Warming in Southeast Brazil due primarily to rising levels of greenhouse gases, study shows
February 12, 2020
By Elton Alisson | Agência FAPESP – The average temperature in Southeast Brazil has risen in recent decades, contributing to increases in the frequency and intensity of extreme weather events such as flooding, drought and heat waves.
For many years, the UN’s Intergovernmental Panel on Climate Change (IPCC) has pointed to a correlation between rising global temperatures over the past 100 years and rising levels of greenhouse gases in the atmosphere as a result of human activities. On the regional scale, specifically with regard to Brazil’s Southeast Region, there is still considerable uncertainty about the causes of this warming. According to experts, factors such as urbanization and shifts in agricultural land use, for example, may also have a significant impact on local temperatures.
Now, researchers affiliated with the University of São Paulo’s Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG-USP) in Brazil and the University of Edinburgh in Scotland have reported decisive evidence that the greenhouse effect was the main cause of a rise of 1.1 °C in the region’s average temperature between 1955 and 2004.
The results are published in the journal Geophysical Research Letters.
The researchers analyzed surface weather observations and 34 simulations of temperatures in Southeast Brazil over a period of 50 years based on models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), used by the IPCC.
They subjected the data to a statistical method proposed in 2017 by French and Canadian scientists for climate change detection and attribution.
“Applied to Southeast Brazil, this detection and attribution method uses data for climate responses to temperature variations due either to natural causes or aerosols or to the greenhouse effect separately,” Humberto Ribeiro da Rocha, a professor at IAG-USP and principal investigator for the project, told Agência FAPESP.
Climate change detection and attribution is the process of assessing whether observed changes in temperature and other factors, including extreme events, are likely to be due to natural variability alone.
Natural causes include solar radiation and volcanic activity. “These natural phenomena control regional climates,” Rocha said. “In addition, the entire climate system is reorganizing because of global warming. The ocean is a clear example: it can warm up considerably during episodes in some parts of the planet, affecting regional climates differently across as huge a continent as South America.”
Attribution also takes into account sampling uncertainty, variability in models and observational error. The contributions of multiple potential causes are evaluated to appropriate levels of statistical confidence.
In this study, the calculations were performed by meteorologist Rafael Cesário de Abreu during his ongoing PhD research at IAG-USP, supervised by Rocha. The results showed that greenhouse gas levels contributed significantly to the 1.1 °C temperature rise detected in the region during the period of 1955-2004.
“This finding corroborates the hypothesis that temperature changes are underway and that they’re strongly controlling regional climate in the Southeast, in line with the global trend,” Rocha said.
Lack of regional studies
Other long-term warming detection studies have been conducted in Brazil, he added, but none produced a regional-scale attribution result such as this one for the Southeast with decisive findings that account for uncertainties and distinguish between different causes. Furthermore, few regional detection and attribution studies have been performed worldwide to analyze the anthropogenic (human-induced) causes of climate change, especially the rise in levels of greenhouse gases.
Previous studies of subnational regions in China, Canada and England used similar statistical methods to those used in the study of Southeast Brazil.
This region is particularly vulnerable to the adverse effects of climate change, as it is home to more than 40% of the Brazilian population, accounts for over 50% of the nation’s gross domestic product (GDP), and contains a wide array of economic activities, the authors of the study note.
“These studies all show that the IPCC’s finding that the average global temperature rose 0.85 °C between 1880 and 2012 doesn’t apply regionally and above all locally, in studies of cities and so on,” Rocha said.
In the city of São Paulo, for example, the average temperature rose approximately 3 °C between 1940 and 2010; however, it cannot be said categorically whether this warming was due mainly to rising levels of greenhouse gases.
“In rural areas, there are other factors that influence warming,” he said. “Deforestation and destruction of savannas can also explain warming but are confined to local-scale effects, which in turn are not really captured by the IPCC’s CMIP5 models.”
The Geophysical Research Letters article “Attribution of detected temperature trends in Southeast Brazil” (doi: 10.1029/2019GL083003) by R. C. de Abreu, S. F. B. Tett, A. Schurer and H. R. Rocha can be retrieved from: agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL083003.
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