The author of today's post, Michael Oppenheimer, Ph.D., is the Albert G. Milbank Professor of Geosciences and International Affairs in the Woodrow Wilson School and the Department of Geosciences at Princeton University. He also serves as science advisor to Environmental Defense.
"In about a century, some of the places that make America what it is may be slowly erased" by rising sea levels, says an Associated Press news story from last week. In the map of Florida below, some of the most vulnerable areas – which include Cape Canaveral and a big chunk of Everglades National Park – are shown in red.
Source: University of Arizona's Department of Geosciences.
It's not just historic sites that are threatened, but people. Rising sea levels could displace millions in heavily populated coastal areas across the world.
Why are sea levels rising, and what do scientists project for the future?
Direct measurements indicate that global average sea level has been rising for at least a century. Since 1993, it's risen about 3.1 millimeters per year. More than half the rise is due to "thermal expansion" from warming oceans. (Like all liquids, water generally expands as it warms.) The rest of the increase comes from water pouring into the oceans as land-based ice melts. Today, most of the additional water comes from glaciers and small ice caps, with the massive ice sheets of Greenland and West Antarctica making smaller contributions.
There's no question that sea levels will continue to rise in the future – scientists can agree on that much. But how much will they rise?
The IPCC's Fourth Assessment Report (AR4) projects sea level rise from now to the end of the century for six different greenhouse-gas emissions scenarios, with each projection given as a range. The lowest projected value is 0.18 meters (7 inches); the highest is 0.59 meters (23 inches). But there is good reason to believe that the AR4 projection is conservative, and sea level rise may be significantly higher.
The AR4's estimate includes the conservative assumption that the recent rate of ice flow from Greenland and Antarctica will continue in the future. But it may well increase, and this would mean higher sea levels. The AR4 acknowledged this, commenting that "larger values [of sea level rise] cannot be excluded" because the flow rate may increase.
Why is ice flow rate important? An ice sheet affects sea level when it melts faster, or when some of its ice flows into the sea. There is evidence that portions of both the Greenland and Antarctic ice sheets are doing the latter at an increasing rate. The causes are unclear and the changes may not continue in the future, but parts of these ice sheets seem to be far less stable than previously thought. Changes such as these are very difficult to model.
Another recent paper described an alternative way of projecting sea level rise [PDF] that doesn't require large-scale modeling. For the past 120 years the rate of sea level rise has changed roughly in proportion with global temperature. If this relationship remains constant, the same emissions scenarios used by the IPCC would cause as much as 1.4 meters (4.5 feet) of sea level rise by 2100 – more than twice the highest estimate from the AR4 models. And sea levels would continue to rise for centuries after that.
Hopefully policymakers will take this to heart. Our choices today will profoundly affect sea levels. As Bill wrote in "How Warm is Too Warm?", if we don't act very soon to halt the rise of greenhouse gas concentrations, we could, over centuries or millennia, trigger the unavoidable disappearance of the Greenland ice sheet. This would eventually raise sea levels seven meters (about 20 feet). How many places do we want to erase?