Rules of thumb about ice sheets and sea level rise:

One of the advantages of being part of a research institute are the fascinating conversations that happen over lunch between colleagues working in different areas. Today was a classic with conversation ranging from the stratospheric effects of the Hunga-Tonga eruption to the different types of snow crystals that form in snow packs and their impacts on sea ice. However, the conversation started with a request to me for some rules of thumb on sea level rise, so here they are: 

The Greenland ice sheet loses on average around 250 to 280 Gigatonnes of ice each year – that’s from all processes including melt and surface runoff, iceberg calving, basal melting and submarine melting.

It varies a bit from year to year but over the last 26 years the ice sheet has either lost ice or been neutral (and it’s had very few neutral mass budget years).

Change in mass of the Greenland ice sheet from the GRACE and GRACE-FO satellites since 2022

The Antarctic ice sheet loses on average about 100 Gigatonnes of ice net each year (probably) from all processes, it receives about 2000 to 2500 Gigatonnes of snow (depending a bit on where you measure Antarctica to end) whereas Greenland receives around ~700 gigatonnes of snow.

The small glaciers and ice caps around the world contribute a bit more to sea level rise in total each year than each of the big ice sheets currently, but they will be quickly exhausted. As there are thousands of small glaciers, most of which are not well monitored, we have to estimate how these are changing using models. It appears that on avergae they add around 0.7 to 1 mm of global sea level rise each year.

Glacier changes are not well measured at most glaciers but this analysis from Copernicus is based on a few that are in Europe.

The thermal expansion of the oceans is still the largest part of currently observed sea level rise but on an annual basis, the cryosphere now often contributes more.

As I’ve elaborated on before, 1 Gigatonne of water is hard to visualise, it is a cube 1 km long, 1 km wide and 1 km high and about 360 Gigatonnes (or km3) raise global average sea level by 1 mm, so Greenland contributes around a half to three quarters of a millimetre to global sea level every year. My old friend Lindsey Nicholson at Innsbruck University has a cool blog (which you should check out here) and shows this visualisation if it helps..

What a gigatonne looks like, visualisation shared by Dr Alex Gardner, JPL from this talk on glaciers and sea level rise

Since the early 1990s sea level rises about 3mm every year, but over the last 5 years it has been closer to 4.5mm per year. The curve over the last 2 decades has followed a quadratic shape rather than a linear shape – put simply, this means sea level is accelerating. The sea rose 10mm from January 2020 to August 2021.

Global mean sea level rise since the early 1990’s as shown in the WMO state of the climate report 2022

An El Nino, which some are warning could occur this year, may cause a temporary pause or at least slow down in sea level rise, even as global air temperatures increase, mostly due to the large amounts of rain that are associated with it, but this will only be temporary.

While the rate (3-4 mm per year) doesn’t sound like very much, every mm counts, increasing the risk of coastal flooding and storm surges affecting coastal communities.

Finally, global sea level rise is not distributed evenly, broadly speaking, the further away from an ice mass you are, the more likely it is to affect your local sea level, so Greenland matters less than Antarctica in Northern Europe.

NOAA’s visualisation of observed sea level rise from satellites in the background and at tide gauge locations (the round dots) since 1993, note the uneven pattern which reflects processes like ocean currents, atmospheric circulation and winds, local relative land movements and gravitational changes due to changing ice masses.

I hope these little rules of thumb help. Feel free to add more (or disagree) in the comments..

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