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Researchers Pry Climate Change Record from Giant Clams

Scientists are looking to strange sea life for records of climate's past--and forecasts of future warming



ISTOCKPHOTO/CEMENTJUNGLE

The long necks protruding from a geoduck's giant shell have led some to believe that the world's largest burrowing clam is an aphrodisiac. Whether or not that is true, the sea creature may offer another—albeit less sexy—benefit for humanity, according to new research. The long-lived geoduck (pronounced "gooey duck") could help us deduce what climate conditions were like hundreds of years ago and more accurately predict future patterns.

"Unlocking climate and ecosystem records in the marine environment is equally as useful as tree rings are in the terrestrial environment," says Bryan Black, a tree-ring analyst at Oregon State University. "Geoducks and trees each capture climate variability from their own unique perspective."

Gaps in climate records continue to force substantial variability into climate projections, including those found in the most recent Intergovernmental Panel on Climate Change (IPCC) report. These limitations are mostly due to the length and global distribution of instrumental climate records. But scientists are now discovering that data from a suite of animal proxies has the potential to fill in some of these holes. And holes in the sand created by geoducks' squirting siphons are one place where researchers have begun digging for those clues.

Incremental records
Parallel arcs on the hinge of the Pacific Northwest clam's shell record growth increments, much like tree rings. Sea surface temperatures—which strongly influence regional climate—are one of the key variables that can be inferred from these records. Hotter temperatures drive metabolism in geoducks; wider increments mean warmer years. The same goes for trees. Inland, old-growth Douglas firs have a longer growing season when the snow melts earlier.

"We can take those different perspectives—each explaining a different part of the story—and put them together to get a full view of what the climate was like in the past, and a more robust forecast for the future," says Black, who was the lead author of a paper on proxies for climate reconstruction recently published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. "And from the ecology side," he adds, "once we know which variables a marine organism is sensitive to, we can better inform how future climate changes could affect it." This information could be especially useful for fishery managers.

The researchers collected and compared more than 30 individual trees and geoducks, at each of several locations in the Pacific Northwest. Through a process called cross-dating, they ensured that annual growth increments were indeed representative of climatic variation rather than an individual's unique experience. "When you are looking at multiple samples, you see some rings are narrow and some are wide," says Carolyn Copenheaver, a forestry expert at Virginia Polytechnic Institute and State University and co-author on the paper. One individual tree, she explains, may have unusually narrow increments compared with its neighbors if it lost branches the previous year. But comparing them as a group, Copenheaver says, helps eliminate that noise and allows scientists to say, "This one was 1977; that one was 1944."

In the end the team found that almost 50 percent of the variability in the sea surface temperatures—obtained from instrumental records dating back to the 1940s—correlated with the variable widths of the tree rings. Once the tree rings were combined with geoducks' incremental record, the amount of variability explained rose to close to 70 percent.

The researchers then used these established relationships to extrapolate further back in time and reconstruct more accurate accounts of sea temperatures over the past 150 years—about twice as far back as written records go in the northeastern Pacific.

"Anytime you get something older than the written record, you are making progress," Copenheaver says.

Critical information for climatologists
"My students always ask," Copenheaver adds, "'Are we experiencing climate change?' And I tell them, 'That's not the important question. The real question is whether or not the change we are experiencing is unprecedented. Have we ever had changes this fast, and to this great extent?'"

Climatologists see power in the new proxy combo to help answer these questions. "This sort of work can help in a very real way," says Michael Mann, director of the Earth System Science Center at The Pennsylvania State University and lead author of the IPCC's third report. "Piecing together complementary information from various archives can help us sort out some of the current uncertainties of model projections of future climate change."

As detailed in the most recent IPCC report, Earth's average temperatures can be traced back more than 1,000 years—primarily through ice core samplings. But what remains unclear is the spatial pattern of that record, explains Mann, who was not involved in the geoduck research. "It's easier to figure out how warm it was in general than to figure out exactly where it was warmest, and where it was not quite as warm," he says. "There is a lot of heterogeneity in the climate system."

A lot of that variation intrinsically involves the oceans. So, knowing what is going on in the ocean is essential for an understanding of the basic dynamics of the climate, Mann says, pointing specifically to the El Niño phenomenon in the Pacific. He notes that El Niño patterns are known to influence rainfall in Africa, tropical cyclones in the Atlantic, winter weather in North America, and droughts in the U.S. southwestern desert.

"We can project meaningfully what the average temperature of the globe will be, but there's quite a bit of uncertainty about how El Niño will change," Mann adds. "It's a little bit of a dirty secret that there is very little consensus on something that fundamental." But, he adds, "using geoducks combined with tree rings can better pin down [El Niño] patterns."

Ugly geoducklings wanted: Dead or alive
The proxy combination is not without limitations. Whereas trees can live 500, even 1,000 years, geoducks' life spans typically don't exceed 150 years. Still, short chronologies can be valuable where the written record is short—like in the northeastern Pacific. And scientists are now looking to dig up dead geoducks in sea sediments to further extend that range, Black says.

This novel approach could also be applied in other regions, and with other organisms. "It may be that these methods find equally, or even more important applications where there are analogous organisms that can be used as climate proxies in the Southern Hemisphere," Mann says. The Southern Hemisphere is mostly water and therefore is relatively short on living records like trees, commonly used in the Northern Hemisphere.

Oregon State's Black has used the proxy technique in other situations himself—tracking the climate through the bones of Pacific rockfish. More recently, he has begun working with freshwater mussel shells, which appear to keep tabs on river flows—another useful climate variable.

Meanwhile, Virginia Tech's Copenheaver is back where she is most comfortable: the forest. "Tree rings are beautiful to look at," she says, "but geoduck clams are like the dullest things I've ever seen. They are so embarrassing—such ugly, phallic things."

Together, however, beauty and the beast—or more aptly, the ugly geoduckling—are recovering climate's missing chapters, and giving life on Earth a better shot at getting to happily ever after.

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