Height Change at Profile 4 from 1993 to 2019

From JIRP's beginnings in the late 1940s, through to the early 1990s, the Taku Glacier thickened, as evidenced by the rising surface elevation at the base of the Camp 10 nunatak. In the summer of 1950, a borehole was drilled in the glacier at the location of Profile 4, with 8" x 8" timbers and other drilling supplies stored at the base of the nunatak at the end of the 1950 summer season. Throughout subsequent decades, the surface of the glacier rose, burying the drilling equipment. In the summer of 2018 the timbers and other equipment were observed to have re-emerged due to the thinning of the glacier. High-precision GPS surveys were first conducted on the Juneau Icefield during the summer of 1993, and by the summer of 1998 it appeared likely that the Taku Glacier's thinning phase had begun.

Profile 4, at an elevation of 1,118 meters (WGS84 ellipsoid; or 1,114 meters mean sea level) is one of the main transverse GPS survey profiles on the Taku Glacier. It is located roughly 4-6 kilometers upglacier from the historical (1946-2017) firn line. This profile has been extensively surveyed during the past 70 years, although it wasn't until 1993 that GPS was used at Profile 4. As a result, this graph presents data only from 1993 to 2019. The profile is surveyed July 25 each year, at which time the surface height is measured at each of the 31 survey points within the profile, and the entire profile's mean surface elevation is calculated.

This graph summarizes the annual (year-to-year) surface height change and cumulative surface height change from 1993 to 2019. The vertical red and green bars show the surface height change compared to the previous year. For example, the surface height of the Taku Glacier at Profile 4 decreased an average of 2.79 meters from July 25, 2018 to July 25, 2019.

The blue line plot is the cumulative surface height change relative to the first GPS survey in 1993. For each year, it indicates how much higher or lower the surface was when compared to the baseline year of 1993. For example, the surface at Profile 4 was 11.69 meters lower on July 25, 2019 than it was on July 25, 1993.

After 27 years of detailed GPS surveying at Profile 4, a pattern showing both positive and negative surface height change from 1993 to 2012 is seen. During these years, annual (year-to-year) surface height alternately varied from +2.03 meters to -1.64 meters, with the cumulative change from 1993 to 2012 being -1.85 meters. However, negative surface height changes have occured every year since 2012. The time period from the summer of 2012 to the summer of 2013 seems to be the inflection point at which the thinning of the Taku Glacier has drastically increased. As of July 25, 2019, the surface elevation of the Taku Glacier at the location of Profile 4 is 11.69 meters lower than it was in 1993.

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