Marine sediments show how part of the northeast has become semi-arid

This phenomenon is related to the shrinkage of the tropical precipitation belt that would have occurred gradually over the past 5,000 years. The results of the study conducted at the University of the South Pacific help to predict the future of the climate in the region (Collection of marine sediment samples near the mouth of the Barnepa River; Photo: Cristiano Chaise)

May 11, 2021

Jose Tado Arantes | FAPESP Agency The rains associated with the so-called tropical convergence zone affect the food and water security of nearly one billion people around the planet. Approximately 11% of Brazil’s population, concentrated in Rio Grande do Norte, Ciara, Piauí and Maranhão, are part of this group. These states have a semi-arid climate over large areas and nearly half of the annual precipitation occurs in just two months of the year: March and April. These rains form part of the equatorial precipitation belt when it reaches its southernmost location, on northeastern Brazil. During the remainder of the year, the belt moves north, to be responsible for the peak rainfall in the coastal region of Venezuela, between July and August.

Predicting future precipitation behavior in semi-arid regions such as those mentioned above is essential in order for society to be able to anticipate potential changes in precipitation patterns, due to current climate change. Study by a professor at the University of São Paulo (USP) Christiano Chessi And the collaborators showed that rainfall in northeast Brazil has decreased systematically over the past 5,000 years, contrasting with an important model of paleontology. This review of what has happened in the past helps create a more realistic picture of what may happen in the future.

Article On this topic is available in the magazine Paleobiology and Paleoclimatology. Work included the support By FAPESP.

“According to the prevailing model, the tropical precipitation belt has moved southward over the past 5,000 years. Our research indicates, instead, that what has happened is that the belt has contracted in its transverse range of oscillations. That is, it began to oscillate within a narrower range.” As Chase says FAPESP Agency.

Valuable information about climate system responses to different conditions is recorded in geological sediments deposited on the ocean floor. In the study concerned, three independent indicators of precipitation were used, through sediments collected from the mouth of the Barnepa River, on the borders of Piauí and Maranhão states.

We analyzed the ratio between the concentrations of the chemical elements titanium and calcium – titanium from erosion of continental rocks and calcium from marine organisms’ shells. In addition, we surveyed the rate of accumulation of continental sediments on the ocean floor and the formation of hydrogen isotopes in the waxes of continental plants found in marine sediments. These three sets of data, along with an analysis of the Numerical Climate Model results, indicate that the tropical precipitation belt has shrunk over the past 5,000 years, rather than the proposed displacement to the south, “says Chaise.

The study also showed that the temperature distribution on the surfaces of the two hemispheres is essential in controlling the position of the tropical precipitation belt in a way that differs from what was suggested in the current model.

According to this model, the belt’s migration to the south would be attributed to the gradual increase in the radiation received from the Sun from the Southern Hemisphere during the summer. As in the northern hemisphere, the opposite happened, such a scenario could have imposed increasing difficulties in the belt’s advance to the north. However, two weaknesses of this model caught our attention. First, the fact that the belt’s position has to be determined by the distribution of surface temperatures in the two hemispheres, which do not necessarily respond in a linear fashion to the distribution of radiation received from the Sun. Second, the evidence supporting the model was located almost exclusively in the northern hemisphere, and lacked southern control over the supposed displacement, ”Chase explains.

According to the researcher, although the radiation received from the sun underwent the described changes, the responses of the two hemispheres to the phenomenon were different. This is due to the difference in the area covered by the continents and oceans in the two hemispheres, where the continents respond faster than the oceans to changes in solar radiation. “Therefore, it is necessary to review the model that has influenced paleontology over two decades,” says Chaise.

Until the end of this century, numerical climate models indicate a reduction in the latitudinal range of the oscillation of the tropical precipitation belt, which will lead to a further decrease in precipitation in the northern part of northeastern Brazil, with potentially severe social and environmental consequences. However, if the large circulation of Atlantic waters weakens significantly, exceeding a critical threshold, as another study by Chessy predicts, the southern Atlantic should warmer more than the North Atlantic, forcing the belt south. “This will have very negative consequences in different parts of the planet, but at the regional level, it will prevent a further decrease in precipitation in the northern part of the northeast,” comments the researcher (Read more on: agencia.fapesp.br/23015/).

Article Mid to late Holocene contraction of the intertropical convergence zone over northeastern South America It can be accessed at https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020PA003936.

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