Recent Wyoming temperature trends, their drivers, and impacts in a 14,000-year context

Wyoming provides more fossil fuels to the remainder of the United States than any other state or country, and its citizens remain skeptical of anthropogenic influences on their climate. However, much of the state including Yellowstone National Park and the headwaters of several major river systems, may have already been affected by rising temperatures. This paper examines the historic climate record from Wyoming in the context of similar to 14,000-year temperature reconstructions based on fossil pollen data. The analysis shows that 24 of 30 U.S. Historical Climatology Network records from the state show an increase in the frequency of unusually warm years since 1978. Statewide temperatures have included 15 years (50%) from 1978 to 2007 that were greater than 1 sigma above the mean annual temperature for 1895-1978. The frequent warm years coincide with a reduction in the frequency of extremely low (<-20A degrees C) January temperatures, and are not well explained by factors such as solar irradiance and the Pacific Decadal Oscillation. Linear regressions require inclusion of atmospheric greenhouse gas concentrations to explain the multi-decadal temperature trends. The observed warming is large in Yellowstone National Park where 21 years (70%) from 1978 to 2007 were greater than 1 sigma above the 1895-1978 mean; the deviation from the mean (> 1A degrees C) is greater than any time in the past 6,000 years. Recent temperatures have become as high as those experienced from 11,000 to 6,000 years ago when summer insolation was > 6% higher than today and when regional ecosystems experienced frequent severe disturbances.