Forecasts and Planning Tools

Climate Change Scenarios

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Introduction

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 (CO2) concentrations range from 549 to 970 parts per million by volume (ppmv) by 2100. This increase is 2 to 3.5 times the pre-industrial (circa 1750) value of 280 ppmv. Numerous research centers around the world have used these projections of future greenhouse gas concentrations in numerical models of Earth's climate system to project future global climate.

The Climate Impacts Group (CIG) recently examined a select subset of these global simulation models, driven by two greenhouse gas emissions scenarios (B1 and A1B), and produced updated scenarios of future climate for the PNW. The CIG is also developing a regional climate model for evaluating regional-scale climate change impacts.

A summary of the new (2008) scenarios is provided below. Annual and seasonal climate change scenario summary data is also available for download.

Future Northwest Climate

Temperature

As with previous assessments of PNW climate change, all scenarios evaluated by the CIG project a warmer PNW climate in the 21st century. In comparison with 20th century PNW climate:

Different estimates of future greenhouse gas emissions have important impacts on the projections beyond the 2050s. For example, the difference between average annual temperature for the B1 and A1B emissions scenarios is only 0.2°F (0.1°C) in the 2020s and about 1°F (0.6°C) in the 2040s. By the 2080s, however, the difference is substantially larger: 2.7°F (1.5°C). The relatively minor difference in the choice of scenarios prior to the 2050s is attributable to the fact that it takes decades for the differences in emissions rates between scenarios to result in large differences in climate.


Table 1: 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". Note that the low and high values below are the highest and lowest values for temperature and precipitation from all of the modeled scenarios and do not necessarily come from the same model.
Changes in Annual Mean
 
Temperature
Precipitation
2020s
Low
+ 1.1ºF (0.6ºC)
-9%
Average*
+ 2.0ºF (1.1ºC)
+1.3%
High
+ 3.3ºF (1.8ºC)
+12%
2040s
Low
+ 1.5ºF (0.8ºC)
-11%
Average*
+ 3.2ºF (1.8ºC)
+2.3%
High
+ 5.2ºF (2.9ºC)
+12%
2080s
Low
+ 2.8ºF (1.6ºC)
-10%
Average*
+ 5.3ºF (3.0ºC)
+3.8%
High
+ 9.7ºF (5.4ºC)
+20%

click image to enlarge

Comparison of observed year-to-year variability with projected shifts in temperature and precipitation

Figure 1 Comparison of observed year-to-year variability and projected shifts in average temperature and precipitation from 20 climate models. The blue bars represent the year-to-year variability in PNW temperature and precipitation during the 20th century. The pink bar represents the historic average for 20th century PNW temperature and precipitation. The orange, maroon, and black lines indicate the projected shift in the historic average for the 2020s, 2040s, and 2080s, respectively. Average temperature could exceed the year-to-year variability observed during the 20th century as early as the 2020s, while future projected precipitation falls within the range of past variability. Source: Climate Impacts Group, University of Washington.

Precipitation

Modest changes in regional precipitation are expected through mid-century, although changes in precipitation are less certain than changes in temperature due to challenges associated with modeling precipitation at the global and regional scale. More specifically:

Some models project increases in precipitation while others project decreases (Table 1). The divergence in model projections results from the fact that precipitation is affected by complex yet sometimes subtle changes in large-scale atmospheric circulation patterns which, in turn, are influenced by many imperfectly understood processes (e.g., ocean currents, tropical circulation, interactions between vegetation and the atmosphere).

It is also important to note that natural year-to-year and decade-to-decade fluctuations in precipitation are likely to be more noticeable than longer term trends associated with climate change. Thus, species or systems that respond primarily to changes in precipitation are likely to have already experienced the range of variability expected in the 21st century. Systems that are tuned to precipitation and temperature, however, are likely to find the conditions of the 21st century different from what they have previously experienced.

How will climate change affect extreme events in the PNW?

Because many key aspects of climate (e.g., windstorms, heat waves) either are not well simulated by models or cannot be studied using monthly mean values which are the standard model output, the CIG cannot speculate how they may change in the future. However, droughts may become more common due to the effects of warmer temperatures and reduced winter snowpack on late summer streamflows. Changes in the intensity of precipitation are uncertain, although a preliminary analysis suggests that average monthly (Nov-Jan) winter precipitation could become more intense by the end of the 21st century. Additionally, ongoing work at the CIG suggests that extreme daily precipitation could increase by the end of the century.

How will ENSO and PDO be affected by projected climate change?

Changes in the behavior of climate patterns like the El Niño/Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Arctic Oscillation (AO) are projected by most models, but models typically aren't able to reproduce the variance observed in those climate patterns during the 20th century. As a result, there is no conclusive evidence as to how climate patterns such as ENSO, PDO, and the AO may change in the future.

