About Pacific Northwest Climate

Climate Change

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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

20th century trends in average annual PNW temperature and precipitation (1920-2000) and April 1 snow water equivalent (1950-2000)20th century trends in average annual PNW temperature and precipitation (1920-2000) and April 1 snow water equivalent (1950-2000)

20th century trends in average annual PNW temperature and precipitation (1920-2000) and April 1 snow water equivalent (1950-2000)

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


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 



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


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).

"Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level."

-- IPCC 2007 WG1, Summary for Policy Makers

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.

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,

For More Information: Earth's Changing Climate

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.

For More Information: Modeling 21st Century Global Climate

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

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

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:

For More Information: Projected 21st Century Climate Change

Changes in 20th Century Pacific Northwest Climate

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

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
20th century trends in average annual PNW temperature and precipitation (1920-2000) and April 1 snow water equivalent (1950-2000)20th century trends in average annual PNW temperature and precipitation (1920-2000) and April 1 snow water equivalent (1950-2000)
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.

For More Information: 20th Century PNW Climate

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
+ 1.1ºF (0.6ºC)
+ 2.0ºF (1.1ºC)
+ 3.3ºF (1.8ºC)
+ 1.5ºF (0.8ºC)
+ 3.2ºF (1.8ºC)
+ 5.2ºF (2.9ºC)
+ 2.8ºF (1.6ºC)
+ 5.3ºF (3.0ºC)
+ 9.7ºF (5.4ºC)
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) (Source: Mote and Salathé 2009)


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

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).

For More Information: Future PNW Climate

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.

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Additional Information on Global Climate Change

Additional Information on Global Climate Change

Many web sites offer explanations of global climate change. The following provide explanations at varying levels of detail. Note that website materials and publications prepared prior to 2007 may not be updated to include the most recent 2007 IPCC assessment.

Comprehensive reports on climate change

Comprehensive sites with lots of information and links:

"Frequently Asked Questions" (FAQs) links

Other Reports of Note

Page last updated March 2008