecosystems and the Department of Ecology, Bozeman, Montana 59717, USA [2] Laboratoire des Sciences du Climat et de l'Environnement (LSCE), CEA CNRS UVSQ, 91191 Gif Sur Yvette, France.
1] Swiss Federal Research Institute WSL, Dendroclimatology, Z├╝rcherstrasse 111, Birmensdorf 8903, Switzerland [2] Oeschger Centre for Climate Change Research, University of Bern, CH-3012 Bern, Switzerland.
Laboratoire des Sciences du Climat et de l'Environnement (LSCE), CEA CNRS UVSQ, 91191 Gif Sur Yvette, France.
Department of Earth and Environment, Boston University, 685 Commonwealth Avenue, Boston, Massachusetts 02215, USA.
Faculty of Earth and Life Sciences, VU University Amsterdam, 1085 De Boelelaan, 1081HV, Amsterdam, The Netherlands.
Global Carbon Project, CSIRO, Marine and Atmospheric Research, Canberra, Australian Capital Territory 2601, Australia.
ARC Centre of Excellence for Climate Systems Science & Climate Change Research Centre, University of New South Wales, Sydney, New South Wales 2052, Australia.
Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, Montana 59812, USA.
College of Engineering, Computing and Mathematics, University of Exeter, Exeter EX4 4QF, UK.


The land and ocean act as a sink for fossil-fuel emissions, thereby slowing the rise of atmospheric carbon dioxide concentrations. 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, considered to be driven primarily by terrestrial ecosystem processes dominated by tropical rainforests. 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 Ni├▒a 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.

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