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Climate Change at Rocky Mountain National Park

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Rocky Mountain National Park (RMNP) protects 415 square miles of spectacular mountain environments in northern Colorado. It is home to a diversity of ecosystems – alpine, subalpine, and montane – that are each uniquely adapted to the climate zone of their elevation. This is why, as I learned during a recent visit,  climate change is a serious issue for the park.

According to the National Park Service, the average annual temperature in RMNP has increased 3.4°F over the past century. A report from a weather station inside the park (Grand Lake), shows that the number of frost-free days has increased from an average of 65 in the mid-20th century to an average of 100 in this past decade.

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In the 20th century, the area including Rocky Mountain National Park experienced a warming trend. The five-year rolling average (thick red line) allows the viewer to look beyond annual variability to focus on long-term trends. (Analysis of PRISM data, original source Daly 2008). Credit: NPS

This warming trend, says the NPS, has caused a number of environmental changes in RMNP. The winter snowpack is melting approximately 2 to 3 weeks earlier, resulting in less water being available for people, plants, and animals during the summer. There has been an explosive increase in the number of mountain pine beetles surviving the now warmer winter months, allowing them to devour more trees. The phenology, or the timing of natural events, can also be thrown out of sync when warm spring weather arrives earlier than normal. Wildflowers that bloom before the arrival of butterflies, for example, can leave the insects with a reduced food source. This puts a kink in how the larger food chain fits together.

In the park’s alpine tundra region, the American Pika is at particular risk. According to scientists, this small furry relative of the rabbit can only live at high elevations in cool, rocky environments. They say it cannot survive in temperatures above 75°F for more than a few hours.  While other species adapted to lower elevations can move upslope as average temperatures rise, the pika has nowhere to go.

American Pika on the rocky terrain of RMNP's alpine tundra region. Image Credit: The Weather Gamut.

The American Pika, a native of RMNP, is sensitive to even small changes in climate.  Image Credit: The Weather Gamut.

Another impact of climate change is the spreading of non-native plant species that can thrive in the now warmer environment of RMNP. While Cheatgrass, a native of Eurasia, is found throughout the western US, it used to be limited to lower elevations.  Now, it is found as high as 9,500 feet in parts of RMNP. In addition to crowding out native plants and changing the look of the landscape, this invasive species is highly flammable. Its presence increases the danger of wildfires – something the West certainly does not need.

While these are just a few examples of the observed and expected impacts climate change is and will have on RMNP, scientists are continuing to research how additional increases in temperature will affect this national treasure.

Why Air Temperature Decreases with Height

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While visiting Colorado recently, I had the opportunity to explore Rocky Mountain National Park, and it was largely a vertical experience. Within its borders are 72 named peaks that reach above 12,000 feet in elevation. Traveling from the Beaver Meadow Visitor Center – elevation 7,840 feet – to the Alpine Visitor Center – elevation 11,796 feet – the drop in temperature was anything but subtle.

The reason for air being cooler at higher elevations is twofold. First, the sun’s rays heat the Earth’s surface, which in turn, radiates that warmth into the atmosphere. As you climb in altitude, there is less surface area of land available to heat the air. Second, as air rises, it expands and cools. This is because air density and pressure aloft are lower than at the surface.

The exact rate at which the temperature decreases with height – the environmental lapse rate – varies with location and daily conditions. On average, however, for every 1000 feet gained in elevation, the temperature drops by about 3.6°F.

Image Credit:British Geographer

Image Credit: The British Geographer

Why U.V. Intensity Increases with Elevation

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One of the most important items on the packing list for my trip to Colorado this past week was sun-block. Averaging 300 days of sunshine per year at a mean altitude of 6,800 feet above sea level, the U.V. index in the Centennial state can range from high to extreme during the summer months.

The U.V. index is a scale that measures the intensity of the sun’s ultraviolet radiation. Readings vary from place to place as local factors affect the amount of U.V. light that reaches the ground. These include, the thickness of the ozone layer, latitude, season, cloud cover, and elevation. Developed by the NWS and EPA in the early 1990’s, it informs the public about the daily health risk of unprotected exposure to the sun.

At high elevations, the atmosphere thins and is less able to absorb U.V. radiation. With every 1000-foot increase in height, according to the National Institutes of Health, U.V. levels increase by about 4%.  So, in Denver, “the Mile High City”, U.V. radiation is about 20% stronger than a location at sea level at the same latitude. Heading up into the Rocky Mountains, where peaks can reach above 14,000 feet, the U.V. intensity soars even higher.

Credit: EPA

Credit: EPA