Science1 publisher3 min readPublished
Alaska's permafrost soils warmed about twice as fast as soils elsewhere in the state over 27 years
Washington State University found soil in Alaska's permafrost region warmed 0.64 degrees C a decade from 1997 to 2023, about double the rest of the state. The authors point to snow cover, an inference from comparing 43 stations across regions where the air itself warmed at different rates.
The Scientist · Science desk
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What happened
- Air over Alaska's permafrost region warmed 1.15 degrees C per decade, faster than elsewhere in the state and well ahead of the soil beneath it.
- Co-author Claire Phillips said winter warming at that depth in permafrost soils reached 2 to 3 degrees C per decade.
- Lead author Erin Oliver said most of Alaska's weather stations went in only in the late 1990s, leaving a short record to work with.
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Why it matters
- contradiction Air warmed fastest in the same permafrost region where soil warmed fastest, so the regional comparison cannot by itself separate a snow effect from an air-temperature gradient.
- constraint Using air trends as a stand-in for ground trends would put annual soil warming in the permafrost north about 1.8 times too high, while missing how depth and season change the picture.
- exposure The warming trend is clearest below the surface, in the frozen ground that roads and buildings are built on and that gives off carbon dioxide and methane when it thaws.
In the permafrost north, the ground kept a little over half of the air's warming. Soil there rose 0.64 degrees C a decade while the air rose 1.15, a ratio of about 0.56 [6][1]. In central and maritime Alaska, by the study's account, soil warming nearly kept pace with the air [5]. "In the Lower 48 or farther south, you would expect that soil temperatures and air temperatures would change at the same rate," said lead author Erin Oliver, a postdoctoral research associate in WSU's Department of Crop and Soil Science [3][17]. "But in Alaska, over half the year, our soils are covered in snow. So how would that affect things?" [4]
The answer to her question comes from a regional comparison. The team divided Alaska into areas of continuous permafrost, partial permafrost and none, and found that soil in the latter two warmed at roughly half the permafrost rate, or about 0.3 degrees a decade [7][8][2]. According to the article, that suggests deeper winter snow at lower latitudes buffers the soil against rising air temperatures [9]. The same split also sorts the stations by air warming, and the air warmed fastest over the permafrost [6]. If the central and maritime stations whose soil nearly matched the air fall in those slower regions, their air must have warmed far more slowly than 1.15 [5][2]. A soil gap that follows an air gap needs no snow effect to explain it. I think snow is a plausible contributor that these figures do not isolate. The article does not report snow depths, air warming rates for the other two regions, or how the 43 stations divide among the three.
Depth is the cleaner result. Every permafrost-region station showed a warming trend at 1.2 meters, against 65 percent at 5 centimeters, where temperatures swing more from year to year [12]. "A neat result was that soil warming rates weren't slowing down deeper in the soil," said co-author Claire Phillips, a research soil scientist at WSU [10][17]. "The seasonal fluctuations do drop out, but the long-term warming trend was actually more obvious at depth." [11] Less noise at depth helps because the record is short. "Most of the state is very remote, it's very expensive to put in these weather stations, and they really only started putting them in the late 1990s," Oliver said [16].
For anyone estimating ground warming from air data, the study shows the soil-air relationship changing with region, depth and season [5][12][15]. Assume permafrost soil tracks the air and its annual trend comes out about 1.8 times too high [3]. Winter at depth may cut the other way. Phillips described "permafrost soils showing 2 to 3 degrees Celsius per decade warming at a depth of 4 feet" in the winter months and said "that's upsetting" [13]. Her figure covers winter only, so it cannot be set directly against the annual air rate of 1.15 [6]. "In the permafrost region, most of the warming was in winter," Oliver said [15].
The study measured temperature. Thaw, unstable roads and buildings, and releases of carbon dioxide and methane are consequences the article draws from that warming [2]. "The soil is storing a lot of heat that would otherwise all be held in the atmosphere," Phillips said [14].
What to watch
- Whether the Frontiers in Climate paper reports snow-depth measurements or per-region air trends that separate a snow effect from the air-warming gradient.
- Winter air warming rates for the permafrost region, to set beside the 2 to 3 degrees C per decade Phillips cited at 1.2 meters.
- Thaw-depth or carbon-flux measurements at the same stations, which would test whether the temperature trends are translating into thaw and emissions.