Washington Department of Ecology
Snowpacks in Oregon, Idaho, Washington and California and are expected be much smaller by mid-century if greenhouse gas emissions continue to rise, according to federal projections released Friday.
The Fourth National Climate Assessment, completed once every four years, asserts that the mild winter of 2014-15 may have foreshadowed the future.
“As a harbinger, the unusually low Western U.S. snowpack of 2015 may become the norm,” according to the report.
The highly anticipated assessment, written by government and university scientists, reports that average temperatures globally and in the U.S. have risen by 1.8 degrees since 1885. The report concludes that it’s “extremely likely” human-released greenhouse gases are the main cause.
In the Northwest — Oregon, Idaho and Washington — average temperatures are projected to rise by mid-century by 3.66 to 4.67 degrees, depending on different levels of carbon emissions. Temperatures in California and five other southwest states are projected to rise by 3.72 to 4.80 degrees.
The heating up is projected to continue in the latter half of the century, with average temperatures in the West expected to be about 8.5 degrees higher than current norms by 2100.
The low snowpack of 2015 preceded one of Washington’s most severe droughts. The warm winter was caused by natural forces, not human-caused climate change, but Washington State Climatologist Nick Bond said the odds of such winters occurring will increase as baseline temperatures rise.
“It wasn’t global warming, but it was a dress rehearsal for it,” he said. “We know the times they are a-changing, and they already have to a certain extent.”
As winter temperatures increase, the percentage of precipitation that falls as snow will decrease. The snow that does stick will melt earlier in the spring, potentially disrupting water-management practices, according to the assessment.
In a high-emissions scenario, the average winter snowpack in the Cascades will have 41 percent less water by 2050 and 90 percent by 2100. The Sierra Nevada, Klamath and Wasatch mountains are projected to have similar declines in snowpacks.
Precipitation is projected to increase in some places but decrease in other places. The northern U.S. is expected to have more precipitation in the winter and spring, according to the assessment.
“If we use our resources wisely, I could imagine for at least awhile, we’ll have enough water to get by,” Bond said.
Growing seasons may be extended, with the time between frosts increasing by a month or two, according to the assessment. The report warns, however, that new invasive weeds may thrive and that increased demand for irrigation could exceed the water supply.
Even in places that do get more rain, soils are expected to be drier because evaporation likely will outpace precipitation, according to the assessment.
THe number of large forest fires has been increasing in the West since the 1980 and that trend is expected to continue. Fires may also be held back if drought and insect infestations stunt forests, according to the assessment.