Whats a gigatonne?

One challenge in science is framing quantities that we are used to dealing with in a way that people who are not used to dealing with them can understand. For global glaciology the challenge is in explaining mass changes from ice to water in the order of 100s of gigatonnes … but what is a gigatonne?

A tonne (t) is the mass of 1000 kilograms (kg) – which for water, occupies 1 cubic metre (a cube of 1m x 1m x 1m)

A gigatonne (Gt) is 1 billion tonnes, which is 1 trillion kilograms – for water this occupies 1 cubic kilometer (1km x 1km x 1km).

Thats not really helpful is it?

Dr Alex Gardner of JPL recently gave a public lecture, which you can watch here. In it he used a cool visualization to illustrate how huge these quantities are, and I asked him for the image afterwards, so here is what 3Gt looks like in the context of Manhattan:

Data from NASA’s GRACE satellites (which are very cool and have been measuring gravity anomalies over the earths surface since 2002) show that the land ice sheets in both Antarctica and Greenland are losing mass. The continent of Antarctica has been losing about 118 gigatonnes of ice per year since 2002 (with a certainty margin of ± 79 Gt per year), while the Greenland ice sheet has been losing an estimated 281 gigatonnes per year since 2002 (with a certainty margin of ± 29Gt per year). You can see the updated graphs of mass changes from Antarctica and Greenland on NASAs website here.

Together, that is equivalent to 133 of the 3 blue cubes in the picture above converted from ice to water each year since 2002.

Thats a lot, right?

Cumulatively, since 2002 up until March 2016, the NASA data shows Antarctica has lost 1507.5Gt and Greenland has lost 3540Gt. Together that is 5047.5Gt converted from ice to water from Antarctica and Greenland since 2002. So thats over 1600 of the 3 cubes in the picture above and I’m back to the situation where its hard to visualize!! See how tricky this is?

For more interesting visualizations, the Danish Meteorological Institute produces the very brilliant Polar Portal, which plots all manner of data about Greenland. You’ll notice some differences in the exact numbers reported. This is because scientists are continually trying to improve the quality of measures of spatial distribution of mass change over the earth determined from the GRACE satellites. Polar Portal also plots its cumulative mass changes relative to summer 2006 rather than from the launch of the GRACE satellites in March 2002. Don’t let this put you off, get frustrated, or doubt the data. Its  absolutely astounding that we can measure mass changes from space and evolving changes in how the exact values are computed is analogous to improving at a sport: The first time you do a 180 on your snowboard you’re happy just to have landed it, and indeed the main achievement has been met, but over time you do more and more and they get incrementally better and better. Refinement. Different people might give you different tips and have different ideas on how you can achieve this refinement. Refining it won’t change the fact that it was a 180 already, but you’ll likely want to get it as good as you can, right? So it is with science, we will keep on working to get the most accurate estimates of the state of our planet as we possibly can.

About lindsey

Environmental scientist. I am glaciologist specialising in glacier-climate interactions to better understand the climate system. The point of this is to understand how glaciated envionments might change in the future - how the glaciers will respond and what the impact on associated water resources and hazard potential will be.
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