Ninth International Conference on Permafrost ... - IARC Research

Continued Permafrost Warming in Northwest Alaska as Detected by the DOI/GTN-P Borehole ArrayGary D. ClowU.S. Geological Survey, Lakewood CO 80225, USAThe U.S. Department of the Interior contributes to theGlobal Terrestrial Network for Permafrost (GTN-P) with a21-element deep borehole array located in northwest Alaska.The majority of the holes are located on the Arctic CoastalPlain (ACP) with the remainder being in the northernfoothills of the Brooks Range (Fig. 1). All but two of theboreholes penetrate the base of permafrost which is situatedbetween 210 and 410 m in this region.The U.S. Geological Survey began monitoring temperaturesin the DOI/GTN-P boreholes in the late 1970s, soon after theholes were drilled. This was done by periodically reloggingthe wells with a portable temperature logging system (PTLS)developed especially for this purpose at USGS. While theoriginal PTLS was limited to an incremental or “stop-andgo”logging mode, improvements allowed the PTLS toacquire “continuous” temperature logs by the mid-1980s.Further refinements have reduced the standard uncertaintyof the ITS-90 temperature measurements obtained with thissystem to less than 3.3 milliKelvin. Early results whichfocused on interpretation of the curvature of the boreholetemperature profiles for past climatic changes were reportedby Lachenbruch and Marshall (1986) and Lachenbruch etal. (1988).Monitoring the thermal response of permafrost tocontemporary climate change has been the primary researchfocus of the DOI/GTN-P monitoring network for the lastdecade. Due to logistical constraints, our current protocolis to relog the entire borehole array every five years with afew key wells being relogged annually. Measurements in thewells show that near-surface temperature fluctuations in NWAlaska were generally small during the late 1970s and mostof the 1980s, except for a short cold period during 1983–84.Permafrost temperatures began warming dramatically in1989. By 2002–03, near-surface permafrost temperatureshad warmed 3 K on average across the array relative to1989. The coastal plain portion of the borehole array wasrelogged again during 2007 in support of the internationalThermal State of Permafrost (IPY/TSP) project. These newmeasurements show that shallow permafrost temperatureshave continued to warm on the ACP since 2002–03; themagnitude of the warming since 2002–03 ranges 0.0 to 1.0K (mean = 0.4 K), depending on local site conditions. Thetotal average (shallow) permafrost warming in this regionsince 1989 is now ~ 3.5 K (Fig. 1).Data from the co-located DOI/GTN-P active layer/meteorological network show that the 2002–03 boreholemeasurements coincided with a peak in mean annual airtemperatures in NW Alaska. Mean annual air temperaturescooled substantially during 2004, but have been rising sinceand are now warmer than those experienced during 2002–03. Permafrost temperatures have not tracked the post-2004air temperature warming due to the effects of changing snowcover.Figure 1. Location of the DOI/GTN-P boreholes in northwestAlaska. Also shown is the magnitude (degrees Kelvin) of shallowpermafrost warming at various well sites on the coastal plainbetween 1989 and 2007.Figure 2. Sample permafrost temperatures measured in one of thecoastal plain DOI/GTN-P boreholes (East Teshekpuk) since 1977.Extrapolated mean annual surface temperatures have increasedabout 3.6 K at this site since the late 1970s, close to the averagefor the entire array.47