We are now living in a time that’s the warmest in the history of modern civilization, according to the latest Climate Science Special Report, part of the National Climate Assessment. Global annual average surface temperatures have risen nearly 1.8F (1C) since 1901. Sixteen of the warmest years on record have taken place during the past 17 years.
Scientists have calculated future scenarios for the coming decades that include sea-level rise, more severe rainfall and an increase in the frequency of heatwaves. Some areas will get drier, others wetter. No matter what the future brings, one thing is clear: Impacts from a warming climate are already being felt across the American West, with changes to ecosystems and water supply.
Scientists have been documenting this for more than a decade. A study published in 2001 showed that spring in the Western U.S. was arriving earlier, as measured both hydrologically (based on snowmelt) and phenologically (based on plant behavior).
“That was a landmark paper because it was suddenly clear that things were already changing in response to warming,” said Philip Mote, a professor in the College of Earth, Ocean and Atmospheric Sciences at Oregon State University and director of the Oregon Climate Change Research Institute. “This was no longer something we were predicting would eventually happen but something that was already happening.”
Water Supply Impacts
If you’re wondering what climate change feels like, look to Oregon. A warm winter in 2015 with temperatures above the 20th-century average meant that most of the precipitation fell as rain instead of snow, explained Mote. And even though total precipitation was near normal, the state still ended up with drought conditions when spring and summer rolled around and there was little snowmelt coming out of the mountain streams.
Wells went dry. Reservoirs failed to fill. At the popular Detroit Lake reservoir, the water level dropped below the boat ramp, ending the motorboat season in May. And there were challenges providing flows sufficient to keep fish populations healthy, said Mote. It was the same time that a combination of warm temperatures and little precipitation gave California its smallest snowpack in centuries.
“In the West we’ve documented, both at lower elevations and up in the mountains, that over the last 60 years there has been a pretty clear decrease in the fraction of precipitation that is falling as snow,” said Mote. It’s a trend that’s expected to increase with more warming.
The places where this is the most pronounced are areas that are close to freezing point, such as Oregon’s Cascade Mountains and parts of California’s Sierra Nevada. By contrast, in the higher-elevation Rockies, “It’s much colder, so even though they have been experiencing the warming trend they are not yet near a freezing level,” said Becky Bolinger, a climatologist and drought specialist at the Colorado Climate Center and a researcher at Colorado State University’s Department of Atmospheric Science. “Some of the impacts in the colder regions are a shift in the timing of snowmelt – it’s melting a little bit earlier. But the effects are more pronounced in the Sierra Nevada or the Pacific Northwest.”
Changes in the timing or the amount of snowpack directly impact water availability and management for downstream communities, as snowpack, essentially a giant reservoir, is often the biggest water storage system on which Western communities rely. If snowpack decreases or melts earlier it may impact water available for fish at crucial times of the year or what’s needed by farmers during drier summer months.
Higher temperatures also impact river flows. In California, spring runoff from the Sierra Nevada to the Sacramento River has declined by 9 percent in the past 100 years, according to the state’s report Indicators of Climate Change in California. Other Western rivers have also shown impacts.
A study in the journal Water Resources Research published in March found that between 2000 and 2014 the Colorado River had its worst drought on record, with flows 19 percent below the 20th-century average. But the researchers discovered that not all of that reduction was due to reduced precipitation – about one-third of the loss in flow was a result of higher temperatures resulting from climate change. “Previous comparable droughts were caused by a lack of precipitation, not high temperatures,” the authors wrote.
There is interest in California now, following its five-year drought, to better understand the role that higher temperatures played.
“One of the things that has come out of drought is an interest in figuring out how the warmth is now an element of the drought to be folded in with the below-average precipitation, and trying to figure out the interplay between the two is an area that researchers are starting to dig into,” said California’s state climatologist Michael Anderson.
But scientists do know that climate change is having some effect on drought, at least in terms of intensity. A 2015 study about California’s drought published in Geophysical Research Letters found that “there is widespread consensus that warmth has intensified the effects of the recent precipitation shortfall by enhancing potential evapotranspiration” and “anthropogenic warming has substantially increased the overall likelihood of extreme California droughts.” Higher temperatures make drought impacts worse.
Since the 1980s, the number of large wildfires in the Western U.S. has risen, according to the U.S. Climate Assessment. Although there are numerous factors that drive wildfires – weather, land management, human activity, elevation, ecology – climate change, too, can have an impact in some places, and wildfire areas are expected to increase with warming temperatures.
One way is by driving an increase in fuel aridity, which is the drying out of vegetation. A paper published in the Proceedings of the National Academy of Sciences found that climate change has contributed to an increase in wildfire danger and doubled the area of land burned since 1984.
Warming temperatures also increase the risk of lightning strikes – 1.8F (1C) of warming translates to a 12 percent increase in a lightning strike.
Wildfire risks and conditions, though, vary greatly by geography and topography across the West.
“In 2015, as Oregon and California were struggling through drought, Washington fared slightly better hydrologically,” said Mote. “But they had really big fires that year, and that’s a direct consequence of having very little snowpack, so the forests dry out much sooner and fire season can be a lot longer and more severe when there is low snowpack.”
This is especially true for northern states such as Montana, Idaho, Oregon and Washington, where a low snowpack followed by a dry summer can lead to fires, said Mote.
In the past 30 years, the length of the fire season has grown by 2.5 months and severity is expected to increase as the climate continues to warm.
This can have big economic costs and societal impacts. This year $2 billion was spent fighting fires. “Every time I talk to public health folks about climate change, the first thing they bring up is fires and air quality,” said Mote. “This past fire season, as well as 2015, big parts of the Northwest were blanketed in smoke for weeks at a time and it led to respiratory problems and people missing work.”
Warming is also impacting the health of forests. Bolinger in Colorado said that one of the biggest impacts from warming she’s seen is the migration of pine bark beetles in national forests in the Western U.S., which are responsible for millions of dead trees. “The warming has allowed them to migrate to different areas – that’s been one of those widespread impacts that we’ve seen in the past couple of decades from that warming trend,” she said.
Research has shown that warming temperatures have helped beetles move to elevations at which they haven’t been before and where they are encountering tree species that don’t have the defenses to fight off the invaders, resulting in swathes of dead trees across Western forests. Over the past 17 years, bark beetles have decimated 54.3 million acres of forest across the West.
Beetle numbers are also increasing because more are surviving the warmer winters when, previously, cold temperatures would have killed the larvae. And in areas impacted by drought, already stressed trees are unable to fend off the insects. Drought and beetles have combined to kill more than 100 million trees in California since 2010.
Forests have also been impacted by warming in other ways. “There has been an incursion of trees into alpine meadows over the last few decades,” said Mote. One example has been in Washington’s Mount Rainier National Park, where it could impact the area’s biodiversity. Alpine meadows also have a key role in storing and releasing water.
“Since 2001, every year has brought more evidence that the world is changing around us,” said Mote. “Some in ways that have been anticipated for decades and some in ways that are very surprising.”