Extreme weather likely to become more frequent and intense in São Paulo

Research shows that heavy rainfall concentrated within a few days followed by long, dry periods may become commonplace in the state in the decades ahead.

By Elton Alisson

Agência FAPESP – The recent climate swings experienced by metropolitan São Paulo, where a few days of torrential rain frequently alternate with long periods of dry, warm weather, are set to become a long-term trend or even worsen in the decades ahead.

These are the findings of a study conducted by researchers at the National Space Research Institute (INPE) and the Natural Disaster Surveillance & Early Warning Center (CEMADEN), in collaboration with colleagues at the Universities of São Paulo (USP), Campinas (UNICAMP), São Paulo State (UNESP) and Taubaté (UNITAU), the Aeronautical Technology Institute (ITA) and the Aeronautics & Space Institute (IAE), among other institutions and universities in Brazil and elsewhere, as part of the thematic project entitled “Assessment of impacts and vulnerability to climate change in Brazil and strategies for adaptation”, which is supported by FAPESP.

The study was published in the journal Climate Research and was part of the Atlas of Temperature & Precipitation Forecasts for São Paulo State, published by INPE in 2014, also as a project outcome.

“We’re observing in metropolitan São Paulo an increase in the frequency of intense rainfall leading to floods and landslides, interspersed with dry periods that can last months,” said José A. Marengo, a researcher affiliated with INPE and currently at CEMADEN.

“Climate models predict that these extreme weather events will be increasingly common in São Paulo and other cities around the world. They may even intensify if urbanization and greenhouse gas emissions continue at the present rate,” said Marengo, who coordinated the study.

The researchers analyzed climate variability in the metropolitan area over the last 80 years, using daily rainfall data for the period 1933-2011 supplied by the Água Funda weather station, which is operated by the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences (IAG-USP). For the period 1973-1997, the researchers also used data from 94 other weather stations located in various parts of the metropolitan area.

The observations showed a significant increase in the total volume of rainfall during the wet season since 1961. This finding may be associated with an increase in the frequency of days with heavy rain and a decrease in the frequency of days with light rain in the area.

While there were practically no days with heavy rain (more than 50 mm) in the 1950s, these days occurred two to five times a year between 2000 and 2010.

Heat islands

According to Marengo, variations in São Paulo’s rainfall regime may be due to natural climate change but may also be related to increasing urbanization, especially in the last 40 years, when the city has increasingly experienced the phenomenon of “heat islands”.

With increasing urbanization, a vast area once covered by dense vegetation of the Atlantic Rainforest has been paved with materials such as asphalt and concrete, which absorb heat and do not retain moisture.

As a result, the weather is very warm during the day, while the accumulated heat is released into the atmosphere at night. The relative humidity of the air in the city is almost always low, and water evaporates rapidly from the ground to form clouds, Marengo explained.

“This accelerated evaporation rate means more water is extracted from the soil, leaving it totally dry, as we’ve seen in the areas where the reservoirs that supply metropolitan São Paulo are located,” he said.

Weather forecasts

To anticipate trends and changes in the pattern of extreme rainfall up to 2100, the researchers produced forecasts of climate change in different parts of São Paulo State, including the metropolitan area, using a technique called downscaling.

Downscaling combines the Eta-CPTEC regional climate model developed by INPE with the global climate models HadCM3 and HadGEM2 developed in the United Kingdom and used by the Intergovernmental Panel on Climate Change (IPCC) for short-, medium- and long-term forecasting with a spatial resolution of 40 km.

“It enables us to make more detailed weather forecasts for areas of São Paulo State such as the Paraíba Valley or Serra do Mar, which can’t be done with a global climate model,” Marengo said.

The researchers ran their model using the IPCC’s SRES A1B scenario for greenhouse gas emissions up to 2100.

Under this intermediate climate scenario, greenhouse gas emissions could reach 450 parts per million (ppm) and could cause a global temperature rise on the order of 3°C by 2100.

The researchers performed simulations for the periods 2010-2040, 2041-2070, and 2071-2100 based on the climatological period 1961-1990, which is the baseline defined by the World Meteorological Organization for climate projections.

The simulations predicted an increase in the frequency and intensity of extreme rainfall in metropolitan São Paulo and in the northern, central and eastern parts of the state in the coming decades.

However, the projections also pointed to a significant increase in the frequency of midsummer droughts in the same areas, suggesting that extreme rainfall will be concentrated within a few days and will alternate with long, dry periods, Marengo explained.

“The projections show an increase in the risk of flooding and landslides in metropolitan São Paulo and in the northern, central and eastern parts of the state,” he said.

“People who live in these areas will experience higher temperatures, changes in the rainfall regime and longer dry periods.”

Climatic vulnerability

According to Marengo, these particular areas may be most negatively affected by climate change because of the high population density.

In addition to the areas mentioned, the Paraiba Valley, Serra do Mar, Baixada Santista and Campinas areas are also expected to experience particularly severe effects of climate change.

“The social and economic impact of rising temperatures, longer droughts and changes in the rainfall regime in these areas will be more severe,” Marengo said. “In the case of eastern São Paulo State, for example, the population density is lower and the impact will be less severe, although these phenomena will occur there too.”

The size of the metropolitan area, precisely where the population is most vulnerable to the consequences of climate change, is projected to increase by 2030; this means a greater risk of climate-induced disasters.

“Population displacements caused by climate change won’t occur in the countryside only because more people now live in urban than rural areas,” Marengo said. “If recent phenomena such as the drought in São Paulo show that we aren’t prepared to deal with the problems caused by climate change, the findings of this study emphasize the fact that these problems will only get worse and that we must consider possible adaptation strategies.”

The paper “Observed and projected changes in rainfall extremes in the Metropolitan Area of São Paulo” (doi: 10.3354/cr01160), by Marengo et al., can be read in Climate Research at www.int-res.com/abstracts/cr/v61/n2/p93-107/.

The paper “Rainfall and climate variability: long-term trends in the Metropolitan Area of São Paulo in the 20th century” (doi: 10.3354/cr01241), by Obregón et al., can be read in the same journal at www.int-res.com/abstracts/cr/v61/n2/p93-107/.