Abstract
The Atlantic Meridional Overturning Circulation (AMOC) and the Amazon rainforest are potential tipping elements of the Earth system, i.e., they may respond with abrupt and potentially irreversible state transitions to a gradual change in forcing once a critical forcing threshold is crossed. With progressing global warming, it becomes more likely that the Amazon will reach such a critical threshold, due to projected reductions of precipitation in tropical South America, which would in turn trigger vegetation transitions from tropical forest to savanna. At the same time, global warming has likely already contributed to a weakening of the AMOC, which induces changes in tropical Atlantic sea-surface temperature (SST) patterns that in turn affect rainfall patterns in the Amazon. A large-scale decline or even dieback of the Amazon rainforest would imply the loss of the largest terrestrial carbon sink, and thereby have drastic consequences for the global climate. Here, we assess the direct impact of greenhouse gas-driven warming of the tropical Atlantic ocean on Amazon rainfall. In addition, we estimate the effect of an AMOC slowdown or collapse, e. g. induced by freshwater flux into the North Atlantic due to melting of the Greenland Ice Sheet, on Amazon rainfall. In order to provide a clear explanation of the underlying dynamics, we use a simple, but robust mathematical approach (based on the classical Stommel two-box model), ensuring consistency with a comprehensive general circulation model (HadGEM3). We find that these two processes, both caused by global warming, are likely to have competing impacts on the rainfall sum in the Amazon, and hence on the stability of the Amazon rainforest. A future AMOC decline may thus counteract direct global-warming-induced rainfall reductions. Tipping of the AMOC from the strong to the weak mode may therefore have a stabilizing effect on the Amazon rainforest.
Highlights
Understanding these tipping elements and their respective dynamics
We examine the effect of increased freshwater flux, which we assume to be dominated by Greenland melt water runoff, on the Atlantic Meridional Overturning Circulation (AMOC) strength
Concerning the long-term correlations between Atlantic sea-surface temperature (SST) and mean annual precipitation (MAP) in the southern Amazon, we uncover a northern tropical Atlantic ocean region for which SST anomalies are negatively correlated with MAP in the Amazon, and a southern tropical Atlantic ocean region with positive correlations between SST anomalies and Amazon MAP (Fig. 3)
Summary
Quantifying the associated critical thresholds has remained challenging This is due to the fact that potentially highly nonlinear interactions between tipping elements, as well as their effect on the overall stability of the climate system, are still poorly understood. We investigate how a weakening of the AMOC could, in the long-term, influence the Amazon rainforest via changes in tropical sea surface temperature (SST) patterns and associated rainfall changes in tropical South America. Simulations of comprehensive climate models suggest that a rising GMT has a decreasing effect on rainfall sums in the Amazon region [11] In this context, the question arises whether a weakening or even shutdown of the AMOC would dampen or further enhance these projected rainfall reductions, and how an AMOC collapse would influence the long-term stability of the Amazon rainforest [12]. The changes of SSTs due to an AMOC shutdown are estimated from a hosing experiment enforcing an AMOC shutdown using the Eddy-permitting HadGEM3 model [10,13,18]
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