Average annual temperature has increased about 2°F since the early 20th century. Winter warming has been characterized by a much below average occurrence of extremely cold days since 1990. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
Colorado’s mountains are the headwaters of four major rivers that supply water to 18 other states downstream. While future changes in annual precipitation are uncertain, warming temperatures would cause earlier snowmelt and runoff, and tend to reduce overall water availability.
Severe droughts have occurred in recent years. Projected warming will increase the rate of loss of soil moisture during dry spells, increasing the intensity of future naturally-occurring droughts. As a result, the frequency of wildfire occurrence and severity is projected to increase in Colorado.
Colorado has a varied climate due to its inland continental location and wide range of topography. Major geographic features include the High Plains in the east, the high mesas and canyons in the far west, and the Rocky Mountains in the central part of the state. Colorado’s semi-arid climate means temperatures vary widely from day to night. Temperatures can also vary dramatically over short distances due to elevational changes. The one hundred highest summits in the Rocky Mountains all lie within the state, and Colorado has the highest average elevation of any state, at 6,800 feet.
Average annual temperatures for Colorado have remained consistently higher than the long-term average (1895–2015) over the past two decades (Figure 1). Since 2000, the state has experienced the highest average spring and summer temperatures in the historical record (Figure 2). The hottest year on record was 2012, with an average annual temperature of 48.3°F. In addition to the overall trend of higher average temperatures, the state has experienced an above average number of very hot days (days with a maximum temperature exceeding 95°F) (Figure 3) and a decrease in the number of very cold nights (days with a minimum temperature below 0°F) since 1990 (Figure 4). Colorado rarely experiences warm nights (days with a minimum temperature exceeding 70°F) due to its high elevation and generally dry climate. The greatest number of warm nights occurred in the 1930s, and, along with other parts of the United States, the state has also seen an above average number in recent years (Figure 5a).
Annual total precipitation is 17 inches on average statewide, but has ranged from a low of 11.85 inches in 2002 to a high of 25.52 inches in 1941. The driest multi-year periods were in the 1930s and 1950s, and the wettest were in the 1940s, 1980s, and 1990s (Figure 5b). The driest 5-year period was 1952–1956 and the wettest was 1995–1999. Local annual precipitation varies widely, from only 7 inches in the middle of the San Luis Valley to more than 60 inches in a few mountain locations. Seasonal patterns also vary; in the eastern plains and valleys, most precipitation falls during the spring and summer, but the mountain peaks receive the most precipitation during the winter months. Historical statewide-average seasonal precipitation amounts have been variable, however, as Colorado has experienced above average fall precipitation since 1980 and below average spring precipitation since 2000 (Figure 5c). Snowfall totals also vary across the state, with the high mountains receiving 150 inches to more than 400 inches per year. Occasional blizzards can have significant impacts. In December 2006, two blizzards dropped heavy snow along the Front Range. The first blizzard on December 20–21 dropped 1–3 feet of snow, shutting down the Denver International Airport for two days during the busy holiday travel season. A second storm over December 28–29 dropped another 1–2 feet of snow across the region.
Colorado is a headwaters state and, thus, changes in precipitation can impact a much larger area than the state itself. Four major U.S. rivers have their source in Colorado: the Colorado, the Rio Grande, the Arkansas, and the Platte. Yearly variations in snowpack depths have implications for water availability across the west as snowmelt from the winter snowpack feeds many rivers and streams. For example, low snowpack levels in 2012 resulted in low spring runoff across the southern Rockies (Figure 6). However, in years with heavy snow cover, snowmelt, in combination with widespread spring rains, has the potential to cause spring flooding. There is no long-term trend in snowpack water over the past 80 years (Figure 6).
Thunderstorms are common in the eastern plains and eastern mountain slopes, with the more intense storms bringing damaging hail and occasional flash floods. The mountains are effective thunderstorm generators, especially during the summer months when humidity is highest. Northeastern Colorado lies in “Hail Alley,” the most hail prone area in the entire country, and locations in this area experience an average of six hail days per year. One of the most damaging hailstorms in the state occurred on July 20, 2009 in the Denver metro area. The storm produced hail greater than one inch in diameter and caused more than $750 million in damages. Locally intense thunderstorms can also cause flash flooding, particularly in the lower foothills east of the mountains. Unlike many areas of the United States, Colorado and other southwestern states have not experienced an upward trend in the frequency of extreme precipitation events (Figure 5d). However, these events can be devastating when they do occur. From September 10 to 16, 2013, heavy rainfall from a nearly stationary weather system caused extensive river flooding across the foothills and the Front Range, some of the worst in state history. More than 15 inches of rain fell in some locations and the storm caused more than $2 billion in damages.
Colorado frequently experiences droughts, which can increase the risk of wildfire. In 2012, the state experienced one of the worst wildfire seasons in its history due to extremely dry conditions. Two fires—the High Park Fire and the Waldo Canyon Fire—were particularly devastating as they burned in the wildland-urban interface along the Front Range. The High Park Fire, caused by a lightning strike, was the second largest fire in Colorado’s history, burning more than 85,000 acres. The Waldo Canyon Fire was the most destructive fire in the state’s history, destroying 346 homes and resulting in two deaths.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records. Increases in heat wave intensity are projected, but the intensity of cold waves is projected to decrease continuing recent trends (Figure 4).
Although projections of overall annual precipitation are uncertain, precipitation is projected to increase in the winter (Figure 7) and possibly decrease in the summer. Although heavier winter precipitation could provide important water for the water-scarce Southwest, projected rising temperature will increase the average lowest elevation at which snow falls (the snow line), with more precipitation falling as rain instead of snow, reducing water storage in the snowpack, particularly at those lower elevations which are now on the margins of reliable snowpack accumulation. Warmer temperatures will also result in earlier melting of the snowpack and increased evaporation of soil moisture, further decreasing water availability during the already dry summer months.Extreme precipitation events are also projected to increase because of increases in the atmospheric water vapor in the oceanic water vapor source regions (due to rising sea surface temperatures) for Colorado’s extreme events.
The intensity of droughts, a natural part of Colorado’s climate, is projected to increase (Figure 8); and with so many river basins originating in the state, any changes in precipitation patterns pose a risk to water supplies for cities and farms across the region. Higher temperatures will increase the rate of loss of soil moisture, leading to more intense drought conditions. Increased drought intensity, along with possibly decreased summer precipitation, will increase the risk of wildfire occurrence and severity.