August in the Mid-Atlantic is notorious for its heat and humidity, which has long driven an annual exodus of official and unofficial Washington to cooler destinations. It's no accident; after all, that Congress takes the month of August - and not say October - off. According to a new analysis by the nonprofit organization Climate Central, by the middle of this century climate change is likely to make August in Washington significantly hotter and more uncomfortable, making the congressional schedule even more enviable.
During the 1980s and 90s, the report found, there were an average of ten August days in D.C. that reached or exceeded 90 degrees (this year, we've had tem). Yet by the 2050s, that number may jump to an estimated average of 19 days - a 90 percent increase.
According to Climate Central, which is a nonpartisan and nonadvocacy group, August days with a high temperature above 95°F have been relatively rare occurrences, with an average of just three such days per year during the 1980s and 90s. But by the 2050s, such days could be much more commonplace, with a best guess average of about ten such days per year during that decade. And the average number of August days at or above 100°F - which was zero in the 1980s and 90s - is projected to increase to an average of three days during the 2050s.
Climate Central's study examined August extreme heat in 20 other major U.S. metropolitan areas as well, and found that extreme heat is likely to become more frequent and intense in nearly all of the cities studied.
For example, at present there are only two cities on the list where more than half the days in an average August exceed 95°F. These are Phoenix and Dallas, which are both well known for their brutally hot summers. But by the 2050's, Climate Central scientists found that Houston, Sacramento, Tampa Bay and Orlando could join them on that list.
Dr. Ben Strauss, Climate Central's Associate Director for Strategic Initiatives, cautioned that the numbers should be taken as best guesses within a range of uncertainty, and that they are estimates of averages. "No matter how close the projections turn out to be, some years will have more hot August days, and others will have fewer," Strauss stated.
The analysis reinforces a broader result from climate science that large changes in extreme weather events can result from a relatively small amount of global warming. The likelihood that a warming planet will lead to more frequent, severe and longer lasting heat waves in the United States (as well as other extreme weather events) was detailed in a federal climate science assessment published last year.
Climate Central's study is based on findings from a dozen computer models that were used to simulate future climate conditions under a scenario of greenhouse gas emissions that is lower than the current trend. So far this decade, global emissions of these gases, such as carbon dioxide, have actually been outpacing the emissions scenario used for the study. All twelve computer models projected increased hot days from the present by the middle of the century.
Disclaimer: Andrew Freedman worked as a graduate student intern at Climate Central this summer, where he helped write the results of the extreme heat study. He did not conduct the technical research (i.e. the modeling) that went into the report.