Recent Accelerated Warming in Western United States Mountains

The mountainous western portion of the United States and North America have been warming since the mid 1970s, by a total of about 1 degree C. This warming has been most pronounced in spring, present but less obvious in winter, and nearly absent in autumn. In summer, a slow rise in temperature was under way until the late 1990s. Over the past 8 years a succession of very warm summers has occurred, with multi-year temperatures well above all similar past multi-year averages. Of further interest is that the major Colorado River drought now underway also began about this time. Contributing to that drought were several springs that featured a month with much above normal temperatures that greatly hastened snow melt at a time of year when snow pack is usually still accumulating. These warming effects have been seen in lower elevation and higher elevation onitoring networks. They do not appear to be a product of observational methodologies. There is some evidence that other parts of the global climate system have behaved somewhat differently over those years. Potential relationships or causes for this apparent acceleration in warming will be discussed.