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Earth sciences

Ancient Waters and their stories

Remains of rainwater in caves help describe the climate for the last thousands of years

eduardo cesarStalagmite…eduardo cesar

Heavy rains of the kind that flooded dozens of cities in the State of Santa Catarina early this year could be connected with weather phenomena occurring in distant regions, with heavy rains in the Amazon Region, and severe cold in the Northern Hemisphere. In a relatively recent period of the Earth’s history, from 100 thousand years ago to our times, abrupt temperature oscillations in the Northern Hemisphere, that repeat themselves at an average of every 3 thousand years, corresponded to rainfall variations in tropical regions at an even greater extent than the El Niño or La Niña, caused every five to seven years by the warming or the cooling of the Pacific Ocean.

Geologists and climate experts from Brazil, the United States and Germany came to these conclusions analyzing rocks that had previously attracted people because of their beauty – stalactites and stalagmites, rock columns that grow from the ceiling of the cave downwards or from the floor of the cave upwards and that have gained scientific value in the last few years because they preserve remains of rainwater that fell thousands of years ago. The results of the analysis of the oxygen from the calcium carbonate of stalagmites in Brazilian caves are placing the tropics into the history of the planet’s climate in the last 500 thousand years; the story is told by means of samples of ice removed from polar regions and from sediment found at the bottom of the sea.  Rainfall records obtained by meteorological instruments that also indicate climate trends, rarely go back more than 150 years.

“We are filling in the missing pieces of the jigsaw puzzle of the global climate in the past”, says geologist Francisco Cruz, a researcher at the Geosciences Institute/ IG of the University of São Paulo/ USP. He is the first author of a research study on the stalagmites found in caves in the State of Rio Grande do Norte; the study was published in Nature Geosciences journal last February.  The research study brought together researchers from USP, from the University of Minas Gerais, from Albany, Massachusetts and Minnesota, in the United States, and from Germany’s Institute for Marine and Polar Research. Together, the research studies on stalagmites in caves in Brazil and China emphasize the climate contrasts north and south of the Equator, showing that, as a consequence of this three-thousand year cycle, Brazil had gone through periods of intense humidity, while in Venezuela, a neighboring country, and in another distant country, China, the climate was drier.

Current studies on caves in Brazil’s Northeast and past studies on caves in the states of São Paulo and Santa Catarina, even though they portray climate variations at specific sites or regions, have helped piece together the ancient history of climate in Brazil and specify the moments when changes were most intense. One of these moments is the onset of the semi-arid climate in the Northeast, which began approximately 4 thousand years, because of the periodic displacement of the Earth’s rotating axis.  The result of this movement was that the Southern Hemisphere began to get more solar radiation than the Northern Hemisphere.  “Six thousand years ago, the climate in the Northeast was very different”, says Cruz, who was born in Natal, the capital city of the State of Rio Grande do Norte.

Records of the chemical composition of rocks found in the caves in the Northeast, which refer to the last 26 thousand years, suggest that the climate in that region began to take on its current characteristics four thousand years ago. He says that the backlands in the Northeast were never as humid as the north eastern coast, but the rainfall in the backcountry was better distributed throughout the year.  The rainy season probably lasted from October to April in the backlands, rather than being concentrated only in the months from March to May, which is the case nowadays. This change might have contributed to the substitution of the relatively thick forests with the “caatinga”, the shrub lands that make up most of the region’s backlands nowadays.

eduardo cesar… and inside view of the Abyssal cave, in the back country of the State of Rio Grande do Norte: 26 thousand years of climate variations registered in the rockseduardo cesar

Most of the stalagmites stopped growing when the climate became drier. “The water evaporated before it reached the caves”, says Cruz. The rainfall dropped to such a low point that the team headed by Cruz, after having explored dozens of caves, found only one stalagmite with calcium carbonate layers that had formed in the last four thousand years.  The only witness of this climate change was lying 15 meters underground, isolated in one of the galleries of the Furna Nova cave, which lies 300 kilometers away from Natal.  The cave was discovered by Jocy Cruz, a technician from the National Center for the Study, Protection and Stewardship of Caves/Cecav of the Instituto Chico Mendes institute.  Jocy Cruz was part of the teams from the universities and defined, without being challenged, what the researchers could and could not remove for lab analyses.

Examining another kind of material – grains of pollen from plants preserved in swamps and on the bottom of lakes – botanist Paulo Eduardo de Oliveira, a researcher from the University of Guarulhos, in the State of São Paulo, had already reached similar conclusions: until four thousand years ago, the caatinga mixed with other kinds of vegetation, forming sparser woods, with trees, shrubs and cactuses, which are not found there nowadays. The story is different in the Amazon Region. “The eastern part of the Amazon Region has signs that there were times when the climate there was drier, but not dry to the point of eliminating the rain forest;  in the western part, there are  no vestiges of climate change in the last forty thousand years”, says Oliveira.

Ancient pollens and sediments also help rebuild the story of human occupation, indicating that the ancient inhabitants of the Amazon Region, for example, cultivated corn and manioc meal, according to a study conducted by Oliveira together with researchers from Florida, USA, and Rio de Janeiro.  The study was published in the Philosophical Transaction of The Royal Society B in 2007. Drawings on the rocks, that have not been fully studied yet, are evidence of human occupation in the Northeast before the onset of the semi-arid climate in the region of Lajedo de Soledade, 30 kilometers away from Rainha cave.  This cave, located in the municipal region of Felipe Guerra, has been thoroughly studied. It was discovered a few years ago by speleologist Ariosvaldo Araújo da Silva, one of the founders of a speleological group in the city of Natal.

