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Houston's hottest days are expected to happen more frequently with global warming. Take a look: http://bit.ly/1mUzJiO

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Story Highlights

  • “Hot days” in your city are likely going to happen much more often if we continue to pump heat-trapping greenhouse gases into the atmosphere.

  • A “hot day” temperature threshold was determined for each city, then we used climate models to predict how much more frequently that “hot day” is expected to occur in the future.

  • The methodology for determining each temperature threshold and calculating future projections is listed below.


Click here for a high-res version

This is the warmest time of year in most of the U.S. And if we continue to pump heat-trapping greenhouse gases into the atmosphere, it’s going to get even warmer – not only in the summer, but throughout the year.

“How warm?” is the obvious question. Well, there are all sorts of different ways to answer that. You can look at trends in average summer temperature. You can look at where your city will “migrate” in terms of summer high temps (Boston goes to North Miami; Las Vegas heads for Riyadh, Saudi Arabia, and so on). This week, we’re coming at it in yet another way, by showing how the numbers of extremely hot days are projected to increase by 2050 and 2100 if emissions continue to grow unabated.

The term “extremely hot” means different things in different places, of course. A reading of 100°F is rare in Madison, Wis., -- and in Miami, which might surprise many. On the other hand, in Phoenix 100°F days are not rare at all. These days, 115°F is considered an extremely hot day there. The mercury matches or tops that scorching number only about once a summer; but by 2100, more than 53 are projected. By contrast, a generally cooler city like Madison gets about 10 days at or above 90°F each year, so the temperature threshold there is lower. By 2100, Madison is expecting more than 67 days of 90°F-plus temps. (Remember, you can access projections and graphics for other cities through our archive site: www.climatecentral.org/climate-matters.)

The graphic shows your city’s extremely hot threshold: the number of days that temperature is matched/topped on average during the period 1986-2005, and the number times that temperature is expected to be reached/topped by 2050 and by 2100. This assumes there’s no significant cutback in greenhouse-gas emissions.

If the world does reduce emissions significantly, the number of projected above-threshold days will be fewer – although still more than we are experiencing today. For example, temperatures are still expected to match or top 115°F in Phoenix at least 2 days per year by 2100 instead of 53 days per year projected with the current emissions trends. And in Miami, there are 8 days expected with temperatures at or above 95°F in the low-emissions scenario, compared to 86 days on the current path.

METHODOLOGY: Average annual number of exceedances was calculated for 90°F, 95°F, 100°F, (and, where applicable, 105°F, 110°F and 115°F). Illustrated above are the average annual exceedances for the temperature which this city currently exceeds between 1 and 15 times each year. If you are interested in seeing how often one of the other temperatures is exceeded each year, on average, please let us know.

To calculate the current average number of times a temperature threshold is exceeded, daily max temperature data was obtained from ACIS for each year from 1986-2005. The number of days matching or exceeding the threshold temperature was summed for each year and then averaged across all 20 years.

Projected exceedances were calculated based on the high emissions scenario (RCP8.5, representing a consensus mean from multiple climate models) used in IPCC's 5th Assessment Report, which represents a continuation of our current emissions trends through 2100. Monthly temperature data was downloaded from CMIP5 downscaled climate projections for the periods: 1986-2005, 2041-2060, and 2080-2099. The projected temperature change for each city was calculated from the difference between the future projections and the present period (1986-2005). This projected temperature change was then added to the observed ACIS temperature data from 1986-2005 at each city and the total number of exceedances was then calculated for each future year and average across all 20 years in each future period.



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