From Inside Climate News (Bob Berwyn):
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…
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.”
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.