Human activities such as agriculture and landscaping along Colorado’s northern Front Range are changing temperatures, river runoff and tree-distribution patterns in Rocky Mountain National Park and the surrounding region, according to a Colorado State University study.
Evaporative cooling caused by irrigation could mean more extreme weather, both in the mountains and in the populated parts of Colorado’s eastern plains, says ecologist Thomas Stohlgren with the U.S. Geological Survey at Colorado State.
Looking at an area of mountains and plains about 125 miles on a side and centered on Fort Collins, Stohlgren and his colleagues found that plowing up and irrigating the arid shortgrass prairie produced local cooling in the summer months they believe could match or exceed predicted global warming.
"The major factor is simply the change in the way that water is distributed on the landscape," Stohlgren said. "We’ve replaced dry prairie with irrigated agricultural crops and landscaping – lawns and golf courses and trees and so on. Remember that this was a dry place, years ago."
Normal rainfall on the eastern plains is less than 20 inches a year, Stohlgren said. With nearly 40 percent of the study area and 55 percent of its grasslands converted to agriculture, rougher surfaces such as plowed fields enhance moisture loss. Increased evaporation cools the area, and up-slope winds carry cooler, moister air into the mountains.
Stohlgren and Jill S. Baron, both scientists with the U.S. Geological Survey at Colorado State’s Natural Resources Ecology Laboratory; Thomas Chase, a Colorado State doctoral student in the Graduate Degree Program in Ecology; Roger A. Pielke Sr., Colorado State professor of atmospheric sciences; and Timothy G. F. Kittel, a research scientist with the National Center for Atmospheric Research in Boulder and with Colorado State’s Natural Resources Ecology Laboratory, reported their findings in the June issue of the journal Global Change Biology.
Researchers ran simulations with the Regional Atmospheric Modeling System developed by Pielke and others at Colorado State. The RAMS model suggested average July temperatures for the Fort Collins area might decrease by one degree Fahrenheit as a result of increased cooling.
The scientists then examined average July temperatures going back as much as 65 years from 16 weather stations in the study area. The data confirmed long-term regional cooling, and four of the stations showed "significant long-term decreases" in July temperatures.
Short-term trends of 30 to 40 years were more significant, Stohlgren said, with all but one station showing cooling. Some reported decreases of as much as 3.5 F per decade.
The researchers also looked at the locations of seedlings of six kinds of conifers in Rocky Mountain National Park. Most of the trees – ponderosa pine, Douglas fir, lodgepole pine, Englemann spruce, subalpine fir and limber pine – grow at specific elevation ranges (and hence in defined temperatures) and require certain amounts of moisture.
What they found is that except for ponderosa pine, conifer seedlings are moving downslope to locations that historically had been drier. They concluded that "seedlings of the five tree species have become established (in areas that) are cooler and wetter now than in the past."
A third confirmation of cooler, moister air in the mountains came from June-August water flow records dating back 30 to 40 years for four rivers draining the Front Range section of the study area: the Colorado, Big Thompson and Laramie rivers and Boulder Creek.
Increased discharges for all four drainages suggest decreasing loss of moisture by trees and other vegetation through the process called transpiration. Researchers attributed trees’ reduced moisture loss to lower summer air temperatures. In addition, they noted, cooler spring and early summer temperatures may be delaying snowmelt into the mid-summer measuring period.
Stohlgren said that while Fort Collins is near the center of the study area and has grown to more than 100,000 residents over the past few decades, it’s not large enough to create a weather-influencing "heat island" like large cities do.
"Our thought was that it could have, but that (heat-trapping) reaction apparently is being overwhelmed by the combination of relatively small houses and big yards," he said. "The cooling effect of landscaping is overtaking the heating effect of asphalt, buildings and so on.
"The town has a very small footprint compared to the millions of surrounding acres devoted to agriculture," he said. "It’s an issue of scale."
The authors warn of possible alternate interpretations and call for more research but say the implications of their findings for Rocky Mountain National Park could be serious.
For example, the report said, cooler temperatures and increased frequency and intensity of thunderstorms could affect recreation and tourism, while more moisture in summer air could increase air pollution and lessen visibility. And even slightly cooler temperatures and increased summer moisture could lead to more grass and other ground-level vegetation in lower-elevation ponderosa pine forests, increasing the chance of wildfires. Nonetheless, Stohlgren downplays the model’s predictive abilities.
"It’s not a forecast or a prediction, but rather an assessment based on assumptions and conditions. What we are trying to assess is the vulnerability of natural systems to rapid environmental change," he said. For the mountains, that could mean cooler summers and more moisture. For Fort Collins and other Front Range cities, the prognosis is uncertainty.
"The effects of this cooling will mean more variability in weather, and it’s the increased variability that scares me more than anything else," Stohlgren said. "We face a higher chance of going from droughts to severe rainstorms, for example, and we may well see more days exceeding record temperatures.
"Our model suggests that if urbanization continues, variability will become even more drastic."