Fewer Freezing Days

Climate change is bringing fewer days below freezing to North America, according to a recent study published in the Journal of Climate. This news probably doesn’t come as a shocker to our readers; however, the recent study offers potentially new insight in how this dwindling number of freezing days will be distributed across the North American map. There’s a potentially troubling story here for the Pacific Northwest, according to the research.

The number of days below freezing is projected to decrease across all of North America by the middle decades of this century, according to the study’s authors, Michael Rawlins and colleagues. All of the map will change. However, the researchers note, the regions at greatest risk of future decreasing days below zero will be a select few, including parts of the Pacific Northwest.

The researchers arrived at this conclusion after reviewing spatially-distributed data from both historical in-the-field measurements and future climate projections. (More details on the study’s assumptions and methods can be found at the bottom of this post.)

The drop in freezing days, the researchers found, will be greatest in the central and western United States, the edges of the North American map that currently, climatically speaking, experience freezing days. This region includes the Pacific Northwest east of the Cascades, which is expected to experience below freezing day declines of more than 60 days by the middle of this century, according to Rawlins and colleagues’ estimates.

The researchers’ findings are troublesome. Freezing days—days with average temperatures below 32 degrees Fahrenheit—are essential for making and keeping snow on our region’s mountains. As a bonus, the frosty weather also keeps forest and agricultural pests in check. However, under the climate change projections the researchers reviewed, Pacific Northwest winters are expected to be about 2 degrees Celsius warmer by the middle decades of this century (defined here as 2041 to 2070) when compared with the historical years 1971 to 2000. Rising temperatures mean less snow and potentially a number of compounding problems from water scarcities to more frequent forest fires. Our regional climate map will change and so will the North American map.

In fact, roughly 6 percent of North America that currently experiences freezing days will no longer experience such days in the climatic future of the middle 2000s, according to the researchers. This climatic cartographic contraction largely occurs in an east-west band across the central US, but also includes some parts of the Pacific Northwest’s Cascade Mountains and Columbia Basin.

What’s more, the researchers note, much of the map will experience an earlier spring thaw and a later autumn freeze. This spells an overall lengthening of the freeze-free season. Across much of the US, including parts of Pacific Northwest, advances in spring thaw will exceed autumn freeze delays in the future. In fact, this trend is already occurring, according to observational data.

So which spots on the North American map are at greatest risk of future declines in days below freezing? According to Rawlins and colleagues, they are areas with annual average mean temperatures between 2 and 6 degrees Celsius (36 to 43 degrees Fahrenheit) and areas that include a slower temperature rise during the spring thaw. Much of the Pacific Northwest falls into both of these categories.

A Note on Methods: The researchers used a suite of regional climate models forced with an emissions scenario in which CO2 levels are projected to increase from 490 parts per million (ppm) in 2040 to 635 ppm in 2070. (For readers familiar with the Representative Concentration Pathways, this scenario tracks just below RCP 8.5, or the “business as usual” scenario, so-named because it assumes CO2 emissions continue on their current trajectory.) In such a future climate, our Pacific Northwest winters are projected to be about 2 degrees Celsius warmer by the middle of this century (defined here as 2041to 2070) when compared with 1971 to 2000. The suite of regional climate models came from the North American Regional Climate Change Assessment Program. The program’s modeling suite combines four CMIP3 global climate models with six regional climate models. Each pairing simulates a 1971 to 2010 baseline climate and a 2041 to 2070 future climate forced under the SRES A2 scenario.

Citation: Rawlins, Michael A., Raymond S. Bradley, Henry F. Diaz, John S. Kimball, and David A. Robinson. 2016. “Future Decreases in Freezing Days across North America.” Journal of Climate 29 (19): 6923–35. doi:10.1175/JCLI-D-15-0802.1.

Photo Caption: Frost. (Photo: Susanne Nilsson, some rights reserved.)

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