GRACE is Beauty in Motion

This week, I heard two talks on the Gravity Recovery and Climate Experiment (GRACE), an experiment being developed by some of my colleagues here in Florida. The goal of the experiment is to measure variations in the mass distribution of the Earth, using a pair of orbiting satellites:

JPL

As the satellites orbit, they pass over regions of greater and smaller mass, causing them to speed up or slow down. These variations will affect the leading spacecraft first, causing the separation between them to change. We can measure these changes with a laser interferometer, just like the ones used by LIGO and LISA. This results in a detailed map of how mass is distributed over the planet:

JPL

The heaviest points (in red) tend to be in mountain ranges, like the Alps and Andes.

You might be wondering (as I did) how this relates to climate. The key is that water is dense stuff, so when it moves around, it can significantly change the pull of gravity. As snow and ice melt, the water will flow to different places. The researchers have made their data available online, so I tried putting together some code to make summary plots.

The data I linked to above records the liquid water equivalent thickness, which is the depth of water over an area that would result in the measured mass per area. It covers 2002 to 2020, giving a planet-wide measurement every month. I plotted the data on a Mollweide projection and animated it in time:

I wasn't able to make it as clean as some of the diagrams the presenters showed, but you can see some seasonal variations, particularly in the Amazon region, and you can see things get significantly redder as time goes on. Looking at a single point in the middle of the North Atlantic shows an alarming trend:

As the glaciers and ice caps melt, that water flows into the oceans, raising the levels. On top of that, warm water expands, so any kind of heat added to the oceans will increase the depth. I hope we can use tools like GRACE to learn how best to reverse trends like this, and how to emphasize how necessary it is!