GOLDEN, Colorado—Even as one of Denver’s longest snow droughts on record—232 days—was forecast to end on Friday, nerves in the Mile High City were frayed after a summer of climate extremes, and a heat wave that has stretched into late autumn. 

Just a few days before the forecast snow, an intense wildfire had flared up deep in a Rocky Mountain canyon along I-70, Colorado’s main east-west interstate, and in the afternoon, a sudden dust storm blasted the region and chilled the air by 20 degrees Fahrenheit in a matter of minutes, temporarily breaking the month-long heat wave the West was mired in. The regional heat wave and drought also fueled an early December wildfire in Denton, Montana, that destroyed dozens of homes and businesses.

Research shows that global warming increases some persistent climate patterns that can intensify extremes like droughts and intense rain storms. In the Rockies and other parts of the West, recent extremes intensifed by global warming have included winter bomb cyclones, unusally large avalanches, lightning storms and wildfires, as well as floods. 

In November, temperatures were between 6 degrees and 9 degrees Fahrenheit above average across a huge swath of the West, from the northern Plains through the Rockies and into the desert Southwest, according to the November 2021 federal monthly climate report. Across the Lower 48 states, it was the seventh warmest and eighth driest November in the 127-year record. 

The continuation of the long-term warming and drying in the West has renewed concerns about regional water and power security. But the trends have been projected for years in recent major reports like the Fourth National Climate Assessment and major climate science reviews by the Intergovernmental Panel on Climate Change, both showing that global warming is making the Southwest more vulnerable to climate threats. 

One of the most immediate concerns is a loss of hydropower production at Glen Canyon Dam, which would set up a potential regional energy crunch. In September, a federal water report showed that could happen as soon as next year, if the reservoir drops low enough that there’s no way to get water to the power turbines. 

In Colorado, October and November seem to be particularly susceptible to the autumn warming trend, said Assistant State Climatologist Becky Bollinger. Taken together, recent climate extremes show how thirsty the overheated atmosphere is, and how overlapping trends of heating and drying intensify each other, she said. Without snow cover to reflect sunlight, the ground absorbs more heat, warming the air and making it more thirsty.

That can lead to dangerous “flash droughts” that aren’t related to a lack of precipitation, she said. 

“When the air is really dry, combined with warm temperatures, sunshine and wind, it makes the atmosphere thirsty, where it really wants to take moisture from the surface,” she said. “We’re seeing those evaporative-demand conditions more often in the summer, and we saw them last October, and they contributed to the huge and devastating fires we saw in Colorado.”

“We’ve had our really hot summers and that is carrying over into the beginning of fall. And it seems like it’s harder for us, north of the Four Corners region, to get monsoon moisture,” she said. “If you get hot temperatures on top of that, it’s always going to dry out the soil before freezing it. When winter starts very dry, it takes more snow to replenish soil moisture, which leaves less available for stream flows needed by farmers and ecosystems, she added.

The dry and warm November has also prevented effective snowmaking at ski areas around the West. Those resort-based economies are crucial to the regional Western economy, and lack of certainty about early winter holiday snow conditions can dampen business.

Autumn Extremes Threaten Trees

As atmospheric carbon dioxide concentrations increase, long autumn heat waves can end with an extreme temperature drop, as the climate engine grinds from neutral straight into third gear, from summer to mid-winter conditions. That can harm trees that are still trying to drink and breathe, as well temperature- and snow-sensitive animals that waited too long to migrate, or donned winter camouflage too early.

The Colorado State Forest Service documented the threat of autumn extremes to trees along the Front Range of Colorado after a sudden 70-degree Fahrenheit temperature drop in October 2019. In subsequent aerial monitoring, state scientists found widespread damage in multiple species, and they tracked a similar event in April 2020, when temperatures dropped from the 70s to single digits in 24 hours.

State entomologist Dan West, who tracks forest changes, said the October 2019 extreme temperature shift mainly affected the Front Range, running north to south along the base of the Rocky Mountains. Urban trees, including deciduous species, were also hit by the freeze. He said many of the conifer species show signs of bouncing back, but he warned that some forests may be in for multiple shocks, with more extreme conditions ahead.

“We are parched and have been much warmer than average, so the threat of another situation of extreme temperature swings is looming,” he said. The damage comes because the trees are still actively “transpiring water due to the warmth,” he said. “And then the temperatures drop out, and the water never fully tapers back down. So, in short, they freeze the cells both inter- and intra-cellularly.” 

The long-term Western autumn warming and drying trends also helped spark the spread of tree-killing bark beetles in western North America, starting about 25 years ago, with the insects taking advantage of a longer breeding season, as well as increasingly drought-stressed trees that were an easy target. 

Colorado state climatologist Russ Schumacher said the two cold snaps were unusual, even in a state known for extremes. In the October 2019 cold snap, temperatures in parts of Colorado hit all-time record lows by large margins. 

“There are some indications that colder low temperatures have become more frequent in parts of western Colorado in the fall, even in a place that has otherwise warmed as fast as anywhere in the country,” Schumacher said. “I don’t think we know what the potential connection with climate change is for this.” 

He said he could think of plausible hypotheses related to changes in tropical cyclones over the Pacific, for example, “but I don’t think anyone has really looked at this carefully.”

Fatter Marmots, Fewer Elk

Whatever the reason, the sudden temperature swings are also apparent in the North Fork Valley, an important agricultural part of Colorado several hundred miles southwest of Denver, with significant damage to grape vines, cherry trees and some other fruit trees, said University of Maryland ecologist David Inouye, who has been studying climate change impacts in the Rockies for more than 40 years.

“Last October in Paonia it got down to 5 degrees on Oct. 27, after having been in the 70s and 80s earlier in the month, and the 70s again a few days after that freeze,” he said. The trees hadn’t had a chance to harden before that freeze. “Fall frosts are coming later, so flowering is lasting longer. Marmots are getting fatter than they used to,” he said. 

The long-term records show that the winter snowpack season, when the ground stays covered with snow, is starting later. That means “plants, and some animals, are exposed to more freezing temperatures, without the insulation provided by snow, he said. “It also means a longer period when weasels, snowshoe hares, and ptarmigan are mismatched with background color, since they turn white for winter.”

Some species that migrate up and down in mountainous areas are also likely to be affected by persistent autumn warmth. Elk that historically were often pushed out of the high terrain by October snows haven’t shown around Paonia yet, even now, in late December. 

“I presume that they’re all still at higher elevations,” Inouye said. “I had a license for the season that ended recently and never saw an elk.”