Metro Atlanta has historically experienced one day a year on average with a heat index of 105 degrees, said Astrid Caldas, senior climate scientist for UCS and study co-author, but without significantly reducing the amount of greenhouses gases in the atmosphere, that number could reach more than 20 days by midcentury.
“Even though people are used to the heat, it is going to be hotter, and it will be hot for a longer period of time,” said Caldas. But some climate scientists have expressed concerns about such dire heat scenarios that may not take mitigating factors into account.
Though there is no set standard for what constitutes an extreme heat event, the frequency of heat events classified as extreme has been on the rise since the mid-1960s, and a growing body of research attributes that rise to the increase in carbon emissions, according to the UCS report.
For the analysis, UCS scientists used a set of 18 climate models to project the frequency of days exceeding three heat index thresholds — 90, 100 and 105 degrees — under two emissions scenarios: “no action,” in which there is no substantial reduction in emissions through late century, and “slow action,” under which emissions begin to decline by midcentury. For a rapid action scenario, the data used was based on standards set forth by the goal of the Paris Agreement.
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If global average warming is limited to the goal of the Paris Agreement, the number of days per year on average with a heat index above 105 degrees in metro Atlanta by late century would be 13, as opposed to 55 if no action is taken to reduce emissions, according to the report.
But Judith Curry, professor emerita of Earth and Atmospheric Sciences at Georgia Tech, cautions against using the most alarming scenarios from climate models. Temperature extremes, particularly in cities, are modified by urbanization, air pollution and irrigation, Curry said.
In June, Curry provided testimony to the House Oversight and Reform Environment Subcommittee in which she stated that the perception of extreme weather events as being more frequent or intense and caused by man-made global warming is symptomatic of what she calls “weather amnesia.”
“If people are worried about heat waves and they think they are bad now, look back to the 1930s, when they were really, really bad,” Curry said.
But Caldas said current conditions are unique and science literature has continued to confirm the relationship between carbon dioxide and global warming along with the source of those emissions as man-made — from the burning of fossil fuels rather than natural sources. “In our view, it is fair and it is important to warn people what can happen and what will happen if we continue on this trajectory,” said Caldas.
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The report points out the toll that extreme heat may have on certain populations, specifically outdoor workers, city dwellers, low-income communities and people who live in areas prone to other natural disasters such as floods, hurricanes or wildfires. Recommendations in the report challenge federal and local governments to take action ranging from setting national standards for protecting the health of outdoor workers under the Occupational Safety and Health Act to implementing and strengthening the Paris Agreement, from which President Donald Trump announced the United States’ intent to withdraw, though the process cannot be initiated until November 2019.
“Extreme heat is one of the climate change impacts most responsive to emissions reductions,” Caldas said. “It is the one climate change that is well understood.”