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Future Climate Extremes in Boulder County

A report by the Rocky Mountain Climate Organization (RMCO) projects large increases in the frequency and extent of extremely hot days in Boulder County, with the extent depending on whether global heat-trapping emissions continue on a high trajectory or are reduced.

“On our current path of steadily increasing heat-trapping emissions, Boulder by the middle of the century is projected to average 38 days a year 95° or hotter. By late in the century, an average of two and a half months of 95 °-plus days are projected—75 a year,” said Stephen Saunders, president of RMCO and lead author of the report. "This would be fundamentally different from the climate we have known in Colorado."

"But we don't have to get this hot," Saunders said. "These projetions powerfully illustrate how different our future will be depending on whether we limit heat-trapping emissions or not. If we bring emissions down to a very low level, we can avoid any further increases in extreme heat beyond the relatively modest increases we face in the next couple decades."

The projections also address future precipitation extremes. Although projections for precipitation are more uncertain than for temperature, the models suggest that heavy storms may become more frequent.

“This shows why we need preparedness actions to address the impacts we could face, from more wildfires and possibly more floods to more heat waves that can threaten people’s health and even lives,” Saunders said.

The following projections show the medians of the projections from multiple climate models, based on two possible levels of future heat-trapping emissions—continued high increases as in recent years, reflecting a business-as-usual approach, and very low increases, reflecting rapid and sustained global reductions in emissions. The projections are shown for mid-century (2040–2059) and late in the century (2080–2099). The full report, however, analyzes two other medium-level emissions scenarios, and two other time periods, 2020–2039 and 2060 – 2079.

 Days per year with high temperatures of 95° or hotter in the City of Boulder and vicinity:

  • In 1970 – 1999, averaged five occurrences per year.
  • With high continued growth in global heat-trapping emissions, in mid-century (2040 – 2059) are projected to average 38 times per year and by late in the century (2080 – 2099) to average 75 days per year.
  • With very low emissions instead, are projected to average 20 times in mid-century and 18 times late in the century.

Days with high temperatures of 100° or hotter:

  • In 1970 – 1999, averaged less than one occurrence per year.
  • With high emissions, in mid-century are projected to average 8 times per year and by late in the century 35 times per year.
  • With very low emissions instead, are projected instead to average twice a year in both time periods.

The average temperature of the 30 hottest days in a year:

  • In 1970 – 1999, averaged 93°.
  • With high emissions, in mid-century is projected to average 99°, and late in the century 104°.
  • With very low emissions, is projected instead to average 97° in both time periods.

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. Storms of a half-inch of precipitation or more in a day:

  • With high emissions, are projected by mid-century to become 16 percent more frequent, and by late in the century 36 percent more frequent.
  • With very low emissions instead, are projected to average 22 percent more frequent by mid-century and 19 percent more by late in the century.

For this and a companion report addressing Larimer County, RMCO analyzed 44 million individual projections for daily temperature and precipitation values for four locations: Boulder and vicinity, Boulder County mountains, Fort Collins and vicinity, and Larimer County mountains. The projections are from the latest generation of downscaled global climate models. A projection for an individual day does not have any particular value, but enough of them over a sufficient period of time enables analysis of how often particular conditions are projected to occur in that period.

The reports were funded by the Colorado Department of Local Affairs, using Community Development Block Grant—Disaster Recovery funding through the Resilience Planning Program. Boulder and Larimer counties were heavily affected by the High Park wildfire in 2012 and the September 2013 flooding that led to federal disaster designations. The purpose of the reports is to help local governments in these two counties better understand and prepare for the increased risks of wildfire and flooding expected to come with further climate change.

 

Figure caption

The figure above shows how the number of days 95° or hotter in Boulder could go from an average of 5 per year late in the last century to 75 per year late in this century. For future periods, the figure shows the range of the middle 80 percent of projections from multiple climate models (the checkered portions of the columns) and the medians (the numerals), for four possible levels of future heat-trapping emissions.

Links

The full report, Future Climate Extremes in Boulder County.

A four-page summary of the report.

The news release announcing the report.

A spreadsheet with the full results of the analysis of 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.