Human alteration of the climate puts at the risk the very values that make Colorado such a special place to live, to work, to enjoy.


Climate change: Already happening

In Colorado, temperatures so far this century have been 1.3°F hotter than in the last three decades of the last century.

 

The figure above shows average annual statewide temperatures in Colorado, compared to a 1971-2000 baseline. In the first 17 years of this century, temperatures averaged 1.3°F hotter than the baseline. Figure by the Rocky Mountain Climate Organization, from National Oceanic and Atmospheric Administration data.


Further changes: Up to us

How much more the climate changes and how much it affects us, in Colorado as elsewhere, will be determined primarily by future levels of heat-trapping emissions.

Heat-trapping pollution already in the atmosphere will result in some further increase in temperatures. About another 0.5°F increase over the next few decades even if all human emissions miraculously stopped. Beyond that, though, future levels of global emissions will determine the extent of future temperature increases and other climate changes.

To assess possible climate futures, scientists currently use four different scenarios of future heat-trapping emission levels, shown below. The different possible futures they represent, ranging from continued high emissions to sharp reductions, would lead to very different levels of impacts.

The figures above show the four current scenarios used for projected future climate change. The scenarios include continued high emission increases (a business-as-usual approach without further emission reductions), the blue lines above; two medium scenarios, the black and red lines; and very low future emissions (leading to global temperatures of about 2°F to 2.5° above preindustrial levels in this century), the green lines. Figure A, on the left, shows annual emissions of carbon dioxide, the major heat-trapping gas; B, in the middle, shows the resulting atmospheric concentrations of carbon dioxide; and C, the resulting temperature increase from heat-trapping pollution, in watts per square meter. Figures courtesy of Detlef van Vuuren.


Temperatures

If heat-trapping emissions keep rising at a high, unchecked rate, Colorado by mid-century could average about 5°F hotter — roughly four times the temperature increase we are now experiencing. And later in the century, temperatures would keep getting hotter.

But sharply reducing emissions could hold Colorado’s temperature increase to about 3°F in mid-century, with no further increase.

The figure below shows (on the left) Colorado’s historic temperatures and (on the right) projections from climate models based on two possible levels of future heat-trapping emissions.

 

 

The figure above shows Colorado’s historic temperatures on the left, and projected temperatures with two alternative levels of future global heat-trapping emissions — continued high emissions in red, and very low emissions in green. For the projections, the brighter colors on top of the columns show the range of the middle 80 percent of individual projections from different climate models, and the numerals in the columns show the medians of the projections. Figure by the Rocky Mountain Climate Organization, from National Oceanic and Atmospheric Administration data.

The brighter part of the projection columns above show the range of the middle 80 percent of all projections from climate models, and the numerals show the medians of all projections.


Extreme conditions

Extreme temperatures are expected to go up more than average temperatures. With continued high emissions, a typical year in mid-century in Denver could have seven days 100° or hotter. The hottest year in mid-century could have 25  100°-plus days.

Near the end of the century, a typical year could have a month’s worth of 100°-plus days. The hottest year, two and a half months’ worth.

Again, though, sharp emission reductions could make all the difference. In mid-century, the hottest year could have ten 100°-plus days, and late in the century only eight.

Even more startling is how many 100-degree days Denver could have in extreme years — in the projected hottest year in each of the upcoming 20-year periods.

In the figures above, the actual frequency of 100-degree days per year in the Denver metro area is shown on the left, and projections on the right. For the projections, the brighter parts of the columns show the ranges of the middle 80 percent of the projections, and the numerals are the medians of all projections. The projections cover four different possible levels of future heat-trapping emissions. For more information, see the Rocky Mountain Climate Organization report, Future Extreme Heat in the Denver Metro Area (2017).


Public health impacts

Among the new risks to public health and safety would be the impacts of extreme heat, which can cause heat strokes, other health problems, and even deaths.

