As temperatures have increased, more winter precipitation has fallen as rain instead of snow, and the snow is melting sooner, reported the study published in the journal Science. The result is that rivers are flowing faster in the spring, raising the risk of flooding, and slower in the summer, raising the risk of drought.
Between 1950 and 1999, the period the researchers examined, the total amount of precipitation that fell in the Rockies, the Cascades, the Sierra Nevada and smaller mountain ranges across the West did not vary significantly. But the portion arriving as snow steadily declined, falling by an average of 4.3% per decade in the nine areas included in the study. Average daily minimum temperatures between January and March climbed an average of 0.34 degrees Celsius per decade.
Human-caused global warming has been shrinking the snowpack across the mountain ranges of the West for five decades, suggesting that the region's long battle for water will only get worse, according to a computer analysis released today.
Based on their simulations, along with historical data on snowpack, temperature and river flow, the researchers concluded that there was a less than 1% chance that the last 50 years were a natural aberration.