Study of shells shows how oceans react to global warming

Research performed at the University of São Paulo may contribute to the refinement of climate models used to project future scenarios.

By José Tadeu Arantes  |  Agência FAPESP – In the context of ongoing global climate change, the study of the past is extraordinarily important to the design of future scenarios. Research on changes that took place during the last deglaciation, between 19,000 and 12,000 years ago, when the global average temperature rose by approximately 3.5º C, is fundamental for gauging the accuracy of the numerical models used to predict what will occur on Earth in the decades ahead, for example.

One such research project, conducted by Rodrigo da Costa Portilho-Ramos from the University of São Paulo’s Geoscience Institute (IG-USP), investigated the changes produced by the global warming of the oceanic upper water column, a domain of the climate system that is particularly hard to study. The results are described in the article “Coupling of equatorial Atlantic surface stratification to glacial shifts in the tropical rainbelt”, published in Scientific Reports, an online journal owned by Springer Nature.

The study was supported by FAPESP through a Young Investigator grant to the project “Response of the Western Atlantic Ocean to changes in the Atlantic meridional overturning circulation: from millennial to seasonal variability”, which was led by Cristiano Chiessi, a researcher at the University of São Paulo’s School of Arts, Sciences & Humanities (EACH-USP). Chiessi is supervising Portilho-Ramos’s postdoctoral research and is a co-author of the article in Scientific Reports.

“Although they may seem homogeneous, the oceans are highly stratified, with different layers in the water column,” Chiessi told Agência FAPESP. “The top layer at the surface is enormously important to the planet’s climate because the photosynthesis produced by phytoplankton occurs there. Whether this surface layer is thicker or thinner, warmer or cooler, or more or less productive directly influences the amount of carbon that is released into or absorbed from the atmosphere, and hence the climate system as a whole. But highly complex climate models, such as those used by the IPCC [Intergovernmental Panel on Climate Change], can’t reproduce water column stratification very well so they tend not to take this influence into account. Our research shows, for the first time, how the upper water column in the oceans varied in the context of the abrupt climate change associated with deglaciation.”

To characterize this variation, Portilho-Ramos collected samples of sediment from the ocean floor – more precisely, fossilized shells of planktonic foraminifera. “Foraminifera are single-celled protozoans found only in marine environments and are highly sensitive to variations in temperature, luminosity, salinity, nutrient supply, water column stratification, etc.,” he said. “Planktic foraminifera live at or near the surface at depths of zero to 800 m and have calcium carbonate shells. After they die, their shells sink to the ocean floor and become fossils that record the natural history of the planet. They’re a key tool for paleoceanographic research.”

The analysis of the shells enabled the researchers to determine the position of the Atlantic intertropical convergence zone (ITCZ) both now and at certain periods in the past.

The ITCZ is a narrow belt of low atmospheric pressure and maximum precipitation that circles the planet in the equatorial region and produces a specific pattern of stratification at the sea surface. In the Atlantic, this belt of maximum precipitation and hence of low salinity is currently positioned slightly north of the equator. It migrates from north to south on a seasonal basis. These migrations, through which the ITCZ tracks the maximum solar energy, depend on the cycle of the seasons and the exchange of thermal energy between the two hemispheres via oceanic and atmospheric circulations. 

This complex system of heat transfer drives the Atlantic meridional overturning circulation (AMOC), which transfers 0.4 petawatts (0.4 x 10¹⁵ W) of energy from the southern oceans to the oceans north of the equator, combined with the atmospheric circulation returning 0.2 petawatts to the south. The ITCZ’s position north of the equator is derived from the asymmetry of this energy exchange.

“Climate models suggest that the cooling of the AMOC due to global warming shifts the ITCZ southward. It even appears to move south of the equator itself. This has been confirmed by paleoclimatic records on land, such as cave stalagmites, and by river-borne sediments in oceans and lakes,” Portilho-Ramos said. “All these records point to a major increase in rainfall in the southern portion of tropical South America during three sudden AMOC cooling events in the past 30,000 years: the first about 25,000 years ago, the second between 18,000 and 15,000 years ago, and the third between 12,000 and 11,000 years ago. Our study is the first to show that these shifts in the ITCZ also caused pronounced changes in the stratification of the ocean.”

Ocean salinity fell drastically during these three events in the high latitudes of the northern hemisphere because of the presence of massive flows of icebergs and fresh water resulting from the melting of the Arctic ice cap into the North Atlantic. This variation in the upper water column’s salinity and density cooled the AMOC, reducing the northward transfer of energy and the southward shift of the ITCZ. This, in turn, increased the amount of rainfall south of the equator. 

“The extra fresh water in the upper water column attenuates wind-driven turbulence at the ocean surface, allowing colder nutrient-rich water to penetrate the photic zone. The layers of the ocean reached by sufficient sunlight foster plant growth, boosting biological productivity and altering the planktic foraminifera community,” Portilho-Ramos said. “Our analysis of foraminifera species enabled us to establish the current position of the ITCZ and the amplitude of the shifts that occurred in the past.”

In the second period of AMOC cooling, between 18,000 and 15,000 years ago, the ITCZ moved in the Atlantic 1 degree south, or about 5 degrees south of its average present position. “Our data corroborated climate model simulations, confirming the shift due to the injection of fresh water into the North Atlantic from melting ice,” Portilho-Ramos said.

“This validates the models,” Chiessi added. “The current climate change process is melting the polar ice caps and injecting fresh water into the North Atlantic. This resembles what happened in the past periods of AMOC cooling. We’re not yet able to predict all the consequences of the process owing to the large number of factors involved, but our study suggests a major slowdown of the AMOC and a southward shift of the ITCZ with a substantial increase in rainfall in the northeast of Brazil during the wet season. Because projections point to a decrease in annual average rainfall in the northeast, the extremes are set to intensify.”