Climate change: Tropical flip-flop connections

Abstract

have done just that. They find that over the past 210,000 years northeastern Brazil underwent episodic changes from its usual semi-arid state to a wetter climate, implying a persistently southward-shifted ITCZ. Unlike the present-day disturbances, which last a few years at most,these past wet episodes typically persisted for several centuries. The authors deduced the presence and duration of the wet periods from the growth patterns and ages of mineral deposits, called speleothems and travertines, in the northern part of Bahia state in Brazil. Speleothems are deposited in caves, and travertines along spring-fed rivers and streams, when calcium carbonate precipitates out of supersaturated ground and spring waters; deposition is a good indicator of abundant rainfall. Northeastern Brazil is usually too arid to support the formation of either type of deposit (as is the case for present-day conditions), but repeated episodes of persistently wetter conditions evidently allowed them to form in the past. The remains of vegetation embedded in the travertines show that semi-deciduous forest abounded during the wet phases, linking the Amazon rainforest in the northwest with the Atlantic rainforest along the eastern coast of Brazil. An intriguing idea put forward by Wang et al. is that the unusual biodiversity of these rainforests can be attributed partly to the floristic exchange made possible by the recurring shifts to a wet climate. The more remarkable aspect of this study, however, is identification of the apparent synchrony of wet periods in northeastern Brazil with climate changes near and far. The timing of the wet periods can be accurately determined using uranium–thorium dating, a system that relies on decay products of radioactive uranium-238.The ages of speleothems analysed by Wang et al. correlate remarkably well with the timing of climate changes in different parts of the Northern Hemisphere — specifically, with weakening of the East Asian summer monsoon; with cold periods over Greenland; and with episodes in the North Atlantic, known as Heinrich events, that are characterized by massive release of icebergs into the open ocean from continental glaciers. Closer to home,Wang and colleagues’results elegantly confirm earlier indications of a southwarddisplaced ITCZ deduced from mineralogical changes in sediments of the Cariaco Basin, off Venezuela. What do these records tell us about the climate system? Cold episodes over the news and views

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