Average annual temperatures have increased approximately 2°F since the beginning of the 20th century. Winter warming has been characterized by rising nighttime temperatures, with the number of very cold nights falling far below average during recent years. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
Snowpack plays a critical role in spring and summer water supplies. Projected rising temperatures will lead to more occurrences of rain falling instead of snow, and earlier melting of the snowpack; this could have negative impacts on critical sectors.
Precipitation varies greatly across this diverse state and throughout the year. Projected increases in winter precipitation and decreases in summer precipitation will change the dry season availability of water, leading to challenges for water management. Both the frequency of wildfire occurrence and wildfire severity are projected to increase in Oregon.
Oregon’s climate varies widely from eastern to western regions of the state. On the western side, temperatures are generally mild due to the Pacific Ocean’s moderating effect. The Pacific Ocean also provides abundant moisture, causing frequent precipitation west of the Cascade Mountains from October to May. Temperatures in the central and eastern portions of the state exhibit a greater annual and diurnal range. The Cascades block the flow of moisture and as a result it is much drier east of the Cascades. Oregon seldom experiences severe thunderstorms, compared to other states in the nation.
Since the beginning of the 20th century, temperatures have risen approximately 2°F, and temperatures in the 1990s and 2000s have been higher than any other historical period (Figure 1). The year 2014 was the 3rd warmest and 2015 was the warmest year since records began in 1895 (1934 was the 2nd warmest). During the period of 2005–2009, the state experienced the largest number of extremely hot days (days with maximum temperature above 100°F) in the historical record (Figure 2). In addition to the overall trend of higher average temperatures, the state has experienced below average numbers of very cold nights (days with minimum temperature below 0°F) over the past two decades (Figure 3). The number of days below freezing was lowest during 2000–2004, but has been near average during the last 10 years (Figure 4a). The state rarely experiences warm nights (days with minimum temperature above 70°F) due to the moderating effects of the Pacific Ocean in the west and the low humidity in the east (Figure 4b). Minimum temperatures are increasing at a faster rate than daytime maximums, however, with the summer of 2015 experiencing the warmest minimum temperatures on record.
Regional precipitation varies widely across the state and from year to year, with areas west of the Cascades also experiencing a large variation in rainfall amounts between seasons. Portions of the Coast Range receive in excess of 100 inches of precipitation annually, while some of the desert areas in the eastern part of the state receive less than 10 inches. Statewide annual precipitation has ranged from a low of about 22 inches in 1930 to a high of about 49 inches in 1996, and precipitation can fluctuate greatly between years. When averaged over 5-year periods, precipitation has ranged from 26.2 inches annually during the driest period on record (1928–1932) to 39.5 inches during the wettest period on record (1995–1999) (Figure 4c). Long-term periods of wet and dry spells can have critical impacts on water supplies.
Unlike many areas of the United States, Oregon has not experienced an upward trend in the frequency of extreme precipitation events (Figure 4d). The number of heavy rain events (more than 2 inches of precipitation) has been highly variable over the historical record (since 1900). The past two decades have included the 5-year periods with the highest frequency of extreme rain events (1995–1999) and the lowest frequency (2000–2004).
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways and under the lower pathway a few projections are only slightly warmer than historical records (Figure 1).
Projected rising temperatures will increase the average lowest elevation at which snow falls (the snow line, or snow level) by about 300 feet per degree of warming. This will increase the likelihood that precipitation will fall as rain instead of snow, reducing water storage in the snowpack, particularly at lower elevations which are now on the margins of reliable snowpack accumulation. Since most of Oregon’s precipitation falls during the winter months,the snowpack at higher elevations is an important source of water during the drier summer months (Figure 5). Higher spring temperatures will also result in earlier melting of the snowpack, further decreasing water availability for critical sectors such as agriculture and recreation.
Although projections of overall annual precipitation are uncertain, winter precipitation is projected to increase (Figure 6) while summer precipitation is projected to decrease. When combined with increasing temperatures leading to a higher proportion of precipitation falling as rain, water available during the dry season from snowmelt is projected to decrease and pose challenges for water management. These changes are of particular concern for areas that rely on hydroelectric power, and for regions that depend on the availability of irrigation water from snowmelt-fed basins. For example, the 2015 snow drought caused hundreds of million dollars in crop losses, as well as negatively impacting local fish populations.
Wildfires are also of particular concern for the state, and have become more severe and costly in recent years. The Long Draw fire in 2012 was the state’s largest wildfire since the 1860s, burning over half a million acres in southeastern Oregon. Drier summers, along with higher temperatures and earlier melting of the snowpack, are projected to lead to an increase in the frequency and severity of wildfires.
Increasing temperatures raise concerns for sea level rise in coastal areas. Since 1880, global sea level has risen by about 8 inches. It is projected to rise another one to four feet by 2100 as a result of both past and future emissions due to human activities (Figure 7). Due to the movement of tectonic plates on the ocean floors, the Oregon coast is rising, a phenomenon known as “uplift.” In some parts of the Oregon coast, the uplift is exceeding the rate of sea level rise; consequently, sea level has dropped in these locations. However, the rate of sea level rise is projected to exceed the rate of uplift along the entire Oregon coast by the mid-21st century, resulting in sea level rise for all locations.
Nuisance floods are events in which water levels exceed the local threshold (set by NOAA’s National Weather Service) for minor impacts. These events can damage infrastructure, cause road closures, and overwhelm storm drains. Nuisance flooding events in Oregon are likely to occur more frequently as global and local sea levels continue to rise.