Much Ado About Arctic Sea Ice?
by Bill Chameides | April 12th, 2010
posted by Erica Rowell (Editor)
The Wall Street Journal's recent "report" on sea ice and global warming's death is an exaggeration. (NASA/N. Untersteiner)
Imagine my surprise when I read in Stephens’s April 6 op-ed that “global warming is dead, nailed into its coffin one devastating disclosure, defection and re-evaluation at a time.”
To support his announcement, Stephens described a new “devastating” development in the world of climate science. It’s about Arctic sea ice. His take on the issue, in my opinion, is a little skewed, more in the category of half-truths than facts. Let’s talk a look.
The Sea Ice Context
The issue is about how much ice floats on the Arctic Sea and what’s behind that extent of ice. These are not just climate-science curiosities — because the ice affects the reflectivity of the Earth’s surface, it’s a key factor in determining how much heat the Earth absorbs from the Sun.
We have data on the ice extent dating back to the 1950s when American submarines began hanging out in that part of the world, but since 1979, satellite-borne microwave sensors have been able to collect much more extensive data. (See more about monitoring et al. sea ice info here.)
The trend is clear: the Arctic’s ice cover is retreating. Perhaps more disturbing, the rate of melting has been larger than what climate models predict (see figure).
During the summer of 2007 the loss of sea ice was so extreme it led some to announce we’d reached a tipping point and summers in the region could be ice-free in as few as 10 years. Such dire predictions were not unanimous but were fairly widespread. And most probably wrong.
Enter Bret Stephens. To make his case for global warming’s demise (would that it were true), Stephens first quotes this from a news story from last fall (which appeared in the Telegraph, not, as he claimed, the Guardian): “the Arctic is now melting at such a rate that it will be largely ice free within ten years.” Then he points to a more recent news story in the Guardian (he got the source right this time) about new findings by “Japanese scientists” that, he claims, attribute “the record breaking loss of ice in the Arctic ocean in recent years … to the region’s swirling winds and is not a direct result of global warming.”
The Actual Findings in a New Paper on Arctic Sea Ice and Winds
OK. But what did the “Japanese scientists” actually report in their paper? Maybe it’s just me, but I prefer to get my scientific information from the scientific literature not the Telegraph or the Guardian.
The paper in question was published in Geophysical Research Letters last week by Masayo Ogi of the Japan Agency for Marine-Earth Science and Technology and colleagues. They do in fact report that winds have played a role in the shrinking of Arctic sea ice, but that’s not especially new (see here, here, and here).
How can winds cause the extent of sea ice to decrease? Arctic winds are dominated by opposing atmospheric pressure patterns known as the Arctic Oscillation. Sometimes the winds tend to pile sea ice up against the Arctic Ocean’s western edge near Siberia; during other phases the winds move ice eastward where it is more susceptible to both melting and being pushed out of the Arctic altogether. (See more detailed explanations here [pdf] and here.)
By correlating historical wind data with the September Arctic sea ice extent, the authors tried to tease out the role of winds in controlling the amount of sea ice at summer’s end. They found that “wind forcing accounts for 50% of the variance of the change in September Arctic sea ice extent [SIE] from one year to the next … and it also explains roughly 1/3 of the downward linear trend of SIE over the past 31 years.”
That’s really interesting and important — winds are playing a significant role in the Arctic’s sea ice decline. Much, possibly half, of the year-to-year variability (like 2007’s extreme dive of sea ice) is driven by winds. (It’s worth noting that the study’s wind-field metric differs from the sea-level pressure data largely relied on in other studies. This in turn has led to some differences in interpreting how different wind patterns interact.)
But guess what? It also means that most of the downward trend in sea ice over the past 30 years cannot be explained by winds.
The WSJ’s Coup de Grace
Trying to account for 2007’s dramatic drop in sea ice cover may have brought some predictions of ice loss too far forward. But if there was some overreach there (the fact is ice is melting and most likely in part because of global warming), there’s also some of that going on in the WSJ’s opinion pages.
Stephens notes that “the extent of Arctic sea ice in March was around the recorded average” for the period from 1979 to today. True enough, but if you take it as I did and as I think most folks would — as implying that there’s not been much loss of sea ice since 1979 — it’s quite misleading.
First of all, the sea ice extent this past March is consistent with a long-term downward trend that shows March sea ice cover dropping by about 2.6 percent per decade.
More importantly, March comes at the end of the winter — when sea ice extent is at its maximum and even thin ice counts as ice coverage. A very different picture emerges from the September record of sea ice extent — when the winter’s thin ice disappears. Suffice it to say, the September data show a significant loss of ice since 1979, declining at a rate of 11.2 percent per decade, relative to the 1979-2000 average (see figure). Notably, 2009 marked the third lowest September ice cover on record.
The moral of this story: half-truths doth lousy coffin-nails make.filed under: Arctic, climate change, faculty, global warming, science
and: Arctic Oscillation, Bret Stephens, ice, sea ice, Wall Street Journal, wind