The point-of-no-return for the Amazon? New insights from high-resolution atmosphere simulations

The Amazon basin contains the world’s largest rainforest. Dubbed the ‘lungs of the Earth’, this biome sequesters vast amounts of carbon dioxide and hosts countless species of flora and fauna. Human activities are endangering this irreplaceable ecosystem, with potential consequences not only for biodiversity and local inhabitants, but also the global carbon cycle and climate (Gedney & Valdes, 2000).
One influential early study with a global coupled climate-vegetation model predicted a drier Amazon, driving forest contraction and a self-amplifying die-back, resulting in the conversion of the whole region to savannah (Cox et al., 2000). More recent studies tend to predict less extreme reductions in rainfall but with substantial uncertainties (Huntingford et al., 2013).
Climate changes in Earth’s history offer complementary lessons on the response of the climate system. During the peak of the ice-age (the Last Glacial Maximum or LGM) a globally cooler climate characterised by lower levels of atmospheric CO2, implied serious stresses for the forests. However, paleo-climate evidence suggests that the Amazon rainforest survived, at least in the Eastern lowlands (Wang et al., 2017). This implies a lower limit on the rainfall during this time. However, climate simulations of the LGM do not agree on whether it was drier or wetter.
More widely, the simulation of convective rainfall -the process responsible for the majority of rainfall in the tropics, has important biases. Global climate models underestimate heavy downpours, have the wrong daily timing of rainfall over land, and have difficulty in reproducing the seasonal changes in monsoons (Dai et al 2006, Marsham et 2013). Over the Amazon, Anber et al (2015) found that daily to seasonal cycles in deep convection and fog contribute substantially to the mean state, yet these processes are represented poorly if at all, in conventional global models. This means that our understanding of the past and future Amazonian hydrological cycle may contain substantial errors.
This study will use the high-resolution Met Office atmosphere model to address this situation and investigate the response of the Amazon hydrological cycle to past and future conditions, providing new insight into the Amazon’s vulnerability to multiple environmental stresses.