摘要：The Atlantic Meridional Overturning Circulation (AMOC), a tipping component of the climate system, is projected to slowdown during the 21st century in response to increased atmospheric CO2 concentration. The rate and start of the weakening are associated with relatively large uncertainties. Observed sea surface temperature-based reconstructions indicate that AMOC has been weakening since the mid-20th century, but its forcing factors are not fully understood. Here we provide dynamical observational evidence that the increasing atmospheric CO2 concentration affects the North Atlantic heat fluxes and precipitation rate, and weakens AMOC, consistent with numerical simulations. The inferred weakening, starting in the late 19th century, earlier than previously suggested, is estimated at 3.7 ± 1.0 Sv over the 1854–2016 period, which is larger than it is shown in numerical simulations (1.4 ± 1.4 Sv).
其他摘要：Abstract Climate change will drive major perturbations of the West African summer monsoon. A zonal contrast in precipitation will develop at the end of the century, with an increase in precipitation over the central Sahel and a decrease in precipitation over the western Sahel. Such a zonal contrast results from the antagonist effects of the fast (due to enhanced radiative warming over land, and over the North Hemisphere, relative to the South Hemisphere) and slow (associated with long-term changes in oceanic circulation) responses of precipitation to increasing greenhouse gases. While such changes have already been assessed, less attention has been given to their temporality, an issue of major importance to promote efficient mitigation and adaptation measures. Here, we analyse the future evolution of precipitation changes decomposed into a fast and a slow response, showing that the fast response dominates the slow one. From this evidence, we highlight that mitigation strategies may be successful at reducing the effect of climate change on Sahel precipitation within a few decades, by muting the fast response. This decomposition also allows for a better understanding of the uncertainty of climate model predictions in Africa.
