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New insights into the variability of the tropical land carbon cycle from the El Niño of 2015/2016

A pivotal question in global change science is what role the biosphere plays in ameliorating the rate of climate change and what risks exist that there may be possible tipping points in the Earth System where the biosphere switches from being a brake to an accelerator of climate change [1]. Of the potential major global tipping points, the fate of the tropical forests is among the most important and iconic, with some studies suggesting that climate change could lead to extensive dieback of tropical forests (and the Amazon forest in particular), resulting in further carbon release, major loss of biodiversity, drying of regional climates and thereby further forest dieback and intensification of global warming [2]. However, these scenarios have been the subject of much debate, with questions about whether existing biosphere models sufficiently capture the consequences for key ecological processes, which are dependent on complex system attributes such as species' functional diversity and demography, and on factors such as forest fire that are poorly described [3].

One issue of particular concern is the interannual variability of the tropical land carbon cycle, in particular during El Nino events. During such events, a pool of warm surface water migrates across from the Western Pacific to the Central and Eastern Pacific, disrupting global atmospheric circulation patterns associated with the Walker circulation and causing warming of the tropics in particular. The El Nino Southern Oscillation (ENSO) is the largest mode of interannual variability both in the climate system [4] and in the global carbon cycle [5]. It has long been noted that El Nino events correlate with a surge in the rate of rise of atmospheric carbon dioxide concentrations (e.g. [6]). Global studies of the spatial pattern of carbon dioxide concentration tend …