Cristiano M. Chiessi
Cristiano M. Chiessi is a Professor at the University of São Paulo, Brazil. Together with his research group and collaborators, he investigates past changes in South Atlantic circulation and South American climate/vegetation mainly during the late Quaternary. He primarily employs isotopic (stable and radiogenic) and inorganic geochemical proxies to marine sediment cores (but also shallow-water corals) in order to answer a variety of questions from glacial-interglacial to multidecadal timescales. Projections of a weakening Atlantic Meridional Overturning Circulation (AMOC), prompted his group to focus on the effects that past AMOC slowdowns had on South Atlantic circulation and South American climate/vegetation. His overall aim is to provide society with more reliable future climate projections.
Tipping elements, rainfall and vegetation: How Atlantic Ocean dynamics may destabilize the Amazon rainforest
The Atlantic Meridional Overturning Circulation (AMOC), a key tipping element of the climate system, plays a critical role in shaping tropical South American hydroclimate by modulating the position of the tropical rainbelt. High-resolution marine sediment cores and climate model outputs reveal that past AMOC slowdowns—particularly during major slowdowns known as “Heinrich Stadials”—led to significant and spatially heterogeneous precipitation changes across tropical South America. Semiarid northeastern Brazil experienced rainfall increases that were synchronous and proportional to reductions in the Atlantic interhemispheric sea surface temperature gradient triggered by AMOC slowdowns. In contrast, the Amazon basin exhibited distinct seasonal precipitation anomalies. Positive rainfall anomalies over western, eastern, and southern Amazonia during Heinrich Stadials were not driven by a strengthened South American monsoon, as traditionally thought, but rather by enhanced moisture transport from the equatorial North Atlantic during austral winter. Crucially, negative precipitation anomalies over northern Amazonia during Heinrich Stadial 1 (the most recent major AMOC slowdown) led to a decline in tropical moist forests and an expansion of seasonal vegetation. Vegetation modeling under both glacial and pre-industrial conditions supports these findings, suggesting that similar ecological shifts could result from a future AMOC slowdown. Given ongoing deforestation and fire activity, such a slowdown in AMOC could act synergistically with anthropogenic disturbances, potentially pushing the Amazon rainforest toward a critical ecological tipping point.
