Lack of rainfall that caused severe drought in Argentina and Uruguay last year was not made more likely by climate change, scientists said Thursday. But global warming was a factor in extreme heat experienced in both countries that made the drought worse, they said.
The researchers, part of a loose-knit group called World Weather Attribution that studies recent extreme weather for signs of the influence of climate change, said that the rainfall shortage was a result of natural climate variability.
Specifically, they said, the presence of La Niña, a climate pattern linked to below-normal sea-surface temperatures in the Pacific that influences weather around the world, most likely affected precipitation.
La Niña usually occurs once every three to five years, often alternating with El Niño, which is linked to above-normal sea temperatures. But La Niña conditions have persisted for most of the past three years, and central South America has been drier than normal for most of that time.
In the last three months of 2022, however, the drought became especially severe. Rainfall totals in central Argentina were the lowest in more than half a century, and yields of wheat, soybeans and other crops fell dramatically in prime growing regions there and in Uruguay. Water emergencies were declared in some areas.
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Like previous studies by the group, this one used observational data and computer models to compare what occurred late last year with what would have likely occurred in a world that had not warmed by 1.2 degrees Celsius, or 2.2 degrees Fahrenheit, as a result of human-caused emissions of carbon dioxide and other heat-trapping gases. The work has not yet been peer reviewed, but the techniques the researchers used have been peer reviewed in previous studies.
The analysis looked at rainfall totals for October to December over most of Argentina, all of Uruguay and a small part of southern Brazil. They found that without climate change there was a 5 percent chance that such a period of sharply lower rainfall would occur in any given year.
But climate change did not increase the chances, they found. If anything, climate models show that region of central South America becoming slightly wetter as warming continues, though the researchers said the effect was insignificant and, as appeared to be the case with this drought, could be overwhelmed by natural variability.
The area also experienced record-setting heat waves last November and December. Extreme heat can make droughts worse by increasing the loss of moisture from soil and plants, said Juan Rivera, a researcher at the Argentine Institute for Snow Research, Glaciology and Environmental Sciences, who worked on the study.
In an earlier analysis, World Weather Attribution found a link between climate change and the December heat wave, saying that it was about 60 times more likely to occur than it would have been in a world that had not warmed.
Paola A. Arias, a researcher at the University of Antioquia in Colombia who contributed to the study, said that other factors, including deforestation, could also have worsened the dry conditions.
Large-scale clearing of forests reduces atmospheric moisture, Dr. Arias said, and Argentina and Uruguay receive much of their moisture from the southern Amazon, where extensive deforestation has occurred.
World Weather Attribution has conducted several dozen studies of droughts, heat waves, floods and other extreme weather events since 2014, and most of them show some influence of global warming.
Friederike Otto, a climate scientist at the University of Oxford who co-founded the group, said that the new research shows “that not every bad thing that is happening now is happening because of climate change.”
“It’s important to show what the realistic impacts of climate change are,” she said.
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