Antarctica losing six times more ice mass annually now than 40 years ago

>Your argument is misleading because it assumes uniform ice loss across the continent

I'm not assuming that. "Average" and "uniform" are two different things. I never said anything about "uniform".

Take a look at the ice loss visualization [1] from NASA's website. On the west side there are ice losses as high as 300m (dark red), on the east side some ice gains of about 100m (light blue). All those losses and gains are estimated with some instruments. If you are in one of the areas where the ice loss was estimated to be about 100m, you look at your initial elevation, final elevation and take the difference. Will you elevation difference be -100m, or -98m? There will be some measurement uncertainty. Overall, uncertainties from different instruments tend to cancel out, that't the law of large numbers. Overall, I doubt we have that many numbers (or so little individual measurement uncertainty to begin with) to end up with a final measurement uncertainty that is so small.

Take a look at article [2] about the estimation of precipitations in Antarctica. It was published in March 2018 in the journal "Polar Science" owned by Elsevier, which as far as I can tell does not publish junk articles. The general tone is that there are large uncertainties in the estimation of precipitation in Antarctica.

"The study of Antarctic precipitation has attracted a lot of attention recently. The reliability of climate models in simulating Antarctic precipitation, however, is still debatable"

"The current method of data collection relies on measurement with limited temporal basis, with distances between measurements exceeding 1300 km (Knuth et al., 2010). This makes instrumental-based measurement of precipitation in Antarctic highly unreliable (Genthon et al., 2003)."

Of course, such an article does not get much press. How would one report it? "New research shows large uncertainties in our understanding of the mass balance in Antarctica" ?

[1] https://svs.gsfc.nasa.gov/30880

[2]https://www.sciencedirect.com/science/article/pii/S187396521...

Why are you talking about Earth's mass? We are talking about Antarctica here. If I weight myself on a scale that has 0.2 pound precision (like the one I have), and then somehow I tell people I lost 0.002 pounds, some people will be quite skeptical of my claims. In the case of Antarctica, how is one supposed to quantify the accuracy of its net ice loss? By comparing it with the mass of an elephant of a blue whale [1] ?

[1] https://www.washingtonpost.com/news/energy-environment/wp/20...

Thanks for taking your time to explain all this. It's quite cool. I spent some time with your link and also with the wikipedia page about GRACE. This is quite an ingenious experiment. First I thought the two satellites were at different altitudes, one in LEO and one in, let's say GSO. The high altitude satellite perceives the Earth more or less like a mass point, while the low altitude satellite is much more sensitive to the distribution of mass, especially at the Earth's surface. You could in principle use several satellites at different altitudes, and do some sort of "triangulations". This way, you could potentially learn about the mass distribution deep inside the Earth. Anyway, the actual way it's done is also cool, a lump of mass, like a mountain pulls faster the leading satellite, Jerry, and the second satellite, Tom, lags a little behind. Later on, Tom goes over the same mass and catches on.

That being said, the complexity of this model sounds absolutely humongous. Complex models are exciting for mathy guys to work with, but are inevitably subject to more model risk. As coders say, the number of bugs is roughly proportional with the number of lines of code.

You're going to say that this numerical calculation is bug free. A numerical calculation with 4 thousand mass element (mascons) and distance measurements probably in the billions, and lots and lots of approximations and contributions from the sun and the moon and the planets. Of course you are not going to say it is bug free. Should we trust ice loss calculations coming from GRACE? To be honest, after I read your explanation and all other things about GRACE, I'm less skeptical. I don't fully trust it, but I don't fully distrust it either.

But as you said, this is not that relevant, since the actual result published was not based on GRACE, but rather on ice flow models. Now these models I take with quite a large grain of salt. Why? Models produce estimates, and the estimates generally include an uncertainty level. The uncertainty reflects only how the uncertainty in inputs propagates to the uncertainty in the final result, not the model uncertainty per se.

Take a look at the figure 1 in [1]. It shows the precipitation in Antarctica over 2.5 decades based on several models. The average number (labeled "multi-model mean") shows very little variability from year to year, and you could infer that very little uncertainty as well. However, this is just an artifact of taking the average of several not very highly correlated time series. The model uncertainty is better reflected by the wide range of the numbers produced by the different models. If you look at this range, it's about 200mm/y, which is 10 times the net ice loss we are talking about here.

Do you see why I'm a bit skeptical?

But anyway, thanks again for taking your time with me. I learned something cool today.

[1] https://www.sciencedirect.com/science/article/pii/S187396521...