November 11, 2004

Krill the Messenger

The Los Angeles Times reports that a decline in krill stock in the oceans around Antarctica could spell doom for the region’s whales, seals, and penguins. Global warming is their suspect, but a close look at the evidence is enough to acquit.

“Antarctic Food Chain in Peril, Study Finds” screams the headline. The Nov. 4 Los Angeles Times cautions that “Krill—the heart of the rich Antarctic food chain nourishes whales, seals and penguins—have declined by more than 80% in the last 25 years in key ocean regions, according to a new study that links the loss to warming temperatures.” The result of the higher temperatures, writes reporter Usha Lee McFarling, “is a diminished ice cover in some parts of the waters surrounding Antarctica.” No ice, no krill, she reports: “Krill larvae require sea ice to survive the winter….Without sea ice, the larvae starve.”

McFarling based her article on a study led by Angus Atkinson of the British Antarctic Survey, published in Nature magazine the same day. Atkinson does indeed relate the observed declines in krill to sea ice conditions, illustrating the relationship with two figures. The first illustration (reproduced as our Figure 1) shows the history of krill density in the Southwest sector of the Atlantic Ocean—a region Atkinson defines as lying just off the tip of the Antarctic Peninsula, the thin arm of the Antarctic continent that is reaching toward the southern tip of South America. That region is one of the most productive krill regions in the Southern Ocean. Certainly there seems to be a decline in krill numbers in this region since the mid-to-late 1970s.

Krill Density

Figure 1. Temporal history of krill density in the southwest Atlantic sector of the Southern Ocean, 1976-2003 (source: Atkinson et al., 2204).

To determine whether sea ice conditions were associated with the observed decline in krill, Atkinson related krill density to the duration of fast-ice (ice connected to the shoreline) measured at the South Orkney islands (a small island chain that lies near the middle of the oceanic study region) the previous winter. He postulated that winter ice serves as prime larvae over-wintering grounds—the more ice, the better the survival conditions and the greater the krill population the following summer; that relationship is reproduced in our Figure 2.

Sure enough, there seems to be strong correlation between the previous winter’s sea ice duration and krill density. Taken together, the two figures give the impression that declining krill populations are related to declines in sea ice duration, which, it goes without saying, is related to global warming—after all, the region around the Antarctic Peninsula has been described by Atkinson as “one of the world’s fastest-warming areas.”

Of course, without the tie-in to global warming—spoken or unspoken—the results do not make headlines in one of the world’s leading newspapers (that krill populations are at the bottom of the food chain that includes everyone’s favorites—whales, seals, and penguins—doesn’t hurt either).

Krill Ice

Figure 2. Relationship between winter ice duration at the South Orkney Islands and krill density. The longer there is ice, the more krill there are (source: Atkinson et al., 2004).

One last figure demonstrates the relationship between krill decline and sea ice loss conclusively: The temporal history of the winter duration of fast-ice observed at South Orkeys. But wait a minute! Where is the key figure in the Atkinson article that shows the recent decline in sea ice duration around South Orkneys? Try as we might, we couldn’t find it in the Nature article. That seemed a bit odd, so we went out to look for the data on the web. After a couple of hours of searching, we finally located it. Our Figure 3 shows what we found. Now we know why Atkinson didn’t include it in his article—there has been no change in the duration of fast sea ice at South Orkeys from 1975 to 2000, the period associated with the steep krill density declines! How can declines in sea ice related to global warming be responsible for the large krill declines when the winter sea ice duration is not declining at the location where the relationship developed?

Ice Duration

Figure3. The history of winter ice duration observed at South Orkney Islands, 1975-2000 (source: Clarke and Harris, 2003).

Furthermore, the record of sea ice duration at South Orkneys actually begins in 1903. Our Figure 4 shows the complete record. It shows that sea ice conditions are variable from year to year, but that since about 1960 the multiyear average has been pretty constant. Contrast that with what took place during the first part of the 20th century. In the early decades, the winter sea ice duration averaged about 200 days per year (or even longer during the period around 1930) and then it declined rapidly to the late 1950s, when it averaged barely more than 100 days per year. If krill populations were as sensitive to sea ice variations as Atkinson and colleagues would have us believe during the latter portion of the 20th century, we can’t imagine how any came out of the first 50 years of the past century alive at all!

Ice Duration

Figure 4. The history of winter ice duration observed at South Orkney Islands, 1903-2000 (source: Clarke and Harris, 2003).

The obvious answer is that the krill population adapts to the changing conditions. Perhaps they do this by changing locations in the Southern Ocean (after all, the majority of the Southern Ocean is actually experiencing rises in sea ice extent and duration), or perhaps by changing at what depth they hang out in the water column, or perhaps by some other mechanism (see World Climate Alert, Vol. 7, No. 14, for another explanation). As Steve Nicol, a krill expert from the Australian Antarctic Division, told the L.A. Times, “Could we really have lost 900 million tons of krill [from a biomass that was once estimated at 10 billion tons] without anyone noticing? I don’t think so. You would expect to see most of the predators in decline, and this doesn’t appear to be happening.”

Sadly, this episode is simply another example of Nature magazine’s reviewers—and the perhaps too trusting news reporters—neglecting to ask the right questions. Or perhaps, they simply look the other way. In any event, it seems that the accuracy of the science comes secondary to the importance of the “fear global warming” message.

References:

Atkinson, A., Siegel, V., Pakhomov, E., Rothery, P., 2004. Long-term decline in krill stock and increase in salps within the Southern Ocean. Nature, 432, 100-103.

Clarke, A., Harris, C.M., 2003. Polar marine ecosystems: major threats and future change. Environmental Conservation, 30, 1-25.

Cavalieri, D.J., Parkinson, C.L., Vinnikov, K.Y., 2003. 30-year satellite record reveals contrasting Arctic and Antarctic decadal sea ice variability. Geophysical Research Letters, 30, doi:10.1029/2003GL018031.




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