Four thousand years ago, as the Northeast began to dry up, it began to rain more in the South and Southeast regions, for the same reason: increased solar radiation in the Southern Hemisphere because of the shift in the oscillation of the Earth´s rotation axis about 23 thousand years ago.  Depending on the inclination of the Earth’s rotation axis, there may be more shade in the Northern Hemisphere and more solar radiation in the Southern Hemisphere, and vice versa.

Four years ago, after examining the stalagmites in a cave in São Paulo and another cave in Santa Catarina (Pesquisa Fapesp nr. 111, May 2005), Cruz observed that stronger insolation had made summers on the Southern coast rainier – and this explains part of the impact of the heavy rainfall that left 80 thousand people homeless in 63 municipal regions in the State of Santa Catarina.  When he presented the findings of his research work, published in Nature journal in May 2005, Cruz commented that the insolation and heavier rains in the South were a natural tendency expected to continue for the next one thousand years, “without taking human interventions on climate into consideration”.

“Nowadays, the entire Southern Hemisphere is subject to greater exposure to the sun’s rays”, says Cruz. Because the continent is warmer, it attracts the trade winds, the humid winds that come from the Atlantic Ocean.  Coming from the east, they provoke rain that propagates from the Amazon Region to the South and Southeast Regions of Brazil.  The heat released because of the rain is the source of a movement of air that goes up to high altitudes, moves to the east, in the opposite direction from the trade winds, and dives in the vicinity of the Northeastern seacoast. “This mass of colder, drier air acts as a barrier to the humidity that would feed the rains in the backcountry of the Northeast”, says Cruz.

FRANCISCO CRUZ/USPLajedo de Soledade: signs of human occupation thousands of years agoFRANCISCO CRUZ/USP

“The data on the stalagmites might allow us to evaluate the capacity of the computer models to reproduce climate variations in the past in South America”,  says Mathias Vuille, a climate scientist from the US’s Albany University, who participated in this study.  He stated that the stalagmites indicate the actual variations of rain in the tropics, but do not explain why it rained more or rained less, whereas computer simulations, obtained by means of mathematical models, provide information on atmospheric mechanisms that caused climate change, but cannot be considered accurate until they are compared with real records.

José Marengo, a climate scientist at the National Space Research Institute, has resorted to geological or botanical registers to obtain a first view of climate conditions in the past. According to him, this material can also lead to risky conclusions: “We need more data to show more accurately what the climate in the past was like”, he says. Climate scientists agree that, in spite of the limitations, studies on pollens and rocks in caves can help differentiate markedly natural effects, at a time when human civilization was incipient, from the ones that may have suffered from the influence of human intervention. In the last 100 thousand years, because of essentially natural causes, the temperature in the polar regions probably remained some degrees above or below previous thresholds for many centuries, while in the tropics it probably rained two or three times more.

At the beginning of this year, the Argentines witnessed the worst drought in the last 50 years and the Australians witnessed their cities being consumed by fires, while temperatures reached record highs of 46,6º Celsius. In the State of Santa Catarina, heavy rains caused flooding and turmoil in dozens of cities; people in Great Britain postponed all their plans because of unusually heavy snow storms. Climate scientists have not stated that the unusual weather conditions at the beginning of this year already reflect permanent climate change, but they have warned that these situations could occur more frequently in the future. In a report prepared in 2007, climate scientists stated that more widespread fires could become more frequent in Australia.

Marengo coordinated the preparation of climate scenarios derived from regional models that projected episodes of heavier rains or sharper temperature variations in Brazil in the second half of the 21st century (Pesquisa FAPESP no. 130, December 2006), or perhaps even sooner. In the week before the Carnival festivities, he watched the river that flows through the city of Guaratinguetá, between Rio de Janeiro and São Paulo, as it overflowed and flooded roads and homes. “I had never seen the river rise so much”, he said, in astonishment. According to him, in Guaratinguetá and in other Brazilian cities, “the climate makes the existing lack of urban planning worse”. Argentina might also experience heavier rain and temperature oscillations, such as the ones registered this year, according to a study coordinated by Marengo. This study is scheduled for publication shortly in the International Journal of Climatology.

Cruz is running against time to discover more data about the climate’s secrets. The top of his desk at the Geosciences Institute is covered with stalagmites – the preferred ones, because of their clear layers and relative regularity – waiting to be analyzed. These stalagmites came from caves in the States of São Paulo, Santa Catarina, Minas Gerais, Goiás, Mato Grosso, Bahia and Tocantins. One of the most valuable pieces is a stalagmite that is nearly 2.5 meters long;  it was found, in 13 pieces, in the Caverna do Diabo, the Devil’s cave, located in the south of the State of São Paulo. This stalagmite registers climate variations that have occurred in the last 600 thousand years. According to Cruz, only Brazil and China have such long and continuous records.

The project
Paleoclima do Quaternário tardio brasileiro a partir das razões isotópicas de oxigênio e carbono em espeleotemas (nº 06/06761-0); Modality: Programa Jovens Pesquisadores Program; Coordinator: Francisco William da Cruz Junior – IG/USP; Investment: R$ 104.113,80