The Washington Climate Change Impacts Assessment: Evaluating Washington's Future in a Changing Climate - Executive Summary

Climate models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) on the whole reproduce the observed seasonal cycle and 20th century warming trend of 0.8°C (1.5°F) in the Pacific Northwest, and point to much greater warming for the next century. These models project increases in annual temperature of, on average, 1.1°C (2.0°F) by the 2020s, 1.8°C (3.2°F) by the 2040s, and 3.0°C (5.3°F) by the 2080s, compared with the average from 1970 to 1999, averaged across all climate models. Rates of warming range from 0.1 to 0.6°C (0.2° to 1.0°F) per decade. Projected changes in annual precipitation, averaged over all models, are small (+1 to +2%), but some models project an enhanced seasonal cycle with changes toward wetter autumns and winters and drier summers. Changes in nearshore sea surface temperatures, though smaller than on land, are likely to substantially exceed interannual variability, but coastal upwelling changes little. Rates of 21st century sea level rise will depend on poorly known factors like ice sheet instability in Greenland and Antarctica, and could be as low as 20th century values (20cm, 8”) or as large as 1.3m (50”). 1: Scenarios 1JISAO Climate Impacts Group, University of Washington, Seattle, Washington 98195-5672 2Oregon Climate Change Research Institute, College of Oceanic and Atmospheric Sciences, Oregon State Univ., Corvallis, Oregon 97331-5503 CHAPTER 1: Scenarios 21 1. Global Climate Models Envisioning global climate in a future with much higher greenhouse gases requires the use of physically based numerical models of the ocean, atmosphere, land, and ice, often called global climate models (GCMs) or climate system models. A common set of simulations using 21 GCMs was coordinated through the Intergovernmental Panel on Climate Change (IPCC), described in the IPCC 2007 report (Randall et al. 2007), and archived by the Program for Climate Model Diagnostics and Intercomparisons (PCMDI). These models typically resolve the atmosphere with between 6,000 and 15,000 grid squares horizontally, and with between 12 and 56 atmospheric layers. All GCMs in the PCMDI archive include a fully resolved global ocean model, usually with higher resolution than the atmospheric model, and nearly all include models of sea ice dynamics and models of the land surface. By calculating energy fluxes between the sun, atmosphere, and surface, these models compute surface temperature distributions that compare well with observations. Details of the models, as well as references, can be found in Table 8.1 of Randall et al. 2007. Simulations of 21st century climate require projections of future greenhouse gases and sulfate aerosols (which reflect sunlight and also promote cloud formation, thereby offsetting greenhouse gases locally), more than 40 of which were produced under the auspices of the IPCC (SRES, Nakicenovic et al. 2000) after considering a wide range of future socioeconomic changes. Three of these “SRES” scenarios were commonly chosen for forcing the GCMs: B1, A1B, and A2. The climate forcing of all scenarios, including B2 and the older IS92a used in the Second Assessment report (Figure 1) is similar until about 2020 owing primarily to the long lifetime of coal fired electric power plants and of the major greenhouse gases. A2 produces the highest climate forcing by the end of the century, but before mid-century, none of the scenarios is consistently the highest. Because more modeling groups ran A1B than A2, and since our focus for this study was on midcentury change, we chose A1B as the higher emissions scenario and B1 as the low emissions scenario for our analysis of 21st century PNW climate. We have analyzed available A2 runs as well, as shown in Figures 8, 11, and 12, but we emphasize A1B and B1. Though B1 is the lowest of the IPCC illustrative scenarios, it still produces changes in climate that many scientists call “dangerous” (Schellnhuber et al. 2006) — a threshold that a growing number of political leaders have stated their intention to avoid. At the high end, scenario A1FI (not shown) results in even higher climate forcing by 2100 than A2 or A1B. Mid-2000s global emissions of CO2 exceeded even the A1FI scenario (Raupach et al. 2007). Whether these exceedingly high emissions will continue into the future is beyond our expertise to judge. On the PCMDI web site (esg.llnl.gov), all modeling centers provided simulations of 20th century climate using observed solar, volcanic, and greenhouse gas forcing. Twenty modeling centers provided simulations of 21st century climate with the A1B scenario, 19 with B1, and 17 with A2, for a total of 56 runs. These form the basis for the analysis presented in most of the other chapters in this assessment report. In some cases several different model runs were provided for each scenario; we chose Run 1 CHAPTER 1: Scenarios 22 except as noted in Appendix A. This set of models is larger than the set available in 2005 when similar analysis was performed for the Northwest (Mote et al. 2005, 10 models) and California (Cayan et al. 2007, 12 models but emphasizing two). Randall et al. (2007) and CCSP (2008) evaluated the models’ fidelity in simulating various aspects of global climate, and also calculated each model’s climate sensitivity. The modeled climate sensitivity is a measure of the model’s response to doubled CO2, and has historically been calculated in two ways: either the “equilibrium climate sensitivity” or the “transient climate response” (TCR, ibid.). The equilibrium climate sensitivity is defined as the globally averaged temperature change in a simulation with a doubling of CO2, in which the simulation is long enough for the global temperature to reach equilibrium. Because the climate system takes a long time to come into equilibrium, the calculation of the equilibrium climate sensitivity was typically performed only in models with a very simple ocean component, which was standard before the mid-1990s. By the late 1990s, most models included a sophisticated ocean, and running such a model to equilibrium would require a great deal of computer resources. The TCR was a more practical metric of models’ sensitivity. The TCR is defined as the global mean temperature change at the time of CO2 doubling in a simulation in which the CO2 increased at 1%/year (roughly IS92a, the black curve in Figure 1). The range of values of TCR reported by Randall et al. (2007) was 1.2-2.6°C (their Table 8.2). 2. Model Evaluation: 20th Century Climate of the