Once again, a large scale remote sensing analysis shows us which way the winds of (climate) change are blowing. This time the focus is on glaciers in western North America.
New research published in Geophysical Research Letters [link] pinpoints the change in mass for every glacier in western North America between 2000-2018. The work, led by my colleagues Brian Menounos (UNBC) and Romain Hugonnet (CNRS, Toulouse), represents a huge advancement in our understanding of recent glacier change in the region [Disclaimer: I am a contributing author to this paper]. Its difficult to overstate the importance – and, until recently, the impossibility – of getting this information at continental scales.
Traditionally, glaciers are measured by hand: spring field trips to measure winter snow accumulation, and late summer field trips to see how much snow was left up high, and how much ice had melted away down low. The glaciological mass balance is simply the difference between total accumulation and total melt, inferred from measurements made along the glacier from top to bottom. But these trips are tough, time-consuming, weather-dependent, and can be expensive to mount. They may also result in biased estimates of glacier mass change, and only a handful of glaciers in western North America have reliable long-term measurements using the glaciological approach.
Our methods here follow a geodetic approach similar to the one used by Brun et al. (2018) to examine glacier mass losses in High Mountain Asia. We re-processed stereopair imagery from the ASTER satellite (2000-2018) to generate high-quality digital elevation models through time, and supplemented this with elevation models derived from Worldview and Pleaides imagery. For each pixel in these stacks of imagery, we calculate the trend between elevation change (dh, in m) and time (yr). From an assumed density of snow, firn, and ice, the changes in elevation are converted to changes in mass.
The take-home messages, as I see them:
- The average rate of glacier mass loss for the entire period, over all WNA, was 452 (+/- 162) kg m2yr-1. With a total glacier area of 14,000 km2, that works out to over 600 million elephants worth of ice per year.
- There is short-term variability imposed on the long-term trend of glacier mass loss. Big increases (x6) in glacier mass loss were observed between early and later years of the study in the southern and central Coast Mountains of BC, which contain the largest volumes of ice in this region.
- A southward shift in the mean position of the jet stream is probably the main factor in #2: this reduced winter precipitation in the central and southern Coast Mountains, and led to more negative mass balances in the last 10 years. Conversely, the jet stream shift produced neutral conditions (and even slight mass gains) in areas that started to get more winter precipitation: the south Cascades and Glacier National Park.
And what can we do with this information? We can develop and test regional scale models of glacier change and make improved projections of their future changes. The elevation changes also have an unexpected richness: off-glacier we can see the impacts of forest fires, mining and forestry activities, and forest regeneration. Stay tuned for future studies that incorporate this data!
Some example maps of glacier mass change rates are shown below. Supplementary information (including tiles of the mass change rates) can be grabbed here.