The Island of Doubt

A new study of Antarctic ice trends by an impressively international group of scientists has raised the alarms bells, and not just in the blogosphere, either. We should always take notice when reputable researchers find things are worse than expected, but let's not put out tenders for the Ark just yet.

The first thing to note about "Recent Antarctic ice mass loss from radar interferometry and regional climate modelling" in the latest edition of Nature Geology (doi:10.1038/ngeo102) is that the abstract begins with "Large uncertainties remain in the current and future contribution to sea level rise from Antarctica." And this study doesn't change that.

What it does do is suggest that anyone trying to figure out what global warming will do to sea levels should broaden their focus from Greenland''s ice sheet to include Western Antarctica. Evidence that the former is on the verge of, or already has entered, a period of rapid collapse, is mounting. What the new findings by Eric Rignot et al point to for the first time is a similar acceleration in ice loss way down under;

In West Antarctica, widespread losses along the Bellingshausen and Amundsen seas increased the ice sheet loss by 59% in 10 years to reach 132 +/ -60 Gt yr^-1 in 2006. In the Peninsula, losses increased by 140% to reach 60 +/- 46 Gt yr^-1 in 2006. Losses are concentrated along narrow channels occupied by outlet glaciers and are caused by ongoing and past glacier acceleration. Changes in glacier flow therefore have a significant, if not dominant impact on ice sheet mass balance.

That's just shy of a total loss of 200 Gt over a decade, which is right up at the high end of the range of previous estimates. According to the Globe and Mail's coverage, that represents an 75 percent increase over what was happening before. For more context, we are also told that

The Intergovernmental Panel on Climate Change, the UN-sponsored scientific body that compiles information on global warming, said last year that studies on the subject have been all over the map.

Some have suggested the ice cap was expanding by 50 billion tonnes a year from 1993 to 2003, while others projected losses over the same period of up to 200 billion tonnes.

And in the Washington Post, this explanation:

"Something must be changing the ocean to trigger such changes," said Rignot, a senior scientist with NASA's Jet Propulsion Laboratory. "We believe it is related to global climate forcing."

It's yet another example of how new data tend to fall on the worse side of yesterday's mid-case scenarios. It is worrisome, but so are a lot of things. It is important to remember that the study only looks at a decade worth of information. As Gavin Schmidt and Stefan Rahmstorf caution over at Real Climate, most subjects in climatology require at least 15 years of data for meaningful signals to emerge from the random-weather-generated noise.

For example, despite some honorable efforts to supply some context for the validity of temperature trend modeling by Roger Pielke Jr. and others, you can't say a lot with just eight years of data:

... if you start to take longer trends, then the uncertainty in the trend estimate approaches the uncertainty in the expected trend, at which point it becomes meaningful to compare them since the 'weather' component has been averaged out. In the global surface temperature record, that happens for trends longer than about 15 years, but for smaller areas with higher noise levels (like Antarctica), the time period can be many decades.

Schmidt and Rahmstorf are discussing temperature, not ice, but the same principle applies. First, let's see what other observations and other models have to say, and let's see if we can't look a bit further back in time, then, if the Rignot study enjoys a fair bit of support, break out the lifejackets.

In the meantime, there's still no reason not to continue pressing the powers that be, and will be, that climate change should be top of their agenda. Sea level rise scenarios, after all, may make for dramatic PowerPoint animations, but they're far from the only threat to civilization posed by a warming planet.