When Denver becomes like Pueblo, and Pueblo becomes like Albuquerque
Colorado climate change report looks into ‘uncharted territory’
by Allen Best
The summer of 2012 was hot in Colorado. Major wildfires in Colorado Springs and in the foothills west of Fort Collins launched the summer, and then days of 90-plus temperatures in Denver became monotonously routine.
It was not the only hot summer in Colorado’s recorded history. The years of 1934 and 1954, both of them marked by drought and dust, also leap from the statistical files.
But here’s what’s important. Those hot, hot summers of the past will likely become the average during the next 40 years as record-breaking heat becomes the norm in Colorado, according to a new report.
Called “Climate Change in Colorado,” the report synthesizes hundreds of studies in an effort to help water managers grapple with shifting temperatures and precipitation patterns. It was prepared by the Western Water Assessment for the Colorado Water Conservation Board.
The report, an update of a similar report issued in 2008, produces no new eye-openers, although it does embrace small nuanced steps such as the effect of dust on snow. Increased heat is clear. Denver will become as toasty as Pueblo, and Pueblo will be like Albuquerque, and so it will be up and down the line.
Much uncertainty remains, however, particularly as regards precipitation. The report is littered with hedge words such as “tendency.”
“We need to keep pace with the science, and just because we face the same level of uncertainty that we had in 2008 doesn’t mean we should stop trying to understand and narrow our risk,” says James Eklund, director of the Colorado Water Conservation Board, the state government’s leading water policy agency.
Wetter during winter?
One key takeaway for Colorado mountain towns is that nearly all the projections indicate increasing winter precipitation by 2050, although not necessarily everywhere. Southern Colorado might end up drier.
In the San Juan and Rio Grande basins, about two-thirds of the projections still show decreases in annual streamflow by 2050.
For the Colorado headwaters and Gunnison River, half of projections show increases and half decreases in annual streamflow.
But even those parts of Colorado with more snow in January are likely to have problems. Spring will arrive more rapidly and streams everywhere will likely have less water in them in late July, August and September.
“The body of published research indicates a greater risk of decreasing streamflow, particularly in the southern half of the state,” says the report.
Warmer temperatures will also result in increased demands for water for landscaping and crops—and a harder hit on existing reservoirs. Ridgway Reservoir will start showing shorelines in June instead of July or August.
Delta projected to become 3.3° to 3.7° F warmer, lengthening growing season by 15 to 22 days. This means crops will need more water, and in the case of Olathe sweet corn an estimated 2.7 to 6.7 inches more water each summer.
One consequence of reduced streamflow will be reductions in water quality and greater difficulty in meeting standards. Warming of lakes and reservoirs also reduces seasonal mixing of the water layers, potentially decreasing dissolved oxygen and leading to excess concentrations of nitrogen and phosphorous.
More frequent wildfires can also lead to increases in erosion and sedimentation rates in basins experiencing high-severity burns. Other problems: excess stream nitrates and turbidity.
Jeff Lukas, the lead author of the report and a senior researcher at Western Water Assessment, points out that temperatures during winter precipitation events will tend to remain well below freezing in most Colorado mountain towns. This, he points out, is different from what the climate models show for other regions of the West, including the northern Sierra Nevada, the Cascades, and even the northern Rockies, where what now falls as snow will increasingly switch over to rain.
“The snowpack in Colorado appears to be less vulnerable to the impacts of the observed West-wide warming trend than other regions,” he writes in the report.
More precipitation, however, does not necessarily mean more snow. Instead, there might be more rain during shoulder seasons. Rain might replace snow at lower elevations.
Lukas points out that the natural range of variability for precipitation is already enormous. This summer was soggy, for example, and 2012 was crispy dry. Whether increased greenhouse gases cause precipitation to trend upward or downward will probably not become evident “until the latter part of the century,” he says.
Little changes, big consequences
Colorado has already been warming, however. Average annual temperatures have risen 2° F during the past 30 years. The heating has occurred …
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