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Climate change

Tropical flip-flop connections

A long climatic record shows that episodic wet periods in northeastern Brazil are linked to distant climate anomalies. The ocean–atmosphere system can evidently undergo rapid and global reorganization.

Every few years, the semi-arid but densely populated region of northeastern Brazil experiences severe drought. In 1997–98, exacerbated by poor distribution of resources, such an event caused great hardship. And in the catastrophic drought of 1877–79, hundreds of thousands of people perished. The causes of these conditions are neither accidental nor local, but part of an orchestrated sequence of large-scale atmospheric and oceanic processes over the entire tropical Atlantic (Fig. 1). These processes lead to changes in the seasonal southward migration of the Intertropical Convergence Zone (ITCZ)1, the rainfall band that spans the Atlantic. When the southward migration of the ITCZ in February–May stops short of northeastern Brazil, the rainy season fails.

Figure 1: Determinants of present-day drought years in northeastern Brazil.
figure1

An anomalous north–south gradient in ocean surface temperatures near the Equator (not shown), with warmer conditions in the north, drives a low-level atmospheric circulation, shown by arrows, that strengthens the trade winds in the southern Atlantic and weakens them in the north. The result is to impede the southward migration of the Atlantic Intertropical Convergence Zone, which increases rainfall in the northern tropical Atlantic (blue) and reduces it in the southern tropical Atlantic and northeastern Brazil (red). The opposite set of conditions, persisting over several centuries, is thought to characterize the wet episodes identified by Wang et al.2.

For those seeking knowledge of past climate changes, however, northeastern Brazil presents an opportunity: what better place to focus on than one where flip-flopping climate is singularly representative of changes throughout the entire tropical Atlantic? Wang et al. (page 740 of this issue)2 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 southward-displaced ITCZ deduced from mineralogical changes in sediments of the Cariaco Basin, off Venezuela3.

What do these records tell us about the climate system? Cold episodes over the North Atlantic during the last glacial period are thought to have arisen from abrupt variations in the Atlantic's thermohaline circulation4, a density-driven circulation that carries warm and salty surface waters to the northern North Atlantic. As these waters release their heat to the atmosphere they become cold and dense, then sink, and return south as a deep current. A weakening or shutdown of this circulation — achieved by freshening the ocean surface waters from glacial meltwater or through other means — reduces the heat transported to the North Atlantic and cools the climate there. These changes in thermohaline circulation are in essence high-latitude processes with no obvious link to the tropics; but recent discoveries have documented the presence of abrupt events in the northern tropical Atlantic, Pacific5 and Indian Ocean6 regions. And Wang et al. now establish their existence in the southern tropics with a dating accuracy that puts the synchrony with events in the North Atlantic, and in other tropical locations, beyond reasonable doubt.

This development significantly modifies our view of abrupt climate change. Shifts in the strength and distribution of atmospheric convection in the tropics dramatically affect the global climate by altering the global atmospheric circulation — analogous to how the El Niño–Southern Oscillation system in the tropical Pacific affects global climate today7. The transport of water vapour will change as a result, possibly affecting the salinity and hence the density of the North Atlantic surface waters that determines the strength of the thermohaline circulation8. Taken to the logical extreme, a competing ‘tropical driver’ hypothesis for abrupt climate changes could easily explain the observed global synchronization — although, as yet, there is no convincing model for how such a driver might operate9.

It is nonetheless clear that a complete explanation of abrupt climate change must incorporate both the thermohaline circulation in the North Atlantic and climate processes that occur in the tropics. Results from simulations with climate models offer tantalizing clues as to how this might come about (see ref. 10 for an example). When the thermohaline circulation is forced to shut down, the models respond with rapid and widespread cooling of climate in the Northern Hemisphere and rearrangement of tropical rainfall. The Atlantic ITCZ seems to be particularly sensitive in this regard, and shifts in a manner consistent with the data of Wang and colleagues. Although it is still not clear how the global connections are set up, the tropical Atlantic may have a pivotal role in linking the high latitudes to the tropics11,12.

So, what lies ahead? We can hope for rapid progress in characterizing the timing and spatial extent of abrupt climate changes, particularly in the tropics and Southern Hemisphere where data remain sparse. In this regard, the future looks bright for studies of speleothems, given the outstanding dating accuracy and time resolution that they offer. But speleothems are, of course, confined to land: marine records lack their advantages but remain essential.

For modelling, perhaps the key lesson offered by Wang et al.2 is a deeper appreciation of global interconnections in climate change. This view resonates with basic notions of how climate adjusts to forcings to maintain energy balance, necessitating changes to the Equator-to-pole temperature gradients and global transports of energy and moisture. But the palaeoclimate records are helping to provide an increasingly detailed view of how this system operates in the real world. Our knowledge of the dynamical adjustments to climate change and their sensitivities across models is still developing, and there is a substantial gap between model simulations and the intricacies of climate changes inferred from palaeodata. Bridging that gap is crucial to reaching the stage where we can make confident predictions about the consequences of our own, self-imposed climate change.

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Chiang, J., Koutavas, A. Tropical flip-flop connections. Nature 432, 684–685 (2004). https://doi.org/10.1038/432684a

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