About Pacific Northwest Climate

Climate Change

Climate change ("global warming") is expected to have significant impacts on the Pacific Northwest (PNW) by mid-21st century. The following provides an overview of past and projected climate change as researched by the Climate Impacts Group (CIG) (for regional change) and research communities around the world (for global change). For more detailed information on global climate change, please see our recommended links.

The Science of Climate Change

Understanding of the science of climate change is based on thousands of papers published in peer-reviewed journals. Since 1988 the role of providing a scientific assessment of the state of knowledge on climate change has been carried out by the Intergovernmental Panel on Climate Change (IPCC), which was established by the United Nations Environment Programme and the World Meteorological Organization to assess the scientific, technical, and socioeconomic information relevant for to understanding the risk of human-induced climate change.

In 2007, the IPCC released its Fourth Assessment Report summarizing the latest findings on climate change science and projections for the 21st century. Key findings of this report are outlined below.

Earth's Changing Climate

The Earth's climate is primarily controlled by the balance of incoming energy from the sun and outgoing energy from the Earth's surface. No significant trend has been observed in incoming solar radiation since the late 1970s when satellite measurements became available (IPCC 2001 WG1 Ch.1). In contrast, atmospheric concentrations of all well-mixed heat trapping greenhouse gases are far greater now than at anytime during the last 650,000 years (IPCC 2007 WG1 Ch.6). By strengthening the greenhouse effect, more outgoing energy is trapped at the Earth's surface. Trends and sources of three primary greenhouse gases, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N20), are outlined in Figure 1 and Table 1.

click images to enlarge

Figure 1 Changes in greenhouse gases from ice core and modern data. Atmospheric concentrations of CO2, CH4, and N20 over the last 10,000 years (large panels) and since 1750 (inset panels). Measurements are shown from ice cores (symbols with different colors for different studies) and atmospheric samples (red lines). The corresponding radiative forcings are shown on the right hand axes of the large panels. (Figure adapted from IPCC 2007 WG1 SPM)

Greenhouse gas	% change 1750-2005	Current (2005) atmospheric concentration	Historical perspective on current concentration	Major sources, human and natural
Carbon dioxide	+35%	379 ppm	Higher than any in the past 650,000 years	Fossil fuel use, deforestation and land use changes, agriculture, cement production, decomposition of organic matter, oxidation of organic carbon in soils, oceans
Methane	+142%	1,774 ppb	Higher than any in at least 650,000 years	Agriculture, fossil fuel use, ruminants (e.g., cows) and manure management, landfills, wetlands, decomposition of organic matter
Nitrous oxide	+18%	319 ppb	Appears to be higher than any in the past 650,000 years	Agriculture, fossil fuel use, animal manure management, sewage treatment, nitric acid production, variety of biological sources in soil and water

Table 1 Change in greenhouse gas concentrations between 1750 and 2005 (IPCC 2007 WG1 SPM, USEPA).

Global Climate Change: 20th Century Observed Changes

There are many global-scale indications that the Earth's climate and various physical and ecological systems are changing in response to rising greenhouse gas concentrations, including those listed below (IPCC 2007 WG1 SPM, IPCC 2007 WG1 TS). While it may not be possible to attribute 100% of these changes to human-caused climate change (many of these changes may reflect the combined effects of natural climate variability and long-term changes in average global temperature), the observed changes provide a consistent picture of a what we can expect from a warming climate.

• Globally averaged temperature has increased. Mean global surface temperature has increased by 1.3°F (0.7°C) since 1906. It is very likely (>90% chance) that most of the increase observed since the mid-20th century is due to greenhouse gas emissions from human activities (ibid). It is also very likely (>90% chance) that the period 1950–2000 was warmer than any other 50-year period in the last 500 years and likely (>66% chance) the warmest 50-year period in the last 1300 years.

• Extreme temperature events have increased. Cold days and, especially, cold nights (the coldest 10% of days or nights) have become less frequent since 1950. Hot days and hot nights (the warmest 10% of days or nights) have become more frequent. A widespread decline in the number of frost days has been observed. Evidence suggests an increase in number and duration of heat waves since 1950.