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Climate Change Streamflow Scenarios

A comprehensive suite of climate change streamflow scenarios for approximately 300 locations in the Columbia River Basin and select coastal drainages is available here (external page) for a wide range of users. The scenarios, provided to the public for free, allow planners to consider how hydrologic changes may affect water resources management objectives and ecosystems. The data sets were developed by the CIG with input from several prominent water management agencies in the Pacific Northwest.

Comparing the 2008 Scenarios with Previous PNW Scenarios

How do the 2008 climate change scenarios differ from the CIG's previous (2005) scenarios? The 2008 PNW climate change scenarios show slightly larger annual temperature increases in all three periods (2020s, 2040s, and 2080s) than the 2005 scenarios (Table 2). For precipitation, the average annual change is about the same but the range of possible precipitation changes is greater in all of the periods analyzed. In other words, the 2008 model results are both drier (at the low end) and wetter (at the high end) than the 2005 scenarios. These differences are largely due to the following:

More detailed information on how the 2008 scenarios may affect PNW resources will become available as the CIG incorporates the new scenarios into future climate impacts assessments.


Table 2: A comparison of the CIG's 2008 climate change projections with the 2005 scenario projections (more 2008 scenario summary data). Note that the low and high values below are the highest and lowest values for temperature and precipitation from all of the modeled scenarios and do not necessarily come from the same model.
 
Annual Temperature Change
Annual Precipitation Change
 
2005 Scenarios
2008 Scenarios
2005 Scenarios
2008 Scenarios
2020s
Low
+ 0.7ºF (0.4ºC)
+ 1.1ºF (0.6ºC)
- 4%
- 9%
Average
+ 1.9ºF (1.1ºC)
+ 2.0ºF (1.1ºC)
+ 2%
+ 1.3%
High
+ 3.2ºF (1.8ºC)
+ 3.3ºF (1.8ºC)
+ 7%
+ 12%
2040s
Low
+ 1.4ºF (0.8ºC)
+ 1.5ºF (0.8ºC)
- 4%
- 11%
Average
+ 2.9ºF (1.6ºC)
+ 3.2ºF (1.8ºC)
+ 2%
+ 2.3%
High
+ 4.6ºF (2.6ºC)
+ 5.2ºF (2.9ºC)
+ 9%
+ 12%
2080s
Low
+ 2.9ºF (1.6ºC)
+ 2.8ºF (1.6ºC)
- 2%
- 10%
Average
+ 5.6ºF (3.1ºC)
+ 5.3ºF (3.0ºC)
+ 6%
+ 3.8%
High
+ 8.8ºF (4.9ºC)
+ 9.7ºF (5.4ºC)
+ 18%
+ 20%

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 for changes in temperature and precipitation, 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.

The availability of different global climate models and forcing scenarios (e.g., B1 and A1B) allows the CIG to evaluate a range of possible future climate conditions for the PNW. Determining which scenario range should be used for more detailed modeling studies (e.g., evaluating climate change impacts on a specific water supply) will depend in part on the sensitivity of a resource to variations in climate and the risks associated with those changes. For example, the implications of drought for a river serving as the primary water source for a dense metropolitan area may be much greater than for a similarly sized river with few demands placed on it.

When relatively little is at stake, resource managers may want to choose climate change scenarios with the least amount of warming (i.e., best case scenarios) for evaluating specific climate impacts. When there is more at stake, or when climate impacts could have irreversible ecosystem consequences, resource managers may want to consider warmer (i.e., worst-case) scenarios. The appropriateness of any one model and/or climate scenario for assessing climate impacts should be evaluated on a case-by-case basis depending on the nature of the study.

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 Other Comments

The meaning of "average" change - Reliability Ensemble Averaging. All references to changes in average temperature or precipitation on this page refer specifically to the REA average. As part of the 2008 climate change scenarios update, the CIG implemented a new approach to evaluating climate change projections for the Pacific Northwest. Reliability Ensemble Averaging, or REA (Giorgi and Mearns 2002), weights regionally-averaged GCM simulations in accordance with each model's ability to replicate 20th century Pacific Northwest climate. The REA value for each season and decade is calculated by weighting each model’s output by its bias (i.e., the model is generally too cool or warm, or wet or dry relative to observed 20th century climate) and distance from the all-model average.

Multi-model averages in weather forecasting, seasonal forecasting, and climate simulations often come closer to observations than single models. REA should produce better results for the future than an unweighted average. For more details on the REA calculation, see Mote et al. 2008, Appendix B.

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.

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).

Last updated August 1, 2008