The Northwest Is Warming, and It’s Not Natural Variability

The northwestern United States has gotten significantly warmer over the last 100 years. Plus, the rate of warming has sped up in three of four seasons.

These long-term regional warming trends are closely tied to increases in greenhouse gases, CIRC scientists have found. This “anthropogenic” (human-caused) atmospheric pollution, the researchers say, is a “significant contributor” to the rising temperatures.

Researchers John Abatzoglou, David Rupp and Philip Mote examined trends in a variety of meteorological variables, including precipitation, daily temperature maximum and minimum, diurnal temperature range, length of freeze-free season, temperature on the coldest and warmest day of the year, growing season potential evaporation, and climatic water deficit. One significant finding is that the largest warming trends are occurring on the coldest day of the year.

The increase of 1.3 degrees Fahrenheit, which is statistically significant, cannot be attributed to volcanic activity or changes in the sun’s output, as some people speculate. Nor can they be attributed to changes in ocean temperature or atmospheric circulation patterns, which account for much of the climate variability in the Northwest region.

A notable exception to the overall warming trend is springtime over the last 30 years, when temperatures showed a slight decrease. This decrease, according to the scientists, can be attributed to the El-Nino Southern Oscillation and Pacific-North American pattern combining to mask the anthropogenic warming signal during this period.  All other seasons, and longer periods of record, show warming trends.


Citation: Abatzoglou, J. T., D.E. Rupp, & P.W. Mote (2014). Seasonal climate variability and change in the Pacific Northwest of the United States, Journal of Climate, 27, 2125-2142, doi: 10.1175/JCLI-D-13-00218.1.

David Rupp is a researcher for OCCRI at Oregon State University. He works on CIRC’s Climate Science and Climate Tools efforts. Interested in climate variability and change, and, in particular, in how these two factors impact the hydrological cycle and water resources, David’s work assessing how well Global Climate Models perform in the Northwest has become the foundation of much of CIRC’s Climate Tools and Community Adaptation efforts, including the Integrated Scenarios and Willamette Water 2100. 


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