• Sea surface temperatures have increased. Warming of the ocean is widespread in the upper 2300 ft (700 m).

• Sea level has risen. Global average sea level rise for the 20th century is estimated at 6.7 in (0.17 m).

• Snow and ice have decreased. Snow covered area and snowpack has decreased in most regions, especially in spring. The largest declines in spring snowpack are found at lower and warmer elevations in western North America and the Swiss Alps. Glaciers and ice caps have experienced widespread mass losses. Sea ice extent has decreased in the Arctic.

The Role of Humans in Earth's Changing Climate

Human activities, through fossil fuel use, agriculture, and land use change, have been the dominant cause of increases in greenhouse gases over the last 250 years (Table 1). Furthermore,

• It is extremely likely (>95% chance) that that human activities, specifically greenhouse gas emissions, aerosols, and land surface changes, have exerted a substantial net warming influence on climate since 1750 (IPCC 2007 WG1 TS); and
• It is very likely (>90% chance) that human greenhouse gas increases caused most of the observed increase in global average temperatures since the mid-20th century (IPCC 2007 WG1 SPM).

Modeling 21st Century Global Climate

Future projections of global climate change are primarily controlled by two factors:

1. changes in global greenhouse gas and sulphur emissions, and
2. climate sensitivity to increased greenhouse gas concentrations.

Changes in greenhouse gas and sulfate aerosol emissions are based on different assumptions about future population growth, socio-economic development, energy sources, and technological progress. Because we do not have the advantage of perfect foresight, a range of assumptions about each of these factors are made to bracket the range of possible futures. These individual scenarios, collectively referred to as the SRES scenarios, are grouped into scenario "families" for modeling purposes.

Climate sensitivity, which refers to the amount of warming produced by a doubling of CO2 above pre-industrial levels (i.e., to 550 ppm), has been estimated from observations to lie between 3.4°F and 7.9°F (1.9-4.4°C) (IPCC 2007 WG1 TS). Different climate models also have a range of sensitivity, yielding a range of warming for the same SRES scenario. Consequently, the IPCC's global climate change projections reflect not a single future but rather a range of possible futures derived from running more than 20 global climate models each with two to three different scenarios for greenhouse gas and sulfate aerosol emissions.

Projected Changes in 21st Century Global Climate

The IPCC projects an increase in global average annual temperature of 3.2-7.2°F by the end of the 21st century (Figure 2). This range is the "best estimate" range; the likely range (>66% chance) of warming for all of the greenhouse gas emissions scenarios is 2-11.5°F, as shown by the gray bars on the right in Figure 2.

Figure 2 Multi-model averages and assessed ranges for surface warming. Solid lines are multi-model global averages of surface warming (relative to 1980-1999) for the A2, A1B, and B1 SRES scenario families, shown as continuations of the 20th century simulations. Shading denotes the ±1 standard deviation range of individual model annual averages. The orange line is for the experiment where concentrations were held constant at year 2000 values. The gray bars at right indicate the best estimate (solid line within each bar) and the likely (>66% chance) range assessed for the six SRES marker scenarios. (Figure source: IPCC 2007 WG1 SPM)

	Projected Increase in Global Mean Temperature	Impact of Emissions Scenario Choice (e.g., B1 versus A2)
Early 21st century (2011-2030)	1.1°F to 1.2°F (0.6°C to 0.7°C)	Choice of emissions scenario makes little difference in this period
Mid-century (2046-2065)	2.3°F to 3.1°F (1.3°C to 1.8°C)	Choice of emissions scenario makes some difference in this period
Late 21st century (2080-2099)	3.2°F to 5.6°F (1.8°C to 3.1°C)	Choice of emissions scenario makes a significant difference in this period

Table 2 Range of projected average annual global temperature increase during the 21st century for the six SRES marker scenarios (IPCC 2007 WG1 SPM).

 

The range of projected warming through the mid-21st century is more well-defined relative to the end of the 21st century given the greater certainty in greenhouse gas emission levels in the near future. Greater confidence exists in near-term greenhouse gas emission levels because we have a better understanding of population trends, patterns of socio-economic development, and technology use (including energy use) in the next few decades, leading to a relatively narrow range of projections through mid-21st century, as seen in Figure 2 and Table 2. It is not until the second half of the 21st century that the broader range of assumptions about future greenhouse gas emissions start to have a significant impact on the range of projected warming.

