First, the chart below shows the sequence of annual mean temperatures at depths down to 3m (click to enlarge). Immediately we're struck by a remarkable trend towards higher temperatures in the past decade or so, with the top 50cm seeing an annual mean temperature above freezing in 2016. There was a big jump between 2013 and 2014, which is not surprising as Fairbanks' mean air temperature was more than 7°F warmer in 2014 than in 2012. And ever since the remarkable transition from cold to warm in May of 2013, Fairbanks has seen persistent anomalous warmth with few significant excursions to the cold side of the climate. The integrated effect of this warmth is clearly visible now in the sub-surface warming trend.
Secondly, as a partial answer to Tracy's question about seasonal thaw of the active layer (i.e. the top layer of permafrost that thaws and refreezes each year), the chart below shows annual maximum temperatures at each depth. (Note that the 2m depth is missing from 2014-2016). From 2007 through 2013, the seasonal thaw did not generally extend down below 50cm, but more recently the active layer has deepened to something like 1m.
Looking at annual minimum temperatures, it is really remarkable to see that by 2016 temperatures below 50cm depth were barely below freezing even at the coldest time of year. It doesn't take too much imagination to envision a complete loss of the permafrost in this particular profile, which would upend the ice/water profile: as it stands now, seasonal thaw sits atop permanent ice, but Smith Lake #1 may soon see seasonal ice atop permanent thaw.
For a bit more perspective on the recent changes, here's the temperature time series from 0.525m depth. March 2017, which was the one really cold month in recent years, produced a minor recovery compared to previous years, but presumably last winter's warmth (not yet reflected in the available data) will have reinforced the warming trend as strongly as ever.
Wow! We certainly would expect to see a positive correlation between a warming climate and soil temperature, but the soil temps at 50 cm are indeed alarming. I've also wondered if some of the unusually wet summer and fall periods in the Interior in recent years aren't also serving as a catalyst for permafrost melting. I appreciate you tracking down data from Dr. Romanovsky and exploring how the below surface soil climate is responding to air temperatures. Folks who live on permafrost may appreciate this recent news piece about measures they can take to try and improve soil insulation in order to combat thawing ground. Ideas include building a wrap around deck for shade, planting trees and shrubs (although defensible space for wildfire is a legitimate concern), not disturbing the surface organic mat (very important!), and ensuring that water run-off from the house is channeled away from the foundation. https://www.alaskapublic.org/2018/09/25/in-fairbanks-building-a-home-on-permafrost-with-an-uncertain-future/
ReplyDeleteThanks for the link, Tracy - interesting and potentially very useful.
DeleteYes, presumably moisture levels in both summer and winter have an effect on the thermal processes. I'd like to try to parse out different climate influences, but it's tricky with a rather short history and the fact that ground temperatures respond so slowly (relative to weather time scales).
Interesting question and treatment of the topic! For "second look" phase of this investigation I highly recommend bringing seasonal snow depth into account. I expect there will be even more profound changes evident in the winter 2017-2018 data, considering the thick snowpack last winter. Might even find that the active layer did not fully re-freeze.
ReplyDeleteThanks for the suggestion. I'm very eager to see the data from last winter - hopefully it will be made available soon.
DeleteWe are having a problem at the Ester fire station with our aquifer/well, and this past summer has arsenic levels well above what is normally naturally observed (9400ppb) and the change occurred very suddenly (time scale of several weeks). Our well is 150ft deep and from what I have heard the well is tapping a water lens on top of intermittent permafrost, not the actual bedrock groundwater. Do you think that permafrost melting or lots of rain this past summer could have caused an arsenic-"enriched" water pocket (possibly due to historic fuel spill creating anoxic reducing environment & liberating arsenic) merging with our well and causing the sudden extreme spike?
ReplyDeleteHi, thanks for the comment - sorry to hear about the problem. This summer was not quite as wet as 2014-2016, but the ongoing rapid pace of permafrost change is undoubtedly going to create all kinds of issues. On the specifics, I'd have to defer to a hydrologist for an explanation, although the hypothesis seems reasonable.
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