U.S. Cities: Too Hot, and Getting Hotter

Excerpts from a new supplement to the National Wildlife Federation’s 2009 report More Extreme Heat Waves: Global Warming’s Wake-Up Call

Many Americans in the eastern and southern United States have been sweltering during summer 2010. As global temperature records have been set for the early summer months, states and cities are also setting hundreds of temperature records. More than 70 million Americans experienced extreme heat during June and July.

Unfortunately, climate models indicate that an average summer in 2050 will have even more days topping 90°F if global warming continues unabated. For example, Washington, DC is on track to have about 50 days of 90°F or hotter weather in summer 2010. By midcentury, an average summer could have 55 to 100 such days, depending on how much we curb global warming pollution.

Record Setting Global Temperatures

According to the National Oceanic and Atmospheric Administration (NOAA ), 2010 brought the hottest June on record. Almost the entire global land surface experienced warmer-than-normal conditions, with especially high temperatures in the eastern and south-central United States. The warm summer continued the heat we already experienced through the first half of 2010, which is the hottest January through June on record.

In 2010, New Jersey, Delaware, and North Carolina had their hottest June on record, while Rhode Island and Delaware had their hottest July. Sixteen other states had Junes or Julys that ranked in the top-five hottest. That means upward of 70 million Americans experienced extreme heat these two months. Hundreds of daily temperature records were broken across the country.

Not surprisingly, this hot spell has brought many days where the thermometer topped 90°F. Our analysis of large cities in the eastern United States shows that most locations have had about twice as many days with temperatures exceeding 90°F than they typically would by the end of July.

For example, Washington, DC , had 39 days with temperatures in the 90s by July 31, 2010, compared to 18 days for the same period in an average year. If conditions continue to stay warm, or even if we return to more average conditions in August, Washington and several other cities are on track to meet or break records for the total number of days exceeding 90°F in a single year. Cities in the south-central United States are also running hot: many have had about 50 percent more days over 90°F than average.

Extreme Summer 2010 Heat Could Be Typical by Midcentury

Summers like the current one, or even worse, will become the norm by 2050 if global warming pollution continues to increase unabated. Alternatively, taking steps to reduce emissions can help avoid some of this increase in extremely hot days. For example:

  • Washington, DC , is projected to have about 55 days over 90°F by midcentury under a lower-emissions scenario and about 100 such days if emissions are higher. For comparison, the city will likely have about 50 days above 90°F in 2010 if August and September have an average number of very hot days.
  • Philadelphia, PA , is projected to have about 40 days over 90°F by midcentury under a lower-emissions scenario and about 60 such days if emissions are higher. 4 Through the end of July, the city had 25 days exceeding 90°F in 2010 and is on track to have about 30 or more such days for the year.
  • St. Louis, MO , is projected to have about 60 days over 90°F by midcentury under a lower-emissions scenario and about 80 such days if emissions are higher.5 This year, the city is on track to have 45 extremely hot days, about 10 more than average.

The take away message is that, for each of these cities and for countless others that have been sweltering the last couple months, summer 2010 could be considered mild compared to the typical summers of the future.

The climate projections for the end of the century are even more dramatic. Much of the country will have twice as many days over 90°F if emissions are not curbed. That means much of the southern United States will have at least three or four months each year with temperatures in the 90s or above.

Posted in Climate Change
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