Sea Level Rise: Why We Care, Part 3

In the last post (Part 2), I started to connect climate change to sea level, focusing on how very small increases in ocean temperature can lead to meaningful changes in sea level via expansion of water.  In this post, Part 3, I want to draw a connection between climate change and the other big process that drives most of the climate driven sea level rise projected for the coming decades:  The addition of new water to the ocean basins.

This component of sea level rise is similarly straight-forward (like the thermosteric effect) and driven by processes that we tend to interact with on a day-to-day basis:  If you warm up ice, it melts.  In this particular case we are particularly concerned with large masses of ice grounded on land, things like glaciers perched on mountains or at the edges of ice sheets:

Global map of glaciers (in blue) from the Randolph Glacier Inventory.  See https://earthobservatory.nasa.gov/images/83918

As well as ice sheets (in white in the map above) in Greenland and Antarctica.  As these masses of ice melt, and since they are perched on land masses, when they melt the meltwater flows into the ocean basins, effectively filling them.  Details about how these big masses of ice melt (and also how quickly) are one of the primary sources of uncertainty in sea level rise projections, and compound uncertainties just about climate change (i.e. even if we knew exactly how certain emissions scenarios would influence future temperature, we still wouldn't know exactly how much melt that change in temperature in either the air or water would cause).

What we do know, though, is that these big masses of ice ARE currently melting in a net, long-term sense.  The GRACE satellite mission launched in 2002, and provides near-continuous monitoring of the gravitational forces exerted by the mass of the Earth.  Here is the thing - as these big masses of ice melt, they lose mass.  Therefore their gravitational attraction changes, which can be measured by this satellite mission.  As a consequence you get data like this for Greenland:

Courtesy of NASA: https://svs.gsfc.nasa.gov/30879

or this one for Antarctica:

Thank you NASA!  https://svs.gsfc.nasa.gov/30880

And both of these allows us to assess how much mass is lost or gained from these big masses of ice (and also where it is lost from...which is also interesting and tells a story).  By way of reference, melting roughly 362 Gt of ice leads to 1 mm of global average sea level rise.  Lets play a quick numbers game:  If these masses of ice continue to melt at the same rate as they have since 2002, contributing something like 1.5 to 2 mm/yr to global average sea level...well that will be hard but probably manageable.   That leads us to something like 1/2 a foot of sea level rise by 2100 (from the ice alone...).  The concern though, is that these ice sheets are only just getting started (and there is some evidence for that), and that the possible contributions from Antarctica alone by 2100 may measured in multiple feet.

Its worth noting that sea ice, which is frequently covered as a climate change indicator, does NOT play a direct role in driving long-term sea level change (which isn't technically totally true...but the influence is very small).  It is frozen in the ocean, and therefore displaces seawater.  However, sea ice likely plays a role in buttressing glaciers and ice sheets, slowing them down a bit as they flow into the ocean.