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The Future of Freezing

US temperatures have been warming over the last century, and climate scientists expect much more of the same in the future. In the American West, spring temperatures have especially come up. Besides sounding rather cheerful, warmer springs mean snow melting sooner. And that, in turn, translates to a whole host of effects, from threatened trout habitat to drier forests to more fires (now and later) — not to mention rivers running lower late in the summer, with consequences for farmers and anyone else who uses water. Climate Central videos explore some of these links in Montana and Washington State.

This map shows climate model projections for how areas with monthly average temperatures below freezing are expected to shrink over time. Scientists’ expectations depend on how much more greenhouse pollution gets in the air.

The table just below summarizes key projection results for affected states, and the sections below that explain how the maps were developed.

Climate model projections

The future projections underlying this animation are based on results of 16 different global climate models — essentially, these are computer programs that represent the climate, each based on slightly different methods and assumptions. By taking the median temperature projection of all these models, we get what amounts to a “best-guess”  of future temperatures. It's like getting help from 16 different experts. These models were used by the world's leading and largest scientific body on climate change, the Intergovernmental Panel on Climate Change (IPCC), in its latest major report.

Because climate models make projections only at a broad regional level, we used established methods (see technical explanation) to add fine, local detail to these future climate estimates — enough detail to make different projections for places as close as ten miles apart. This is important because people want to know climate forecasts for the specific places they live, work and visit; and we all know that even nearby locations can be quite warmer or cooler, wetter or drier than each other.

We also allowed for the fact that year-to-year changes can go in either direction — that is, even if the trend is toward warmer temperatures in general, a given year might be colder than the one before. So to reveal trends more clearly, the temperature shown for a given year at each map location is actually the average of projected temperatures for the same month in ten consecutive years around and including the focal year.

It is important to remember that climate model outputs are always projections and never predictions; we can use them to anticipate general trends, but never to foretell the exact temperature or precipitation at a particular place and time.

Emissions scenarios

To make projections about future climate change, we must first make an assumption about the future rate of accumulation of greenhouse gases, like carbon dioxide, in the atmosphere. We cannot know what the exact buildup will be over the next century, because this will depend on many things, including human actions. Therefore, a standard set of “scenarios” is used as input to climate models, covering a range of possibilities from high emissions (continuing expansion of fossil fuel use; high population growth) to low emissions (swift shift toward low- or no-carbon energy sources like nuclear, wind and solar; low population growth).

We compared climate change projections on this page based on one standard future scenario from the high emissions end of the range, and one from the low end. However, in the seven years since scientists developed the current set of standard scenarios, greenhouse gases in the atmosphere have increased somewhat faster than even the highest emissions pathway considered, so climate change projections depending on them—like the ones illustrated here—may be conservative.

The technical labels for the two emissions scenarios used in the climate projections here are SRES B1 (low) and SRES A2 (high) (see the original scenario definitions and report and a snapshot contrast).