Climate Impacts Research Consortium

Temperatures Projected To Rise During 21st Century

Temperature Projections

Projected annual temperature simulations for the Pacific Northwest to the year 2100. Projected annual temperatures are shown as measured against the historical annual average, shown as the dashed, zero line. The information here represents the output of 40 simulations, or computer-run experiments, showing what our region’s future temperatures are projected to look like under climate change. All the simulations point toward rising temperatures. The question is to what extent.

This graph answers this question by representing a range of uncertainty using two greenhouse gas emissions scenarios: RCP 4.5 and RCP 8.5, a medium- and high-emissions scenario represented here by yellow and red, respectively. In the RCP 4.5 emissions scenario, growth in greenhouse gases is less than RCP 8.5, leading to a slower increase and eventually a leveling off
of temperatures.

In the high-emissions scenario, RCP 8.5, a steady growth in greenhouses leads to a steady upward trend in temperatures. (For more info on emission scenarios, see Human Choice, Warming, & Emissions—The Representative Concentration Pathways.) Historical simulated temperatures are represented in gray. Keep in mind, this isn’t the actual historical record taken from in-the-field instruments; instead, it represents how the computer models simulated the climate over the historic period given observed atmospheric forcings. This graph comes from CIRC’s Integrated Scenarios project. (Figure source: David E. Rupp; data source: Rupp et al. 2016, adapted.)

  Thinking Deeper:

Why the Range in Projections?  

The disparity from 1 to 8 degrees Celsius (2 degrees to 15 degrees Fahrenheit) is due in large part to human choice. We can choose to cut our collective carbon footprint, or we can choose to keep things business as usual. In our research, we represent this choice by using two emissions scenarios: RCP 4.5 and RCP 8.5. (To learn more, see Human Choice, Warming, & Emissions—The Representative Concentration Pathways.) RCP 4.5 assumes humanity starts cutting emissions of CO2 and other greenhouse gases in the coming decades, and hence that temperature increases will start to level off. RCP 8.5, on the other hand, assumes we continue emitting at our current rate with warming continuing to rise and not leveling off.

The other reason for the disparity, or range, in our temperature projections has to do with the scientific community’s fundamental lack of understanding just how the climate system will respond to high levels of greenhouse gases. That’s because while all the relevant physical processes of the Earth’s atmosphere and land are represented in the computer models that climate scientists employ, these models, called global climate models (GCMs), nonetheless represent these physical processes somewhat differently from model to model. The end result is this: each individual model tends to produce different amounts of climate change for the same amount of greenhouse gases. (For details on GCMs, see our Modeling & Analysis page.) This does not mean that the results of GCMs are pie-in-the-sky predictions or simply educated guesses; they’re really well-reasoned, physically-based estimates.

This uncertainty between models is one reason our researchers like to employ multiple GCMs when making climate projections. For instance, temperature results for CIRC’s Integrated Scenarios project sprang from data from some twenty different GCMs combined with two RCPs. This ensemble approach—as well as the degree to which the models and scenarios agreed—means we can say with a high degree of certainty that the Pacific Northwest will be warmer in the future. We just can’t say for certain how much warmer. This high degree of certainty is known in climate research as confidence. As we explain later, this same high degree of confidence does not extend to Precipitation projections for our region.