Working to keep the West special

More Hot Days and Heat Waves in Fort Collins

The Rocky Mountain Climate Organization and the City of Fort Collins have released a new report, Extreme Heat in Fort Collins, documenting recent increases in hot days and heat waves and projecting their future levels. Annual rates of both single days 95 degrees or hotter and three straight days of 90 degrees or hotter have tripled so far this century, compared to 1961-1999 rates. In the future, these hot days and heat waves will be even more common, especially if emisisons continue increasing on their current path.

An example of the data from the report, for three straight days of 90 degrees, is shown below. See the box to the right for an explanation of what is shown here.

(For an explanation of the figure, see the box to the right.)

Single extremely hot days have also increased, with the annual frequency of 95-degree Fahrenheit days tripling in the past 14 years, compared to the last four decades of the 20th century. If heat-trapping pollution continues going up at a medium-high rate, those very hot days could double again by 2050 and yet again by the end of the century, to 13 times the 1961-1999 rate,

“One possible future has extreme heat as the new normal, and the other still has a great climate that keeps this such a special place to live,” said RMCO president Stephen Saunders, the lead author of the report. “This is what we have at stake as humans change the climate and why Fort Collins’ award-winning leadership in reducing climate-changing pollution matters.”

One of the reasons that more extreme heat matters is that in the last decade more Americans died from the effects of excessive heat than from any other weather-related cause, such as tornadoes, hurricanes, or floods.

The report also includes new information identifying the linkage of extreme heat to Colorado’s most destructive wildfires in the past two years, which all began on very hot days:

  • The High Park fire west of Fort Collins in 2012, which destroyed 259 homes and killed one person, started on a day that in Fort Collins had a high temperature of 93°, according to the daily temperature data analyzed for this report.
  • The Waldo Canyon fire west of Colorado Springs in 2012, which burned 347 homes and killed two people, started when Colorado Springs had a record-tying high temperature for that date of 100°.
  • The Black Forest fire north of Colorado Springs in 2013, which burned 488 homes and killed two people, started on a day that in Colorado Springs had a record high for that date of 97°.

Lucinda Smith, director of the Environmental Services department in Fort Collins, said, “The City is already engaged in evaluating risk and vulnerability to the provision of City services and infrastructure from a changing climate. The information contained in this unique report will help the City plan for and prioritize actions to minimize the impacts of predicted heat increases.”

Nolan Doesken, state climatologist and senior research scientist at Colorado State University’s Department of Atmospheric Science, said, "The higher frequency of very hot temperatures is indicative of what we all may be dealing with more often in the near future. I appreciate that our city is thinking about potential future consequences from heat, drought, wildfire and floods and being proactive today.”



The full report is here.

A RMCO news release on the report is available here, with a summary table of the actual and projected increases in hot days and heat waves above thresholds of 90, 95, and 100 degrees.

A City of Fort Collins news release is here.

News coverage by the Fort Collins Coloradoan is here and by the Boulder Daily Camera is here.

Quick Facts

Both hot days and heat waves have increased.

For the annual frequency of three straight 90-degree days, shown in the figure to the left, the rate in the first years of this century (2000-2013) was 2.6 times the rate in the last 39 years of the last century (1961-1999).

Another way of expressing that change is the linear trend from 1961 through 2013--the straight line that statistical analysis says is the best fit with the data. The linear trend is a 533 percent increase over 53 years. (The linear trend is not shown in the figure to the left, but trends are shown in the full report.)

For single hot days, the annual frequency of 95-degree days increased from 1961 through 2013 at a statistical rate of 1,069 percent.

Consistently for both single hot days and heat waves, the higher the temperature, the more their frequency increased.

The figure to the left also shows projections for future frequencies of three straight 90-degree days. The projections are for continued increases in the rate of three straight 90-degree days, with the increases projected to be much greater if future emissions of heat-trapping pollution are on about their current path ("medium-high" in the figure) than if they are lower.

The figure shows the ranges of the multiple projections. The solid boxes show the middle half of all projections for a time period, the solid line the average of those projections, and the dashed extensions of the boxes the range of the projections from the 10th percentile to the 90th percentile.

With lower future emissions, the average projection is that by mid-century those heat waves will be 3.5 times as frequent by mid-century as in 1961-1999. By the end of the century, 4.4 times.

But with medium-high future emissions, these heat waves could be 5.2 times as frequent, and by the end of the century 9.1 times.

Single 95-degree days are projected to go up even more, and could occur six times as often as the historic rate by mid-century and 13 times as often by the end of the century.

A high-resolution version of the figure from this page can be obtained for use by the news media use and by others by emailing RMCO.