Tree Rings Show Unprecedented Rise in Extreme Weather in South America

Araucaria araucana trees in northern Patagonia, Argentina, some of which were used in the study. Some trees can live 1,000 years. (Ricardo Villalba, Argentine Institute of Snow, Glacier and Environmental Sciences, at the National Research Council for Science and Technology)

Scientists have filled a gaping hole in the world’s climate records by reconstructing 600 years of soil-moisture swings across southern and central South America. Along with documenting the mechanisms behind natural changes, the new South American Drought Atlas reveals that unprecedented widespread, intense droughts and unusually wet periods have been on the rise since the mid-20th century. It suggests that the increased volatility could be due in part to global warming, along with earlier pollution of the atmosphere by ozone-depleting chemicals. The atlas was published this week in the journal Proceedings of the National Academy of Sciences.

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Recent droughts have battered agriculture in wide areas of the continent, trends the study calls “alarming.” Lead author Mariano Morales of the Argentine Institute of Snow, Glacier and Environmental Sciences at the National Research Council for Science and Technology, said, “Increasingly extreme hydroclimate events are consistent with the effects of human activities, but the atlas alone does not provide evidence of how much of the observed changes are due to natural climate variability versus human-induced warming.” The new long-term record “highlights the acute vulnerability of South America to extreme climate events,” he said.

Coauthor Edward Cook, head of the Tree Ring Lab at Columbia University’s Lamont-Doherty Earth Observatory, said, “We don’t want to jump off the cliff and say this is all climate change. There is a lot of natural variability that could mimic human-induced climate change.” However, he said, armed with the new 600-year record, scientists are better equipped to sort things out.

The South American Drought Atlas is the latest in a series of drought atlases assembled by Cook and colleagues, covering many centuries of year-by-year climate conditions in North America; Asia; Europe and the Mediterranean; and New Zealand and eastern Australia. Subsequent studies building on the atlases have yielded new insights into how droughts may have adversely affected past civilizations, and the increasingly apparent role of human-induced warming on modern climate. Most recently, followup analyses of North America have suggested that warming is driving what may be the worst-ever known drought in the U.S. West.

Coauthor Edward Cook, head of the Tree Ring Lab at Columbia University’s Lamont-Doherty Earth Observatory, said, “We don’t want to jump off the cliff and say this is all climate change. There is a lot of natural variability that could mimic human-induced climate change.” However, he said, armed with the new 600-year record, scientists are better equipped to sort things out.

A continuing decade-long drought has hit much of Chile and western Argentina. Here, Natalia Edith Codoceo Flores amid the ruins of the Chilean village of Gualliguaica, which was flooded by a new reservoir in the 1990s; the water has now almost entirely receded. (Photo by Francesco Fiondella/International Institute for Climate and Society, taken April 29, 2013.)

The South American Drought Atlas is the latest in a series of drought atlases assembled by Cook and colleagues, covering many centuries of year-by-year climate conditions in North America; Asia; Europe and the Mediterranean; and New Zealand and eastern Australia. Subsequent studies building on the atlases have yielded new insights into how droughts may have adversely affected past civilizations, and the increasingly apparent role of human-induced warming on modern climate. Most recently, followup analyses of North America have suggested that warming is driving what may be the worst-ever known drought in the U.S. West.

At the same time, southeastern parts of the continent are seeing heavier than normal rains. Walter Baethgen, who leads Latin American agricultural research for Columbia University’s International Research Institute for Climate and Society, says his own studies show that the La Plata basin of Uruguay has seen more frequent extremely wet summers since 1970, with corresponding increases in crop and livestock production. But the frequency of very dry summers has remained the same, which translates to bigger losses of expected yields when they do come along, he said.

“Everything is consistent with the idea that you’ll be intensifying both wet and dry events with global warming,” said Jason Smerdon, a climate scientist at Lamont-Doherty and a coauthor of the study.

Using newly developed tree-ring records from Peru, Brazil, Bolivia and Colombia, the group is now working to expand the atlas to cover the entire continent, and extend the climate reconstruction back 1,000 years or more, said Morales.

The authors wish to dedicate the study to the memory of the late María del Rosario Prieto, their coauthor, and active promoter of environmental history studies in South America.

— Kevin Krajick, Earth Institute