Forecasts and Planning Tools

Climate Change Scenarios

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** New climate change scenarios available (November 2005) **

New climate change scenarios were released in October 2005. The content of this page will be updated soon to reflect the new scenarios. In the mean time, visitors are encouraged to read more about the scenarios in the following documents:


Although climate can vary naturally and will continue to do so in the future, human inputs of greenhouse gases are almost certain to cause continued warming of the planet. This warming has potentially significant implications for the Pacific Northwest (PNW) that warrant consideration in resource planning and management.

Estimates of future carbon dioxide concentrations range from a doubling of pre-industrial values to an increase by a factor of 3.5 by 2100. Using these projections of future greenhouse gases, numerous research centers around the world have used numerical models of Earth's climate system to project future global climate. The Climate Impacts Group (CIG) has examined some of these models and produced scenarios of future climate in the PNW (see Table 1 below). For details about these calculations, see our publications (Hamlet and Lettenmaier 1999; Mote et al. 2003).

Future Northwest Climate

On average, global climate system models project a future rate of warming of roughly 0.9ºF (0.5ºC) per decade for the PNW (Table 1). This is substantially more than the 0.4ºF (0.2ºC) increase per decade observed during the last half of the 20th century.

Table 1: Changes in PNW climate from eight climate models for the 2020s and 2040s (from the 1990s)
  Temperature change Precipitation Change
  Annual Oct-Mar Apr-Sept
+ 0.9ºF (0.5ºC) +2% - 4%
+ 2.7ºF (1.5ºC) +8% +4%
+ 4.7ºF (2.6ºC) +18% +14%
+ 2.7ºF (1.5ºC) -2% - 7%
+ 4.1ºF (2.3ºC) +9% +2 %
+ 5.8ºF (3.2ºC) +22% +9%

The projections for PNW climate are derived from eight coupled global-atmosphere climate models: CCSR, CGCM1, CSIRO, ECHAM4/OPYC, GFDL, HadCM2, HadCM3, and NCAR PCM3 (Mote et al. 2003). The models assume an annual increase in equivalent carbon dioxide concentrations of approximately 1% per year. Changes are benchmarked from the average for the decade of the 1990s.

Additional details from the models about future changes in PNW climate include:

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Climate Change Streamflow Scenarios

To assist water resources planners and decision makers in assessing their vulnerability to projected future climate change, the CIG has developed a climate change streamflow scenarios tool for converting regional historical streamflow records into projected future streamflows for the 2020s and the 2040s. The streamflow scenarios, which are free to the public, can be directly incorporated into existing planning methods (such as critical period analysis) in place of the historic streamflow record.

Planning for Climate Change

Decisions made today can shape future vulnerability to a variety of stresses, including climate change. An examination of the possible impacts of future climate changes provides valuable information that can be used to inform planning in the PNW. The CIG provides numerous resources, including climate change scenarios, the climate change streamflow scenarios tool, and consultancy-based climate impacts studies, to support the inclusion of climate change information in PNW resource management and planning.

What about uncertainty? Continued research on the global climate system and PNW environment will continue to expand our understanding of climate change impacts. The absence of "perfect information" should not, however, prevent planning for climate change. Good decisions can be made in spite of the uncertainty associated with 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 climate impacts, combined with an analysis of a resource's vulnerability to these impacts, will support prudent approaches to planning.

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Caveats and Cautions

Regional signal. Global climate models are not designed to simulate regional climate. Important regional features like mountain ranges and estuaries are missing. The pattern of changes in temperature, however, is expected to be (and has been in the past) fairly similar across the whole region.

Climate variables. Models simulate observed patterns of temperature better than observed sea-level pressure (a representation of atmospheric circulation and common weather features), and they simulate sea-level pressure better than precipitation. Consequently, we have highest confidence in projections of temperature change, less confidence in projections of changes in atmospheric circulation, and lowest confidence in simulations of precipitation. Other details of climate that are badly simulated in present climate, such as changes in the frequency or intensity of storms, are probably unreliable in future climates as well.

Rates of increase of CO2. Most of the simulations we examined were performed several years ago by modeling centers that chose a 1%/year scenario for growth of greenhouse gases. While the 1%/year scenario leads to unrealistically high values by the end of the 21st century, the scenario’s CO2 concentrations for the first half of the 21st century are within the range of more sophisticated modeling approaches and therefore serve as a good guide for assessing climate impacts through mid-century.

Contrasts within the region. Simulations with a regional model (a climate model with very high spatial resolution) suggest a few important respects in which climate change may differ from the projections of the global models. For example, warming may proceed more quickly at higher elevations than in the lowlands owing to snow-albedo feedback: when snow cover is reduced, it enhances absorption of solar radiation and warms the surface more (Leung et al. 2003).