(Part of the How to Talk to a Global Warming Skeptic guide)

Objection: The Antarctic ice sheets are actually growing, which wouldn’t be happening if global warming were real.

Answer: There are two distinct problems with this argument.

First, any argument that tries to use a regional phenomenon to disprove a global trend is dead in the water. Anthropogenic global warming theory does not predict uniform warming throughout the globe. We need to assess the balance of the evidence.

In the case of this particular region, there is actually very little data about the changes in the ice sheets. The growth in the East Antarctic ice sheet indicated by some evidence is so small, and the evidence itself so uncertain, the sheet may well be shrinking.

But even this weak piece of evidence may no longer be current. Some recent results from NASA’s GRACE experiment, measuring the gravitational pull of the massive Antarctic ice sheets, have indicated that on the whole, ice mass is being lost.

Second, ice-sheet thickening is not inconsistent with warming! Warmer climates tend toward more precipitation. The Antarctic is one of the most extreme deserts on the planet. As it warms, we would expect it to receive more snow. But even a whopping warming of 20 degrees — say, from -50 degrees C to -30 degrees C — would still leave it below freezing, so the snow wouldn’t melt. Thus, an increase in ice mass.

While on the subject of ice sheets: Greenland is also growing ice in the center, for the same reasons described above. But it is melting on the exterior regions, on the whole losing approximately 200 km3 of ice annually, doubled from just a decade ago. This is a huge amount compared to changes in the Antarctic — around three orders of magnitude larger. So in terms of sea-level rise, any potential mitigation due to East Antarctic Ice Sheet growth is wiped out many times over by Greenland’s melting.