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Raising the Estimate of Sea-Level Rise
The effect of post-glacial rebound was overlooked in the West Antarctic
Picture a plastic bowl. Put a large piece of ice in it—one tall enough that it rises high above the bowl’s rim. Now melt the ice. The bowl will catch most of the water, but not all of it. Since the ice is tall, the bowl will reach full capacity before the block melts completely. With no place else to go, the extra water will spill over the lip and onto the counter.
This is an approximation of how climate scientists have long modeled the collapse of the West Antarctic Ice Sheet (WAIS), an expanse of ice roughly the size of Mexico currently melting in a bowl-shaped stretch of bedrock below sea level. Should the two-million-square-kilometer ice sheet fully collapse, some water would stay in the geological bowl. The rest would flow into the open ocean, where scientists previously estimated it would raise global mean sea level by a little more than three meters within 1,000 years of the collapse. But three meters substantially underestimates the problem, according to a recent study by earth and planetary sciences doctoral students Linda Pan and Evelyn M. Powell. Previous models forgot an important piece of the climate puzzle: the dynamics of the Earth’s mantle.
Rather than “just sitting there like a salad bowl,” says the study’s senior author, Baird professor of science Jerry X. Mitrovica, the West Antarctic’s low-viscosity mantle will rise as the mass of the ice pushing down on it decreases. The bowl’s center rises faster than its sides during crustal rebound, reducing the bowl’s volume and pushing water out. This “water expulsion mechanism” is expected to force the equivalent of one extra meter of water into the ocean over the course of 1,000 years, raising projected global mean sea-level rise attributable to WAIS from a little over three meters to slightly more than four. In other words, the worrisome millennium-timeframe prediction is actually up to a 30 percent underestimate. This isn’t just a concern for future generations. Using a climate projection in which humans do little to curb greenhouse gas emissions, Pan and Powell found that, over the next 80 years, crustal rebound would continually augment WAIS’s contribution to global mean sea level by 20 percent, or one additional centimeter by 2100. Because of this study, every projection of sea level rise due to WAIS will have to be revised upward.
The scientific community is not the only one feeling the reverberations of these findings. Even if a centimeter feels small, 20 percent more water flowing out of the ice sheet over the next eight decades could alter coastal climates and weather patterns. Even in the next 10 years, with sea level rising by only several millimeters, oceanside communities will become “far more vulnerable” to natural disasters like hurricanes, Mitrovica says. (The horizontal extent of additional beach erosion is typically 50 to 200 times the rise in sea level; see “The Great Global Experiment,” November-December 2002, page 34.) Long term, the study’s projected four-meter sea-level rise would be “significant enough on its own without storms or tides to cause huge coastal damage and flooding,” he points out. And because crustal rebound takes about 1,000 years, “sea-level rise doesn’t stop when the West Antarctic Ice Sheet stops melting,” says Pan. The continuing uplift will push more water into the open ocean for hundreds of years after WAIS collapses.
Furthermore, the sea-level rise caused by this melting and uplift will not be distributed evenly around the world. Global mean sea level is only an average. Like a “fingerprint,” Mitrovica says, the effects of each melting ice sheet are unique, moving more water to some parts of the globe than others due to the gravitational influence of the ice sheets themselves (see “The Plastic Earth,” September-October 2016, page 46). For instance, North America would see less than the average projected global sea level rise if the Greenland Ice Sheet were to melt. With WAIS, however, luck runs out for the United States. Should WAIS collapse or significantly melt, North America would be “right in the bullseye,” of meltwater, Powell says. “Those of us on the east or west coast will get more than our fair share.” Thanks to the melt “fingerprint” and the effects of crustal rebound, the northeast Pacific Ocean would receive more than five meters of rise: much more than the four-meter average—and enough to irreparably alter coastlines.
“What the solid Earth does, in most climate simulations, is an afterthought,” says Mitrovica, but this study highlights that, in addition to obvious climate-change factors such as temperature and weather patterns, the planet underfoot can influence sea level as well. “This isn’t just an academic exercise,” he says. Studying the solid Earth “really is providing a lot of insight into what’s going to happen in the future.”