Leaking, Thinning, Sliding Ice

This year, the New York Times, Wall Street Journal, Los Angeles Times, and other prominent news outlets around the world granted an abnormally high level of media coverage to scientific news about the world’s great ice sheets. The news conveyed was not good.

Through unprecedented new images, field measurements, and modeling capabilities, we now know that Greenland and Antarctica, remote as they are, have already begun the process of redefining the world’s coastlines. More than a billion people—and untold impacts to economies, ecosystems, and cultural legacies—will be altered, displaced or lost in the coming generations.

Five studies in particular commanded especial attention. One showed that the floating ice shelves ringing Antarctica (which do not affect sea level directly, but do prevent billions of tons of glacier ice from sliding off the continent into the ocean) are thinning, their bulwarking ability compromised. Another found pervasive blue meltwater rivers gushing across the ice surface of Greenland through the use of drones, satellites, and extreme field work. A major NASA program (called “Oceans Melting Glaciers” or OMG) showed that the world’s warming oceans—which thus far have absorbed most of the heat from rising global greenhouse gas emissions—are now melting the big ice sheets from below, at the undersides of marine-terminating glaciers. A fourth study used historical air photographs to map the scars of 20th century deglaciation around the edges of the Greenland ice sheet, showing that its pace of volume loss has accelerated. A fifth, very long time-horizon study used advanced computer modeling to posit that the massive Antarctic ice sheet may even disappear completely in coming millennia, should we choose to burn all known fossil fuel reserves.

That last scenario is extreme. But should we choose to bring it to reality, the world’s oceans would rise an additional 200 feet. To put 200 feet of sea level rise into perspective, the entire Atlantic seaboard, Florida, and Gulf Coast would vanish from the United States, and the hills of Los Angeles and San Francisco would become scattered islands. Even five or ten feet of sea level rise would change or imperil the existence of coastal populations as we currently know them. Included among these are major cities like New York, Newark, Miami, and New Orleans in the USA; Mumbai and Calcutta in India; Guangzhou, Guangdong, Shanghai, Shenzen, and Tianjin in China; Tokyo, Osaka, Kobe, and Nagoya in Japan; Alexandria in Egypt; Hai Phòng and Ho Chi Minh City in Vietnam; Bangkok in Thailand; Dhaka in Bangladesh; Abidjan in Côte d'Ivoire, Lagos in Nigeria, and Amsterdam and Rotterdam in The Netherlands. The risk is not simply of rising water levels, but also the enhanced reach of storm surges (as illustrated by the hurricane Katrina and superstorm Sandy); and of private capital and governments ceasing to provide insurance coverage for flood-vulnerable areas.

Viewed collectively, these studies and others like them, tell us four things that are interesting and important.

The first is that ice sheets are leaky, meaning it seems unlikely that increased surface melting from climate warming can be countered by significant retention or refreezing of water within the ice mass itself.

The second is that the pace of global sea level rise, which has already nearly doubled over the past two decades (and currently rising approximately 3.2 mm/year, on average), is clearly linked to the shrinking ice volumes of ice sheets.

The third is that warming oceans represent a hitherto unappreciated feedback to sliding ice.

The fourth is that the process of ice-triggered sea level rise is not only ongoing, but accelerating. Many glaciologists now fear that earlier estimates of projected sea level rise (about 1 foot if we act aggressively now to curb emissions, about 3.2 feet if we do not) by the end of this century.

Sea level rise is real, it’s happening now, and is here to stay. Only its final magnitude remains for us to decide.