What Do We Know, and When Did We Know It?

Those of us who live near the coast (that’s about 39% of the US population) or who in some way depend on products that come from the ocean or are delivered via ports (that’s just about all of us) have a stake in what happens with sea level rise. The problem, I suspect, is that many of us have a fairly hazy understanding of exactly what’s going on; we’ve heard so many different predictions and projections and conflicting information over the last decade.

It also depends on where we are: Many factors, including circulation patterns in the oceans, differences in water temperature and salinity, rainfall over land and sea, and land subsidence, can influence how much and how quickly the waters rise in a particular area. Planners are doing the best they can in terms of long-term zoning and infrastructure protection, figuring out how to deal with more frequent flooding or saltwater intrusion or whatever other problems they expect to face. But having more information never hurts.

A report released this month can help straighten some of this out. It’s called “Rising Seas in California: An Update on Sea-Level Rise Science,” and before you protest “But I’m not in California,” let me say that there’s likely something here for you if you live near any coast. The specific projections and recommendations for California won’t necessarily apply, but the report’s overview of the basic science of sea level rise—how the heck are they making these predictions, anyway?—is useful and clearly expressed.

A great deal of what happens in California, or anywhere else, depends on what happens in Greenland and Antarctica. How fast are the ice sheets melting? Will they continue to melt at a predictable rate or experience catastrophic collapse? The report’s authors describe how these things are being monitored now, satellite data they expect to have in the near future, and the need for actual measurements—not from satellites but via airplane and ship—from the margins of the ice sheets. A NASA mission is currently gathering some of this information in Greenland, and, the report notes, the same needs to be done for Antarctica. The report also explains the models scientists are using to make predictions, including their limitations and how they’re improving, in an understandable way.

The ways that melting ice sheets affect us, it turns out, aren’t always intuitive or predictable. “As a retreating ice sheet loses mass to the ocean,” the report explains, “its gravitational pull on the surrounding ocean is reduced. Within about a thousand miles…the reduced gravitational pull on the ocean causes the sea-surface (and relative sea level) to drop, even though the ocean has gained volume overall.” But starting at a distance of about 4,000 miles from the ice sheet, “the change in relative sea level is greater than expected from the extra water added to the ocean,” which means that coastlines in the Northern Hemisphere experience more sea level rise as Antarctic ice melts, and those in the Southern Hemisphere are more affected by melting ice in Greenland. Who knew?

There’s also some sobering information about ice shelves. “Another underappreciated consequence of the loss of marine-based ice… is that it can only re-advance (regrow) if confined ice shelves can be reestablished. The shelves are required to buttress the grounding line, allowing it to migrate seaward on its reverse-sloped bed. Because ice-shelf melt rates are so sensitive to a warm ocean…the ocean will have to cool down before the ice shelves can reform. Because of the large thermal ‘inertia’ of the ocean, this could take centuries to several thousands of years, after greenhouse gas concentrations return to their preindustrial levels.”

The report, and the working group behind it—part of the California Ocean Protection Council Science Advisory Team—are specifically updating previous guidance for coastal planners in California. As you’d expect, the projections become less definite the farther we look into the future. As the authors of the report note, our understanding of sea level rise is constantly evolving; it’s not just that the data and the models continue to change and improve, but also that the results of our previous policies will show whether we’ve made wise decisions. For all these reasons, they recommend that projections of sea level rise—and the accompanying approaches we take to deal with it—be updated or at least reevaluated every five years.

There’s a dollar value associated with those predictions and levels of risk, too—not only for cities and ports but, in a very real sense, for individual homeowners. Those in coastal areas prone to more frequent flooding are most obviously at risk, but all of us in any part of the country who buy flood insurance through the National Flood Insurance Program (NFIP) share the burden, as this article in the New York Times Magazine makes clear. For some of the people interviewed in the article, an understanding of what the “hundred-year flood zone” really means came too late, but the more we learn about how to predict coastal flooding—of how the sea is likely to behave—the better for all of us.