Apple Harvest

[In this series, BNA’s climate blog takes a closer look at U.S. regions covered in the draft National Climate Assessment. In addition to the Northwest, regions covered in the assessment include the Southeast and Caribbean , Great Plains , Northeast , Midwest , Southwest , Alaska and the Arctic, and Hawaii and U.S. Affiliated Pacific States.]

Compared to other regions in the United States, the Northwest likely will be spared the worst effects of climate change, especially in regard to sea-level rise and agriculture, according to the draft National Climate Assessment.

However, the Northwest region will be impacted by changes to water resources, including ocean acidification, and an increase in wildfires.

As detailed in an Energy and Climate Report article published Jan. 11, the draft, released by the National Climate Assessment and Development Advisory Committee, is not yet an official federal document. It will become so once it’s submitted to the National Science and Technology Council sometime this fall. The council is expected to review, modify, and approve a final draft by Jan. 30, 2014.

A national assessment on climate science and climate change impacts is due to Congress every four years under the Global Change Research Act of 1990. However, only two reports have been completed since the law passed, one in 2000 and the other in 2009.

The Northwest in the draft national assessment includes Idaho, Oregon, and Washington state.

Temperature, Precipitation

By 2100, temperatures in the Northwest are expected to increase by up to 3.3 degrees Fahrenheit if greenhouse gas emissions decline substantially. However, if emissions continue to rise, the temperature may increase by up to 9.7 F. 

In comparison, the draft estimates that the amount of warming by 2100 on average in the United States is expected to be between 3 to 5 F under a scenario involving substantial reductions in emissions and 5 to 10 F assuming continued increases in greenhouse gas emissions.

Changes in precipitation in the Northwest are less certain, with projections ranging from a 10 percent decrease and to an 18 percent increase by 2100. On a seasonal basis, model projections of regional precipitation also vary widely ranging from modest decreases to large increases in fall, spring, and winter, according to the draft.

However, climate models consistently show that if greenhouse gas emissions continue to increase, annual rainfall is expected to decrease during the summer by as much as 30 percent by 2100, especially in the Southeastern portion of the region.

Sea-Level, Ocean Acidification

So far, the Northwest coastline—thanks to tectonic uplift which is pushing the land upward—has experienced a slower rate of sea-level rise, compared to the global average rise of 8 inches. Globally, sea level is expected to rise another 1 to 4 feet by 2100.

According to the assessment, sea-level is projected to change along the Northwest coast as follows:

• between a decrease of 2 inches and an increase of 9 inches by 2030,
• between a decrease of 1 inch and an increase of 19 inches by 2050, and
• between an increase of four and 56 inches by 2100.

However, the scenario would change if a major earthquake strikes in a subduction zone, which is an area of the Earth’s crust where tectonic plates meet. Such an earthquake is expected within the next few hundred years, according to the draft assessment, and would “immediately reverse centuries of uplift and increase relative sea level about 40 inches or more.”

Other uncertainties remain. For example, El Nino conditions could temporarily increase sea levels by anywhere from 4 to 12 inches across the region, according to the draft.

Ocean acidification, which is caused by the absorption of atmospheric carbon dioxide, threatens “culturally and commercially significant marine species,” in the Northwest region, which has among the most acidified oceans in the world, the draft said. Furthermore, few options exist for "ameliorating projected ocean acidification.”

Water Resources

Climate change has already changed the timing of snowmelt in the Northwest, and it will continue to do so, further reducing the supply of water, according to the draft assessment.

Watersheds in the region are generally defined in the draft assessment as those derived mostly from winter rainfall, those derived mostly from snowmelt, and those derived from a mix of rain and snowmelt.

Basins with significant snow accumulation have shown the largest response to climate change, so far, due to decreased spring snowpack, earlier spring snowmelt, and a decrease in summer flow. This trend is expected to continue, the draft assessment said.

Areas with rain dominant basins will likely be at increased risk for flooding due to anticipated increase in the number of days of heavy downpours.

Meanwhile mixed-basins, such as the Yakima River, could change from deriving most of its stream flow from snowmelt to a rain-dominant basin by 2080 if emissions continue to increase through mid-century, the draft said.

Overall, the effect of climate change on water resources in the Northwest is expected to lead to reduced hydropower production (the region supplies 40 percent of the nation’s total) and decreased supply for irrigation, municipal, and industrial use. The region's freshwater fish, including salmon, steelhead, and trout will also be threatened.

The draft found that the region’s extensive water resource management structure that's in place now can lead to successful climate change adaptation.

But, several factors will need to be addressed—in the Columbia River basin, for example—including the over allocation of existing water supply, conflicting objectives, and limited management and operational flexibility, the draft said.


Warmer and drier conditions have already led to an increase in the number of wildfires in Western forests since the 1970s, the draft said. Together with insect and disease outbreaks, some forests will disappear by the 2080s.

Relative to the 1916-2007 period, the draft assessment said the amount of burned areas could quadruple by the 2080s, and the probability of a very large fire-year would increase from 1-in-20 chance to 1- in-2 chance.


Compared to other risks the Northwest faces due to climate change, agriculture is “perhaps best positioned to adapt,” the draft assessment said. For example, fully irrigated potatoes (55 percent of the U.S. supply is produced in the Northwest) are only expected to decline by 2 to 3 percent by the end of the century due to the fertilization effect of carbon dioxide that is expected to offset direct climate-related losses.

Apple production is projected to increase by 6 percent in the 2020s, 9 percent in the 2040s, and 16 percent by the 2080s, again because of the fertilization effect of CO2. However, higher temperatures may prevent the chilling effect some trees need for maturation, which would reduce the projected increase in yields, the assessment said.

Other field crops, such as wheat, and other types of fruit trees that are dependent on irrigation water in snowmelt-fed basins likely won't fare as well, the draft said, with some crops likely to see up to a 25 percent loss in yield by the 2100.

Adaptation measures could address agricultural impacts, especially if climate change occurs more slowly and “substantial investment” or significant changes in farm operations are made, such as shifting to new varieties of crops.

However, rapid climate change could present difficulties, the draft said, especially in regard to risk aversion among farmers, which could “hamper responsiveness to climatic changes.”