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More Extreme Heat Projected in Denver Metro Area

Climate change is projected to lead to much more extreme heat in the Denver metropolitation area, according to a report released on June 8, 2017, by the Rocky Mountain Climate Organization and the City and County of Denver's Department of Environmental Health.

According to the analysis, with continued high heat-trapping emissions, the Denver metro area is projected to experience an average seven days a year at 100° or hotter in typical years by mid-century (2040-2059), and a full month’s worth—34 days a year—late in this century (2080-2099).

The single hottest year in the mid-century period is projected to have 25 days 100° or hotter, and the hottest year late in the century to have 72.

“This would be fundamentally different from the climate we know here, which only rarely hits 100 degrees,” said Stephen Saunders, the president of RMCO. "The record number of 100 degree days in Denver is six, in 2012. So if emissions continue rising, in mid-century the average year is projected to have more 100 degree days than the hottest year recorded here. And it could get worse -- much, much worse -- from there.

"But the great news is that if we sharply reduce emissions, we can keep these extremely hot days from getting more frequent, and by the end of the century we could even have fewer than we have had in recent years," Saunders said.

Commissioned by Denver Environmental Health, the analysis provides information to help Denver implement its Climate Adaptation Plan, released in 2014.

“The recent pullout from the Paris Agreement by the U.S. only accelerates our efforts to continue to protect Denver’s community from the impacts of climate change,” said Bob McDonald, executive director of Denver Environmental Health. “These projections powerfully illustrate how much difference it makes in Denver whether future heat-trapping emissions continue increasing or are reduced.”

In the following key projections for the metro area, the indicated values are the medians of the projections from multiple climate models based on two possible levels of future heat-trapping emissions. The first reflects a business-as-usual trajectory with continued high increases as in recent years; the second scenario reflects very low emissions from rapid and sustained global reductions in emissions. Projections are shown for mid-century (2040–2059) and late in the century (2080–2099).

Days 100° or hotter in typical years:

  • The years 1967–1999 averaged 0.3 such day per year across the metro area.
  • The first years of this century, 2000-2016, averaged 1.8 100-degree-plus days, already nearly a three-fold increase.
  • With continued high emissions, the median projection for mid-century is an average of seven days per year and by late century 34 such days.
  • With very low emissions instead, the median projections are two such days both in mid-century and late in the century.

Days 100° or hotter in extreme yearsthe hottest single year in each 20-year period:

  • In 2012, Denver’s hottest year on record, there were six days 100° or hotter.
  • With continued high emissions, in the hottest year projected in the 2040-2059 time period, there would be 25 such days. In the hottest year in 2080-2099, there are projected to be 72 such days.
  • Included in the projections above are that the hottest year in mid-century year would have 11 consecutive days 100° or hotter, and the hottest late-century year would have 38 straight.
  • With very low emissions instead, the median projections are two such days both in mid-century and late in the century.

The report also shows that the frequency of days 100° or hotter has already increased at the Stapleton Airport weather station, the metro area's weather station with a long-standing record, from 0.7 day per in 1967-1999 (somewhat higher than the metro-wide average of 0.3 per year) to 1.8 days per year so far this century (2000-2016).

The analysis shows that for the metro area's average temperature of the 30 hottest days in a year:

  • From 1970–1999, the 30 hottest days per year averaged 93 degrees.
  • With continued high emissions, the median projection is for an average of 99 degrees in mid-century, and 104 degrees late in the century.
  • With very low emissions instead, the median projection is 97 degrees in both time periods.

RMCO also released a separate report to Denver Environmental Health of precipitation projections, which suggest that heavy storms may become more frequent. Storms with less than a quarter-inch of precipitation in a day are projected to have little change in their frequency, regardless of emissions levels. For storms of a half-inch of precipitation or more in a day:

  • With continued high emissions, such storms are projected by mid-century to become 15 percent more frequent, and by late in the century, 31 percent more frequent.
  • With very low emissions instead, such storms are projected to average 25 percent more frequent by mid-century and 17 percent more frequent by late in the century.

The precipitation projections are generally considered less reliable than the temperature projections, for reasons explained in the precipitation report.

Explanation of above figure

The figure above shows the number of days 100° or hotter in the Denver metro area. Actual numbers from the Stapleton Airport weather station (on the left) were 0.7 per year in 1967-1999 and 1.8 per year in 2000-2016. For the entire Denver metro area for future periods (on the right), the figure shows the range of the middle 80 percent of projections from multiple climate models (the brighter colored portions of the columns) and the medians (the numerals), for four possible levels of future heat-trapping emissions.


The full report on extreme heat.

A 4-page summary of the extreme heat report.

A companion report on precipitation projections.

The joint news release by RMCO and Denver Environmental Health on the reports.

A spreadsheet of full results on both extreme heat and precipitation from the analysis, covering scores of temperature and precipitation projections for four different assumptions (scenarios) for future levels of heat-trapping emissions, and for each scenario for four 20-year periods: 2020-2039, 2040-2059, 2060-2079, and 2080-2099.