The Centers for Disease Control reports that extreme heat can be dangerous to people’s health – even fatal. Heat waves result in increased hospital admissions for heat-related illness, as well as cardiovascular and respiratory disorders. And any cities across the United States have seen large increases in death rates during heat waves.

In the Denver area, where only half the houses now have air conditioning, the health effects of many more 100-degree days could be serious.  As the public health chapter of the Colorado Climate Change Vulnerability Study points out, the health effects of extreme heat are “rarely discussed in Colorado” because our historic climate has kept heat waves from having effects as severe as in other parts of the country — but this may actually make us more susceptible, because we are less prepared to respond to extreme heat events. Coloradans who are particularly vulnerable to extreme heat include children, adults with diabetes or cardiovascular disease, and seniors, according to the Colorado Health Institute.


Snow and water impacts

River flows in the Colorado River, our state’s largest water source, are now dropping four percent for each 1°F increase in average temperature. That could be a 40 percent reduction in this century.

Farmers and ranchers, who need supplemental water from reservoirs, ditches, sprinklers, and stock ponds to make a go of it here — and who use most of Colorado’s water — are especially at risk.

Already, the winter snowpacks that provide most of our water are shrinking, and projections based on future climate change generally show future decreases in water supplies in all Colorado rivers.

The character of droughts in the West has already begun changing, as water shortages now result from the evaporative effects of higher temperatures. In the Colorado River, the effect has been measured — that  four percent reduction for each 1°F. The effects could be almost unimaginable. This river drains one-twelfth of the contiguous United States, and its water is so precious that it is taken out of that river basin to meet the water needs of the largest cities in each of seven states — Denver, Phoenix, Los Angeles, Las Vegas, Albuquerque, Salt Lake City, and Cheyenne.

Not yet well understood is the other side of the equation — the extent to which higher temperatures will increase the demand for water. But in a hotter climate, cities, farms, and ranches will all need more water,  putting water supplies and demands further out of balance.


Wildfire and forest impacts

Wildfires have already increased across the West as the climate changes, and scientists say they likely will continue getting more frequent and more extensive if emissions are unchecked. Other risks include widespread tree mortality as hotter and drier conditions exceed what our forest’s trees can tolerate.

The Waldo Canyon fire in 2012, which burned a record 259 homes and killed one person, and the Black Forest fire in 2013, which burned a new record of 488 homes and killed two people, each started on days of record-setting or record-tying high temperatures along the Front Range. Not surprisingly, then, even higher temperatures would mean more wildfires. To cite just one of many studies, a National Academy of Sciences report projects that just a very modest 1.8°F increase in average temperature could lead to a 656% increase in area burned in this part of the West.

As documented in a report by the Union of Concerned Scientists and the Rocky Mountain Climate Organization, Rocky Mountain Forests at Risk, the U.S. Forest Service projects that medium-high emissions could by 2050 eliminate most of the pine, spruce, and fir trees that cover our mountains, and nearly half of our aspens, as the climate becomes unsuitable for their survival.


Road and building

Just in Boulder County, high emissions could cost an extra half a billion dollars by mid-century to maintain and repair roads, bridges, and non-residential buildings.

The only detailed analysis yet done of infrastructure impacts in Colorado is a new analysis done for Boulder County. It projects an extra cost of $464 million by 2050 to operate and maintain local infrastructure — not counting people’s homes — if we have cotinuing high emission increases. With a medium level of future emissions, the extra costs could still be $376 million.  The biggest single impact could be road damages from high heat, driving a 60 percent increase in maintenance costs.


Other impacts

Many studies document a wide range of potential Colorado impacts from climate change.

The single best compilation of current information on the full range of our impacts from climate change is the Colorado Climate Change Vulnerability Study (2015), a report to the state government by the University of Colorado Boulder and Colorado State University.

For information on underlying changes to our state’s climate, the best source is Climate Change in Colorado (2014), a report to the Colorado Water Conservation Board by the Western Water Assessment Program at the University of Colorado Boulder. Although intended primarily for water managers, it has basic climate change information useful for many purposes.


These impacts can largely be avoided — see What Can Be Done.