*Journalist Bob Berwyn will discuss global warming in the Rocky Mountains and the Rocky Mountain Climate Watch series on the KGNU public radio program A Public Affair Wednesday (Sept. 17) at 8:30 a.m. Listen online by visiting KGNU’s website and clicking on the “Tune In Online” button (left sidebar).
FRISCO — Juggling Colorado’s already stretched water resources is going to get even tougher in the decades ahead.
Rising temperatures will cut river flows and increase demand from thirsty plants and towns, a new state report concludes, projecting that average temperatures across Colorado will climb by at least 2.5 degrees Fahrenheit in the next 35 years, and by much more than that if there are no cuts in greenhouse gas emissions.
The report, released last month by the Colorado Water Conservation Board, outlines clearly how global warming has already affected water resources in Colorado:
— Warmer temperatures and other changes (dust on snow) mean that snowpack is melting earlier, on average, by one to four weeks compared with 30 years ago. This creates a strain for farmers and other users who draw water directly from rivers.
— Colorado has seen no long-term increase or decrease in total precipitation or heavy rainfall events. Climate models are split about Colorado’s future precipitation, showing a range of possible outcomes from a 5 percent decrease in precipitation to an 8 percent increase by midcentury.
— Climate models tend to show a shift toward higher midwinter precipitation across the state.
— Hydrology models show a wide range of outcomes for annual streamflow in Colorado’s river basins, but an overall tendency towards lower streamflow by 2050, especially in the southwestern part of the state.
The part about more midwinter snow is not a typo. The best available information now suggests a slight boost in precipitation from December through February, but whether that comes as snow or rain depends very much on elevation, and how fast the overall climate warms, said the Western Water Assessment’s Jeff Lukas, the main author of the report.
If some high country ski resorts see a bump in snowfall, you can still blame it on global warming. Warmer air holds more water vapor, and even in warmer climate, those clouds will give up some of that moisture as they’re forced up over the high peaks of Colorado.
The models don’t suggest any other big changes in other parts of the climate system (for example the lifting mechanism that forces air to rise and condense moisture), said Colorado State Climatologist Nolan Doesken.
“Unlike other parts of the country where temperature is the primary limiting factor for snow, we will still be plenty cold enough throughout the midwinter season,” Doesken said.
Teasing out a global warming signal in mountain ecosystems isn’t easy. There’s huge year-to-year variability, even without any changes forced by heat-trapping pollutants like carbon dioxide and methane. Tree ring records spanning thousands of years show longer and drier droughts than anything we’ve seen, but scientists are convinced they’re starting to see a pattern.
“There are so many nuances about this, but we’ve got a signal emerging from noise,” said Lukas. “The precipitation record is still noisy, but we can’t discount that we’re seeing a trend caused by anthropogenic global warming … The global models just don’t get it right for Colorado. It’s pretty difficult to jump from that broad scale to saying there will be more snow in Colorado,” he said.
Regardless, even if there is a bump in winter snow and rain, the warmer temperatures will suck up the moisture and then some. The end result could be a parched Colorado, unless water managers seriously pursue the path of adaptation and mitigation.
[scribd id=240083616 key=key-s7cF4YCyqWz4Ju3iqzOz mode=scroll]