The following changes in climatic conditions and impacts have been observed in Alaska:
- Higher temperatures are contributing to earlier spring snowmelt, reduced sea ice, widespread glacier retreat, and permafrost warming.
- Between 1970 and 2000, the snow-free season increased by approximately 10 days across Alaska, primarily due to earlier snowmelt in the spring.
- Increased evaporation, combined with thawing of permafrost, has resulted in declines in the area of closed-basin lakes over the past 50 years.
- Increasing storm activity in autumn in recent years has delayed or prevented barge operations that supply coastal communities with fuel.
- High-wind events have become more frequent along the western and northern coasts. The same regions are experiencing increasingly long sea-ice-free seasons and hence longer periods during which coastal areas are especially vulnerable to wind and wave damage.
- Coastal erosion is causing the shorelines of some areas to retreat at average rates of tens of feet per year. The ground beneath several native communities is literally crumbling into the sea.
- The rate of erosion along Alaska’s northeastern coastline has doubled over the past 50 years.
- The Bering Sea pollock fishery has experienced major declines in recent years.
The following climate-related changes are projected for Alaska:
- Average annual temperatures in Alaska are projected to rise about 3.5 to 7°F by the middle of this century and 5 to 13°F by the end of the century.
- Higher temperatures are expected to continue to reduce Arctic sea ice coverage, which will increase coastal erosion and flooding associated with coastal storms and alter the timing and location of plankton blooms, which is expected to drive major shifts of marine species.
- Storm activity is expected to increase in the Bering Sea. An increase in the frequency and/or intensity of Arctic Ocean storms is also expected.
- Increases in evaporation due to higher air temperatures are expected to lead to drier conditions overall and an increased likelihood of summer drought.
- Marine ecosystems will experience continued perturbations, including northward shifts, with consequences for the commercial fishery and for the food supplies of indigenous populations.
See National Climate Assessment: Alaska Region
Hawai’i and the Pacific Islands
The following changes in climatic conditions and impacts have been observed in Hawai’i and the Pacific Islands:
- Mean sea levels are rising, particularly in the Western Pacific.
- The number of very warm nights has increased by 10 or more per year for Hawaii from 1950 to 2004.
- All four major Hawaiian Islands (O‘ahu, Kaua‘i, Maui, and Hawai‘i Island) have experienced more severe winter droughts since the 1950s, defined by a longer annual number of consecutive dry days.
- Over the past century, rainfall has decreased across much of the Pacific region. There has been a slight increase in rainfall in the westernmost Micronesian islands.
- Pacific Island habitats and species distributions have changed. For example, increasing temperatures are facilitating the upward migration of mosquito-borne diseases that cause mortality in Hawaiian native forest birds.
- Ocean heat content is rising and ocean chemistry is changing.
The following climate-related changes are projected Hawai’i and the Pacific Islands:
- Air and ocean surface temperature are expected to increase.
- The number of heavy rain events is likely to increase.
- In the Pacific Islands, the rainy season may shift from winter to summer.
- Hurricane (typhoon) wind speeds and rainfall rates are likely to increase with continued warming.
- Islands and other low-lying coastal areas will be at increased risk from coastal inundation due to sea level rise and storm surge, with major implications for coastal communities, infrastructure, natural habitats, and resources.
- The availability of freshwater is likely to be reduced, with significant implications for island communities, economies, and resources.
See National Climate Assessment: Hawai’i and Pacific Islands Regions
The following changes in climatic and related conditions have been observed in the Northwest:
- The region experienced an average temperature increase of 1.5°F over the last century, with some areas having an average increase of up to 4°F. Higher cool season temperatures have resulted in more precipitation falling as rain rather than snow and leading to an earlier snowmelt.
- The April 1 snowpack has declined substantially throughout the region. The average decline in the Cascade Mountains was about 25 percent over the past 40 to 70 years, with most of this due to the 2.5°F increase in cool season temperatures over that period.
- The timing of the peak spring runoff has been shifting over the past 50 years with the peak of spring runoff shifting from a few days earlier in some places to as much as 25 to 30 days earlier in others.
- A low oxygen dead zone off the coast of Washington and Oregon is believed to be driven by climate change.
- Ocean acidification is occurring along the Northwest coast.
The following climate-related changes are projected for the Northwest:
- Temperatures are projected to increase another 3 to 10°F by 2100.
- Increases in winter precipitation and decreases in summer precipitation are projected, though these projections are less certain than those for temperature.
- Heavier winter rainfall suggests an increase in saturated soils and, therefore, an increased number of landslides on coastal bluffs, which will be especially problematic in areas where there has been intensive development on unstable slopes. Sea level rise will exacerbate these conditions.
- Further declines in the region’s snowpack are expected, with variations due to latitude, elevation, and proximity to the coast. A decline in the April 1 snowpack in the Cascades of 40 percent is projected by the 2040s.
- The trend in the earlier timing of the peak spring runoff is projected to continue, with shifts anticipated of 20 to 40 days. However, major shifts in the timing of runoff are not expected in areas dominated by rain instead of snow.
- Extreme high and low streamflows are also projected to change. Increased winter rainfall is expected to lead to more flooding in some areas, and low flows in the late summer are projected to decrease further.
See National Climate Assessment: Northwest Region
The following changes in climatic conditions have been observed in the Southwest:
- The average temperature in the Southwest has increased about 1.5°F compared to a 1960-1979 baseline period.
- As of 2009, much of the region remained in a drought that began around 1999; the most severe western drought in the last 110 years, which has been exacerbated by record warming.
- Precipitation generally decreased during the summer and fall in the Southwest, while winter and spring have had increases in precipitation.
The following climate-related changes are projected for the Southwest:
- By 2100, the average annual temperature is expected to rise from about 4 to 10°F above the 1960–1979 baseline period; summer temperature increases are expected to be greater than the annual average increase in some areas.
- There is an increased probability of drought.
- Increased flood risk in the Southwest is likely to result from a combination of decreased snow cover on the lower slopes of high mountains and an increased fraction of winter precipitation falling as rain and therefore running off more rapidly. The increase in rain on snow events will also result in rapid runoff and flooding.
- Changes in the timing and amount of river flow during the winter and spring in the Sacramento-San Joaquin River Delta is expected to double the risk of flooding by mid-century and increase it by a factor of eight by 2100.
- There is presently no consensus on how the region’s summer monsoon (rainy season) might change in the future.
- Limitations imposed on water supply by projected temperature increases are likely to be made worse by substantial reductions in rain and snowfall in the spring months, when precipitation is most needed to fill reservoirs to meet summer demand.
See National Climate Assessment: Southwest Region
Find more resources on the Western Regional Resources List.