How unusual was 2015 in the 1984–2015 period of North Cascade Glacier Annual Mass Balance?

Abstract. In 1983 the North Cascade Glacier Climate Project (NCGCP) began annual monitoring 10 glaciers throughout the range, to identify their response to climate change. The annual observations include mass balance, terminus behaviour, and accumulation area ratio (AAR). Annual mass balance (Ba) measurements have been continued on 7 original glaciers that still exist. Two glaciers have disappeared: the Lewis Glacier and Spider Glacier. Foss Glacier was discontinued in 2014 as it has separated into several sections. In 1990, Easton Glacier and Sholes Glacier were added to the annual balance program. This comparatively long record from glaciers in one region conducted by the same research program using the same methods offers some useful comparative data to place the impact of regional climate warmth of 2015 in perspective. The mean annual balance of the North Cascade glaciers is reported in water equivalent thicknesses to the World Glacier Monitoring Service (WGMS). From 1984–2015 the mean Ba is –0.54 ma^-1, ranging from –0.44 to –0.67  ma^-1 for individual glacier's. This is equivalent to the WGMS global average for this period of –0.56 ma^-1. The cumulative loss of 17.2 m w.e. and ~ 19 m of ice thickness represents more than 30 % of the volume of the glaciers. In 2015 the mean Ba of nine North Cascade glaciers was –3.10 m w.e., the most negative in the 32 year record, with 2005 the previous maximum loss at –2.84 m. The mean AAR of 3 % was likewise a minimum, previous minimum was 16 % in 2005. The correlation coefficient of Ba is above 0.80 between all glaciers including the USGS benchmark glacier, South Cascade Glacier. This indicates that the response is regional and not controlled by local factors. The similar mass balance losses in alpine glacier regions globally suggest global climate change is the principal driving force.