Also blogging at Climate411.
A recent paper in Nature Geoscience reported large increases in ice loss from West Antarctica over the past 10 years. It’s a sobering result that’s in line with earlier, independent studies.
This paper was covered by a Kossack who noted that skeptics often claim that "Antarctic ice is increasing, not decreasing".
Today I thought I'd explore the science behind that claim: why do some people say that Antarctic ice is growing?
"Antarctic ice" includes the massive ice sheet and surrounding sea ice. It's important to specify which of these you're talking about, because they have different effects, involve different processes, and show different trends. The Antarctic ice sheet is shrinking. Antarctic sea ice shows a non-significant upward trend.
The Antarctic Ice Sheet
There are three regions of the Antarctic ice sheet: the East Antarctic Ice Sheet (EAIS), the West Antarctic Ice Sheet (WAIS), and the Antarctic Peninsula. Most studies indicate that the EAIS is unchanged or growing slightly, while the WAIS and the Peninsula are losing ice.
Measurements of the entire ice sheet from 1993 to 2003 have ranged from 50 gigatons (Gt) growth per year to 200 Gt loss per year, according to the IPCC Fourth Assessment Report (IPCC AR4). But overall, the trend is towards loss of ice. Even with the higher uncertainty for Antarctica compared to Greenland, the AR4 concludes that "losses from... Antarctica have very likely contributed to sea level rise".
So why do some people imply the opposite?
Inhofe's recent "report" quotes Duncan Wingham as saying: "72% of the ice sheet covering the entire land mass of Antarctica is growing." It’s hard to know for sure, but if Wingham was referring to the EAIS, he’s correct that some older studies inferred increased snow accumulation there. What’s easy to miss is that Wingham said parts of the Antarctic ice sheet are growing rather than shrinking.
More than that, the most recent studies have found no evidence of increased snow accumulation on the EAIS overall.
And then there is the question of model projections versus observations. The IPCC AR4 says "Current global model studies project that the Antarctic Ice Sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall."
This means the models made two predictions: more snow and little melt. But the IPCC also points out that recent observations don’t match the model projections. Antarctic ice is, in fact, shrinking. The ice sheet models were wrong on both counts.
Antarctic Sea Ice
Sea ice in the northern hemisphere is decreasing rapidly (see for example, here and here). In contrast, Antarctic sea ice has shown a slight (but statistically insignificant) upward trend. In fact, last year sea ice extent in the southern hemisphere set a record high. What's going on?
When the sea ice extent record was announced, I asked Michael Oppenheimer (a lead author of an IPCC AR4 chapter) for his take on the situation. Here's the summary of our conversation.
The sea ice situation around Antarctica is much more complex than that in the Arctic (not that the Arctic is simple). First, the temperature trends are complex. Except for the Antarctic Peninsula, where it's definitely warming, there is no clear trend (possibly in part because there are so few weather stations).
However, even with warming, the effect on sea ice might differ from the north. One explanation is that extra precipitation leads to enough freshening of the surface ocean so that the freezing point increases. Thus, sea ice may expand even with a slightly warmer ocean surface.
For more about research on Antarctic sea ice, check out this NASA site.
Final Thoughts
Scientists' understanding of Antarctica has some catching up to do, compared to many other regions around the world. As more research projects are undertaken, and more data are collected and analyzed, scientists will fill in the gaps. The remaining questions (and sometimes counterintuitive patterns) are what make science so interesting. But don't forget that these sorts of "scientific uncertainties" don't take away from our overall understanding of global warming and its causes, which is extremely solid.
Further Reading
As always, the IPCC report has virtually everything you'll ever want to know about climate science. Chapter 4 is devoted to changes in snow, ice, and frozen ground. Detailed discussion of ice sheets starts in Section 4.6 on page 361. Sea ice is discussed in Section 4.4, page 350.