Increase in the rate and uniformity of coastline erosion in Arctic Alaska B. M. Jones,1 C. D. Arp,1 M. T. Jorgenson,2 K. M. Hinkel,3 J. A. Schmutz,1 and P. L. Flint1 Received 2 October 2008 revised 22 December 2008 accepted 7 January 2009 published 14 February 2009. [1] Analysis of a 60 km segment of the Alaskan Beaufort Sea coast using a time-series of aerial photography revealed that mean annual erosion rates increased from 6.8 m a 1 (1955 to 1979), to 8.7 m a 1 (1979 to 2002), to 13.6 m a 1 (2002 to 2007). We also observed that spatial patterns of erosion have become more uniform across shoreline types with different degrees of ice-richness. Further, during the remainder of the 2007 ice-free season 25 m of erosion occurred locally, in the absence of a westerly storm event. Concurrent arctic changes potentially responsible for this shift in the rate and pattern of land loss include declining sea ice extent, increasing summertime sea surface temperature, rising sea-level, and increases in storm power and corresponding wave action. Taken together, these factors may be leading to a new regime of ocean-land interactions that are repositioning and reshaping the Arctic coastline. Citation: Jones, B. M., C. D. Arp, M. T. Jorgenson, K. M. Hinkel, J. A. Schmutz, and P. L. Flint (2009), Increase in the rate and uniformity of coastline erosion in Arctic Alaska, Geophys. Res. Lett., 36, L03503, doi:10.1029/2008GL036205. 1. Introduction [2] Rates of coastal erosion in the Arctic are known to be among the highest in the world due in part to the ice- bonded nature of the coastal sediments [Reimnitz et al., 1988 Jorgenson and Brown, 2005]. Historically, mean annual erosion rates along the Beaufort Sea coast in Alaska are as high as 8 m a 1 for exposed ice-rich bluffs [Jorgenson and Brown, 2005] (Figure 1). The coastline here has a northern exposure with bluffs up to 6 masl consisting of ice-rich, near shore marine, glacio- fluvial, alluvial, and aeolian deposits that lack protective barrier islands [Reimnitz et al., 1988 Jorgenson and Brown, 2005]���an ideal setting to support very high erosion rates and an opportunity to study the direct effects of changing Arctic conditions along the ocean- land interface. [3] The Beaufort Sea is traditionally ice-free for three to four months of the year, and it is during this short time period that all coastline erosion occurs. In general, erosion of the ice-rich bluffs located along this coastline involves the formation of a thermo-mechanic erosional niche, col- lapse of bluff materials once niche propagation exceeds bluff strength, deposition of the failed block at the bluff toe, and mass wasting of the block through thermal and mechanical degradation [Reimnitz et al., 1988]. The number of thermo-mechanical niche forming episodes per annum, and thus the erosion rate, is a function of ice-free season duration, the number and type of storms impacting the coastline, sea-level, and summertime sea surface tempera- ture (SST). Localized spatial variation in niche formation is affected by bluff height, degree of ice-richness, and sedi- ment composition, as well as near-shore bathymetry and the presence of barrier islands [Reimnitz et al., 1988 Jorgenson and Brown, 2005 Solomon, 2005]. Further, this landscape is characterized by large, elongated thermokarst lakes, with 70% of the land surface affected by thermokarst lake processes [Hinkel et al., 2005], creating much of the spatial variability in ice-richness along the coast. [4] Understanding contemporary erosion rates is impor- tant because amplified Arctic climate change is leading to rapid and complex environmental responses in both terrestrial and marine ecosystems [Richter-Menge et al., 2008], including record reductions in Arctic sea ice extent [Stroeve et al., 2008], sea-level rise [Richter-Menge et al., 2008], warming of sea-surface [Steele et al., 2008] and permafrost [Brown and Romanovsky, 2008] temperatures, and increasing terrestrial permafrost degradation [Jorgenson et al., 2006] all of which are potential drivers that may affect the rate and pattern of coastline erosion in the Arctic. Any increases in already rapid rates of coastal retreat will have ramifications on Arctic landscapes that provide important freshwater and terrestrial wildlife hab- itat [Flint et al., 2008], subsistence grounds for local communities, the loss of cultural sites that archive human settlement in the Arctic [Jones et al., 2008] represent loci for resource extraction infrastructure [Houseknecht and Bird, 2006], and potentially impact the global carbon budget by transferring organic carbon from terrestrial to marine storage zones [Hayes et al., 2007]. To quantify and better understand how the terrestrial-marine interface has responded to changing environmental conditions over the past 52 years, we delineated a time-series of coast- lines from available aerial photography to measure ero- sion rates for a portion of the Beaufort Sea coast in Alaska. 2. Methods [5] In this study, remote sensing and Geographic Information System (GIS) techniques were combined to delineate coastline positions from available historic and contemporary aerial photography, archived by the U.S. GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L03503, doi:10.1029/2008GL036205, 2009 Click Here for Full Article 1 U.S. Geological Survey, Anchorage, Alaska, USA. 2 ABR Inc., Fairbanks, Alaska, USA. 3 Department of Geography, University of Cincinnati, Cincinnati, Ohio, USA. Copyright 2009 by the American Geophysical Union. 0094-8276/09/2008GL036205$05.00 L03503 1 of 5

