Using the Past to Predict Global Warming’s Future

To see the future, Maureen Raymo looks back. Three million years back, to be exact.

Raymo, a College of Arts & Sciences research professor of earth sciences, and her colleagues are trying to predict just how high the oceans will rise as global warming melts polar ice sheets. It so happens that three million years ago, the level of carbon dioxide—the most influential gas, atmospherically speaking—was about the same as today. Mid-Pliocene temperatures were about three degrees warmer than ours, so as our mercury creeps toward that ancient temperature under the influence of carbon dioxide and other greenhouse gases, knowing how much ice melted back then might shed light on how much could melt in the future. And how many coastal cities could become underwater parks.

So far, efforts by paleoclimatologists like Raymo have come up with wildly varying estimates for Pliocene sea levels—anywhere from 15 to 100 feet higher than today’s levels, depending on the techniques used and where on Earth they’re looking. Raymo says those variances indicate a knowledge gap that is downright embarrassing. A more accurate answer, she says, will tell us whether global warming today is “going to be a modest disaster or a full-out catastrophe.” She has been trying find that answer with a team of researchers working in, of all places, the Australian desert.

Research like Raymo’s has assumed greater urgency as indicators point to unexpectedly dire ice melts in Greenland and Antarctica. The New York Times recently reported that during the 20th century, sea levels rose a mere seven inches or so, a rate scientists thought might prevail over the coming century as well. New findings, however, point to a rise of anywhere from three to six feet by 2100, lending serious credence to Raymo’s disaster-to-catastrophe creep.

How disastrous might a three-foot rise prove? The Times warns that it could cause routine coastal flooding in Brooklyn and Queens, put 15 percent of greater Miami underwater, push the Atlantic one mile further inland over parts of North Carolina, and potentially devastate cities from London and Cairo to Venice and Shanghai. At six feet, the sea rise could send tens of millions of Asians fleeing oncoming waters and turn thousands of dry square miles of the American landscape to seascape.

One way to figure out where the sea’s edge lapped three million years ago is by studying the telltale remnants of ancient marine life: fossilized coral beds, clams, sand dollars, and other remains that scientists can date. The Australian desert, which was once a great sea, makes for fertile fossil finding. Raymo, who with her team spent the last two summers in the desert digging for clues, courtesy of grants from the National Science Foundation and the U.S. Geological Survey, says finding marine fossils was a breeze. They could stop their car almost anywhere on the Roe Plains in western Australia, walk off the road a bit, and pick them up.

Next, she hopes to do similar research from Spain to Africa to India. But getting meaningful, accurate estimates of ancient sea levels, she says, will take years, and need “a lot more people than me to be involved.”

A more precise measure of Pliocene sea levels would tell us how much ice stayed put in Greenland and Antarctica during that time, and how much might stay as our mercury creeps up toward that of the Pliocene period.

In fact, Raymo attributes the imprecision of Pliocene sea level estimates in part to a professional paradox. “Most of the people who are interested in ancient sea levels are paleoclimatologists who work in the oceans,” she says. “And the answer’s on land.”

Raymo is working with science journalist Daniel Grossman to publicize her Pliocene research on a yet-to-be-created website.

Colleague Mark Siddall of the University of Bristol, in Britain, says Raymo has always carved a unique path in paleoclimate research. “Her efforts will greatly improve sea-level estimates during the Pliocene,” he says. “She will give us a critical case study for understanding future climate states due to human-made greenhouse forcing.”

Rich Barlow can be reached at barlowr@bu.edu.