Other important characteristics of projected 21st century global warming include the following:

• Changes in temperature will vary spatially. Temperature changes are expected to be greatest over land (approximately twice the global average temperature increase) and at high northern latitudes (IPCC 2007 WG1 Ch.10).

• Changes in precipitation will vary spatially and seasonally. Precipitation is very likely (>90% chance) to increase at high latitudes, but likely (>66% chance) to decrease in the subtropics (IPCC 2007 WG1 TS). It will take longer for changes in mean precipitation to rise above the range of natural variability than changes in mean temperature.

• More intense precipitation events are expected. More intense precipitation events are expected, particularly in the tropics and high latitude areas where mean precipitation is expected to increase (IPCC 2007 WG1 Ch.10).

• There is more confidence in temperature changes. There is greater confidence overall in changes in temperature than changes in precipitation given the difficulties in modeling precipitation (IPCC 2007 WG1 Ch.8).

• Sea level is projected to increase. The globally averaged range of sea level rise by the 2090s for several emissions scenarios (B1 to A1F1) from the 5% to 95% range of model results is 7 to 23 in (0.18 to 0.58 m). (IPCC 2007 WG1 TS)

• Declines in snow cover and ice extent will continue. Snow cover and ice extent will continue to decrease as a result of warmer temperatures.

Changes in 20th Century Pacific Northwest Climate

The climate of the PNW has changed during the past 100 years. Observed 20th century changes include:

• Temperature has increased. Average annual temperature increased 1.5°F (0.7-0.8°C) in the PNW between 1920 and 2003. The warming has been fairly uniform and widespread, with little difference between warming rates at urban and rural weather monitoring stations. Only a handful of locations recorded cooling. Although the warmest year was 1934, the warmest decade was the 1990s (figure 3a). (Mote 2003)

• Trends in winter season and daily minimum temperatures have been largest. Temperature trends from 1916-2003 were largest from January-March. Minimum daily temperature rose faster than maximum daily temperature through the mid-20th century. In the second half of the 20th century, minimum and maximum temperature rose at about the same rate. (Mote 2003, Hamlet and Lettenmaier 2007).

• Decadal variability has dominated annual precipitation trends. Annual precipitation increased 14% for the period 1930-1995 for the PNW region. Sub-regional trends ranged from 13%-38% (Mote 2003). However, these trends are not statistically significant and depend on the time frame analyzed. Decadal variability is therefore the most important feature of precipitation during the 20th century.

• Cool season precipitation variability has increased. Cool season precipitation in the PNW is more variable from year to year, displays greater persistence, and is more strongly correlated with other regions in the West since about 1973 (Hamlet and Lettenmaier 2007).

• April 1 snow water equivalent (SWE) declined at nearly all sites in the PNW between 1950 and 2000. The declines are strongest at low and middle elevations, and can be explained by observed increases in temperature and declines in precipitation over the same period of record (Mote et al. 2003, Hamlet et al. 2005, Mote 2006, Mote et al. 2008). Low elevation declines at individual stations in the Washington and Oregon Cascades are frequently 40% or more (Mote et al. 2003, Mote et al. 2005) (figure 3b). The linear decline in April 1 SWE for the Washington Cascades is roughly –15% to –35% (mostly around –25%) for a variety of starting points between 1916 and 1970 and ending in 2006 (Mote et al. 2008).

