The historic rain that shattered records across Colorado’s northern Front Range last September was not made more likely or intense by the effects of climate change, according to a study published today that was led by a Boulder-based meteorologist.
“There’s clear evidence that, overall, our greenhouse gas emissions are making the planet warmer and moister, but we found such climate factors had little appreciable effect on the frequency of heavy five-day rainfall events in this area during September,” said Martin Hoerling, a research meteorologist in the Earth System Research Laboratory at the National Oceanic and Atmospheric Administration in Boulder.
Hoerling is the lead author on “Northeast Colorado Extreme Rains Interpreted in a Climate Change Context,” to be published today in the Bulletin of the American Meteorological Society.
The study found that climate change would not make such storms more likely along the Front Range during the month of September. It also indicated they may even become less likely — even as climate models show an increase in the intensity of five-day precipitation events in other regions around the globe by the end of the 21st century.
Also, in Colorado’s limited historical weather record, the 2013 storm was not without precedent.
Researchers said it bore many similarities to a “strikingly similar” September 1938 storm, “long before appreciable climate change,” which struck this area.
In footprint and duration, it also resembled the June 1965 Cherry Creek flood, which, at about $3 billion in current-day dollars, is still the state’s most costly.
The new study made use of a NASA climate model containing information about how climate factors such as ocean temperatures, sea ice extent and greenhouse gas levels have varied since the late 19th century.
Run many times, the model produced occasional heavy September rain events in a 30-year period from 1870 to 1900, as well as the 30-year period from 1983 to 2013.
Comparing the two 30-year periods, researchers found that lower amounts of sea ice, warming oceans and added greenhouse gases did not increase the likelihood of a storm such as Colorado witnessed last year.
The report concedes that, “A weakness of our study is that results are based on a single model, and thus require confirmation using additional models.”
Storm coincided with high Pacific temps
Hoerling said Colorado’s historic weather last year was triggered by a rare weather pattern that combined an abundance of atmospheric water vapor transported north from the gulfs of California and Mexico, and a persistent upslope wind pattern that continued to drive the rain across the landscape.
Kevin Trenberth, a distinguished senior scientist in the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, did not give the study high marks.
“This is not a study that asks the right questions, it does not perform the needed analysis and gets the wrong answers,” Trenberth wrote in an email.
“The study is correct that the set up of weather systems was rare, and was not caused by climate change. But I think there is no doubt that those extremely high SSTs (sea surface temperatures) and record water vapor amounts likely would not have occurred without climate change.”
The sea surface temperatures off the west coast of Mexico, south of Baja, west of Guadalajara, were briefly over 30 degrees celsius at that time and were 1 degree celsius above normal, which made it the hottest spot for the ocean in the western hemisphere, Trenberth said.
Region known for variable weather
In the immediate wake of the storm, many people drew a conclusion that there must be a link between climate change and such a memorable weather event. Hoerling hopes they will view his group’s findings with an open mind.
“I hope the general public will greet the findings favorably, that the scientific community is actually doing the specific analysis, in a timely way with modern tools, at the place that this happened and at the time that it happened,” rather than adhering to a “broad-brushed” view of what global trends might yield in a specific region, Hoerling said.
“It should satisfy someone who got washed out of their home, and their basement filled with water,” Hoerling said. “This was the first serious effort to look at this storm; why did this happen, and should I be worrying about it happening more frequently in the coming years, as I stay here on the Front Range?”
While the study’s findings suggest the answer is no, it doesn’t mean that it couldn’t happen again, anytime the weather patterns combine in a similar fashion to what was witnessed in 2013.
“What this teaches me is that our weather in this region can be extremely variable, and has the capacity to produce extreme weather of a type that you may never have experienced, even if you have been a native of Colorado all your life,” Hoerling said.
“I think the lesson from the analysis is, I think we were surprised by something which we hadn’t experienced, because our data records are too short — not because our climate has so radically changed.”
The study’s co-authors are Klaus Wolter, Judith Perlwitz, Xiaowei Quan, Jon Eischeid, Hailan Wang, Siegfried Schubert, Henry Diaz, Randall Dole.
Contact Camera Staff Writer Charlie Brennan at 303-473-1327, brennanc@dailycamera.com or twitter.com/chasbrennan.
