An interesting new paper has just been published in Nature Geoscience by Romain Millan, Jeremie Mouginot, Antoine Rabatel and Mathieu Morligheim on the velocity and thickness of the world’s glaciers. They make a revised estimate of global glacier ice volume. They aren’t looking at ice in the Greenland or Antarctic ice sheets, just glaciers worldwide.
This new paper provides an update on the global glacier ice volume. It gives us a new estimate of sea level equivalent of ice in all glaciers worldwide. The sea level equivalent is how much global sea levels would rise if all the ice melted and it is one way to visualise or understand ice volume. It is perhaps more useful than just estimating ice volume in cubic kilometres or gigatonnes. I’ve previously written about this here.
The authors have generously made all the data available and accessible and you can explore it yourself in this Mapbox here: https://ige-vis.univ-grenoble-alpes.fr/glaciers/index.html
Calculating the global glacier ice volume
This paper uses an improved method to account for ice above flotation (rather than just ice above sea level), and this therefore improves the estimate of global ice in tidewater glaciers (that is, those glaciers that terminate in the ocean). Ice below sea level, and indeed below flotation, does not raise global sea level when it melts and so is not considered in sea level equivalents (SLE).
The schematic below illustrates this point.
Millan et al. also use the new ice velocity dataset to make an improved estimate of the bed of the world’s glaciers (i.e., glaciers in the Randolph Glacier Inventory; Pfeffer et al. 2014). This is important because we don’t have many direct observations of the bed below the world’s glaciers. Mountain glaciers tend to be up mountains; they’re high up and steep, and often heavily crevassed. In situ radar measurements tend to be the best way to map the bed of the glacier, but this usually means dragging a heavy radar across the ice. As a result, only a few glaciers worldwide have a detailed bed topography mapped out.
Millan et al. 2022 use their new observations of global glacier velocity to calculate ice thickness. Surface motion of glaciers and ice surface slope can be directly related to ice thickness (as long as the glacier deforms in simple shear, which is the case for most mountain glaciers). The new estimates of ice thickness are calibrated against global databases of ice thicknesses.
A reduced global ice volume?
This paper reveals a somewhat reduced estimate of global glacier ice volume (sea level equivalent) of 257 ± 85 mm SLE. This is partly due to the improved modelling of the bed, with better constraints of glacier velocity, and partly due to the use of ice above flotation instead of ice above sea level. In Antarctica, some islands and ice caps are excluded (they were included in previous global estimates), because they are rather connected to the ice sheet. Their ice volume should therefore be included with the ice sheet, and not with global glaciers.
If these areas are included, the ice volume is similar to previous estimates: 311 ± 99 mm SLE (Farinotti et al. 2019 consensus estimated 324 ± 84 mm SLE). So, within uncertainty of previous estimates and a 4% reduction in ice volume. The excluded polar islands and ice caps would need to be included in ice sheet estimates instead of with glacier estimates (the two are dealt with separately).
Reduced ice for water resources
The really important finding of the paper for human societies is that there is less ice in the low latitudes, mainly in South America, where glaciers provide a dependable and very important downstream water supply. The Andes are mountain ‘water towers’, providing water to downstream communities.
Cities like La Paz in Bolivia (2.2M inhabitants) are very dependent on the water provided for glaciers, which reliably release water, providing a buffer for droughts. As the glaciers shrink, the amount of water they can provide downstream declines, meaning that droughts are more likely (Millan et al., 2022).
These glaciers have a comparatively low ice volume (compared with, for example, Alaska) so negligible importance for sea level, but critical for downstream water supply in the dry season (Millan et al. 2022).
The news is better in other places; there is a slight increase in the estimate of water held in Himalayan glaciers, which postpones slightly the timing of ‘peak water’. New hydrological modelling will be needed to determine the exact impact.
How will this affect global sea level forecasts?
From 2000 to 2019, melt of global glaciers accounted for 21% of observed sea level rise (Hugonnet et al., 2021). Glacier melt is an important component of sea level rise, and is projected to remain so til 2100 under present day government policies (Edwards et al., 2021). So, if there is less ice than we thought in glaciers, does that mean that there will be less sea level rise?
Unfortunately, probably not. The new estimate only means that there is 4% less ice in global glaciers given that the excluded polar islands and ice caps will be included in the projections from ice sheets. The ice isn’t gone, just included elsewhere.
There may be a small decrease in sea level rise from glaciers once these new data are included in projections (like this one from Edwards et al. 2021), but I doubt it will be very different (the biggest uncertainty is still how much carbon will be emitted). The recent estimate of sea level rise from glaciers from Edwards et al. 2021 suggested that global glaciers will contribute 13 cm to sea level rise by 2100 AD under present-day policies and committments (25 cm if the Greenland and Antarctic ice sheets are included). That leaves plenty of global glacier ice volume left.
This is because the places with the biggest ice volumes (like Alaska and Arctic Canada and the Greenland periphery), and therefore the biggest contributions to sea level rise from glaciers (Hugonnet et al. 2021), will still have lots of ice left even 200 years from now. The updated analysis reduces the sea level equivalent in Alaska and Western Canada by 4%, for example. The ice not included in Antarctica will still need to be accounted for in ice-sheet projections.
On those multicentennial timescales, it is ice from the Antarctic and Greenland ice sheets that will come to dominate sea level rise.
- What is the global volume of land ice and how is it changing
- Calculating ice volume and sea level equivalents
- What is the future of the world’s glaciers
- Glaciers as a water resource
- Mountains as water towers
- Observing glacier change from space
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Farinotti, D., Huss, M., Fürst, J.J., Landmann, J., Machguth, H., Maussion, F., Pandit, A., 2019. A consensus estimate for the ice thickness distribution of all glaciers on Earth. Nat. Geosci. 12, 168–173. https://doi.org/10.1038/s41561-019-0300-3
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Millan, R., Mouginot, J., Rabatel, A., Morlighem, M., 2022. Ice velocity and thickness of the world’s glaciers. Nat. Geosci. https://doi.org/10.1038/s41561-021-00885-z
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