• Timing of peak runoff has shifted. Timing of the center of mass in annual river runoff in snowmelt basins shifted 0-20 days earlier in much of the PNW between 1948 and 2002 (Stewart et al. 2005). The greatest trends occurred in the PNW, including the mountain plateaus of Washington, Oregon, and western Idaho. These findings are corroborated by modeling studies which show similar changes in runoff timing (Hamlet et al. 2007)

While it is premature to assume that anthropogenic (i.e., human caused) climate change is exclusively driving these trends, these trends cannot be fully explained by climate variability alone. Additionally, the trends are consistent with observed global changes and projected global and regional climate change impacts.

click images to enlarge Figure 3 20th century trends in (a) average annual PNW temperature (1920-2000) and (b) April 1 snow water equivalent (1950-2000). These figures show widespread increases in average annual temperature for the period 1920 to 2000 and decreases in April 1 snow water equivalent (an important indicator for forecasting summer water supplies) for the period 1950 to 2000. The size of the dot corresponds to the magnitude of the change. Pluses and minuses indicate increases or decreases, respectively, that are less than the given scale. Figure source: Climate Impacts Group, University of Washington.

Future Climate Change in the Pacific Northwest

Global climate models scaled to the PNW project an increase in average PNW temperature on the order of 0.2°-1.0°F (0.1°-0.6°C) per decade throughout the mid-21st century with a best estimate average of 0.3°C (0.5°F) per decade (Table 3). Temperature increases occur across all seasons with the largest increases in summer. Changes in annual precipitation are less certain. Most of the models analyzed by CIG project decreases in summer precipitation and increases in winter precipitation with little change in the annual mean.

Changes in Annual Mean
	Temperature	Precipitation
2020s
Low	+ 1.1ºF (0.6ºC)	-9%
Average	+ 2.2ºF (1.2ºC)	+1%
High	+ 3.4ºF (1.9ºC)	+12%
2040s
Low	+ 1.6ºF (0.9ºC)	-11%
Average	+ 3.5ºF (2.0ºC)	+2%
High	+ 5.2ºF (2.9ºC)	+12%
2080s
Low	+ 2.8ºF (1.6ºC)	-10%
Average	+ 5.9ºF (3.3ºC)	+4%
High	+ 9.7ºF (5.4ºC)	+20%

Table 3 Average changes in PNW climate from 20 climate models and two greenhouse gas emissions scenarios (B1 and A1B) for the 2020s, 2040s, and 2080s. All changes are benchmarked to average temperature and precipitation for 1970-1999. Model values are weighted to produce the "average". (Download scenario data)

 

Other important characteristics of projected 21st century PNW warming include the following:

• The rate of warming will be faster. The best estimate rate of warming in the PNW through the mid-21st century -- 0.5°F (0.3°C) per decade -- is three times the rate of change per decade observed in the PNW during the 20th century (0.15°F [0.8°C] per decade). The per decade rate of change for the second half of the 21st century is dependent on the choice of emissions scenarios.

• Precipitation changes are projected to be small. Precipitation changes are projected to be small compared to the interannual and decadal variability observed during the 20th century. The majority of models show increases in winter precipitation and reduced summer precipitation. Analysis of future storm tracks indicates a basis for more confidence in wet season increases, particularly in the second half of the 21st century (Salathé 2006).

• Sea surface temperatures are expected to increase. Coastal sea surface temperature (SST) helps determine the biological and physical conditions of the marine environment and estuaries of the PNW. Climate models project warming in summer SSTs for the 2040s on the order of 2.7°F (1.5°C). This change is somewhat less than the warming projected in the 2040s for PNW land areas (3.5°F [2.0°C]), but is significant relative to the small interannual variability of the ocean.

How will climate change affect climate variability? The El Niño/Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) influence PNW climate and natural resources on seasonal to interannual scales. How will climate change affect ENSO and PDO? The answer is not clear given the difficulty of simulating these patterns in complex global circulation models. While modeling of present-day ENSO events has improved since the 2001 IPCC assessment, the IPCC concluded in its 2007 Fourth Assessment Report that "there is no consistent indication at this time of discernible changes" in ENSO intensity or frequency in the 21st century (IPCC 2007 WG1 Ch.10).

Planning for Climate Change

The decisions we make today shape our vulnerability to climate change. Examining the possible impacts of climate change provides information that can be used to inform planning in the PNW. Good decisions can be made in spite of the uncertainty associated with these projected changes, just as good decisions are made in spite of uncertainty about other factors, such as future economic conditions or rates of population growth. Careful consideration of the range of projected impacts, combined with an analysis of a resource’s vulnerability to these impacts, will support prudent approaches to planning.

Additional Information on Global Climate Change