Geological Survey, Earth Resource Observation and Sci- ence (EROS) data center, from 15 August 1955 (1:55,000 scale, B&W), 19 July 1979 (1:63,360 scale, CIR), and 18 July 2002 (1:40,000 scale, CIR), and acquired recent imagery on 17 July 2007 (1:50,000 scale, true-color) for a 60 km segment of exposed, north-facing coastline along the Alaskan Beaufort Sea coast. The 2002 imagery were orthor- ectified and served as the base in order to geo-correct the other datasets at a spatial resolution of 2.5 m with mean RMS values of 2.6, 3.0, and 2.4 m, for image mosaics from 1955, 1979, and 2007, respectively. The coastline was defined as the ocean-land interface and delineated using a semi-automated classification technique described by Jones et al. [2008]. Erosion rates were determined for the three available time periods using the U.S. Geological Survey, Digital Shoreline Analysis System (DSAS) [Thieler et al., 2005] at 100 m increments along the 60 km segment of coastline for each time period. The dilution of accuracy (DOA) associated with the coastal retreat rates was estimat- ed by (equation (1)): DOA �� Eg 2 �� Ep1 2 �� Ep2 2 ����RMS1 ��2����RMS2 ��2 q��������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������� DT ��1�� where Eg is the positional accuracy of the 2002 orthophotos (determined to be ��5 m with differential global positioning system data collection), Ep1 and Ep2 represent the pixel resolution of the imagery from a particular year, RMS1 and RMS2 are the root mean square errors associated with the registration of an image mosaic from a particular year, and DT is the time interval for a given time period (modified from Hapke [2005] and Lantuit and Pollard [2008]). Thus, the annualized error associated with the erosion rate measurements is ��0.3, ��0.3, and ��1.1 m a 1 for the time periods 1955 to 1979, 1979 to 2002, and 2002 to 2007, respectively. [6] To compare potential factors causing changes in erosion rates for these time periods, we compiled available datasets describing storm events, directional fetch relative to sea ice extent, and sea surface temperatures. Storm events for the late-open water season (August���October) were determined from the terrestrial observational record at Barrow, Alaska from 1955 to 2007. Storm events with the ability to perform geomorphological work were defined by wind speeds greater than 10 m s 1 for a minimum duration of 6 hr [Atkinson, 2005]. Effective storm power values (speed2*duration) were calculated by summing core storm events for wind directions toward the study coastline for each of our time periods. Ice-pack mediated fetch was measured for the same five wind directions from Lonely, Alaska using median September ice extent positions derived from microwave data from 1979 to 2007 to represent relative open-water fetch distances [Fetterer et al., 2002]. Summertime SST data for the western Beaufort Sea, calcu- lated on a yearly basis for July���September, and defined as the mean temperature over the upper 10 m, were compiled by Steele et al. [2008]. 3. Results and Discussion [7] Results reveal that mean annual erosion rates for the study coastline increased from 6.8 m a 1 (1955 to 1979), to 8.7 m a 1 (1979 to 2002), to 13.6 m a 1 (2002 to 2007) (Figures 2a���2c), indicating that erosion rates may be accelerating. In addition, 24% of the coastline exhibited rates greater than 10 m a 1 in the first time period, whereas this number increased to 43% and 65% in the 1979���2002 and 2002���2007 periods, respectively. Our erosion rate measurements for the earlier time periods agree quite well with rates compiled through previous investigations [Reimnitz et al., 1988 Jorgenson and Brown, 2005], indicating confidence in the imagery and methods used. We also better resolve recent observations that suggest that land area lost doubled relative to historic patterns beginning in the 1980s [Mars and Houseknecht, 2007]. Our more detailed spatial and temporal analysis reveals that it has occurred more recently. Further, observations of coastal retreat along the Beaufort Sea coast west of our study area near Barrow also indicate increased erosion since 2003 [Aguire et al., 2008]. [8] Not only have erosion rates accelerated recently, but during image analysis it appeared that a shift occurred in the magnitude of erosion rates along bluffs consisting of dif- ferent degrees of ice-richness. Thus, we divided the study Figure 1. Historic erosion rates along the Beaufort Sea coast in Alaska. Historic mean annual erosion rates associated with 48 regional segments along the Beaufort Sea coast from Barrow to the Canadian border [Jorgenson and Brown, 2005]. The study coastline is bounded by the black rectangle. L03503 JONES ET AL.: HIGHER AND MORE UNIFORM ARCTIC EROSION L03503 2 of 5