On Aug. 26, NASA held a media teleconference regarding current predictions on sea level rise, highlighting the risks to coastal populations in low-lying areas, and the inherent problems in creating reliable global models. A panel of experts from NASA's recently-founded Sea Level Change Team tells us that ocean levels are inexorably on the rise, but gaps in our understanding and ability to survey risk regions mean we don't know just how fast the change will take place.
"People need to be prepared for sea level rise, we're going to continue to have sea level rise for decades and probably centuries, it's not going to stop, the question is how fast is it going to be?" states Josh Willis, climate scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. "If you live on a coastline, or you have some economic dependence on a coastline, we have to be prepared for rising seas, it's not a question of how much, but rather when."
The board stated that the rise in ocean levels is coming from three distinct sources. The first is thermal expansion, in which ocean water expands as it is heated, taking up more volume and causing sea levels to rise. This effect has been exacerbated by greenhouse gas emissions, of which the ocean absorbs over 90 percent of the resultant heat.
The second source is ice loss from the Greenland and Antarctic ice sheets, while the final third is from melting mountain glaciers. Ice sheets and glaciers can be lost from contact with warmer air, the creation of icebergs, or from interaction with warm sea water. It is estimated that the Greenland ice sheet alone has lost around 303 gigatons of mass per year for the last decade.
We are aware of this thanks to a number of scientific instruments wielded by NASA and its partners. A notable contributor to our knowledge has been the Jason 1 & 2 and TOPEX/Poseiden satellites, whose altimeters have allowed for incredibly precise measurements. Simultaneously NASA's GRACE satellite has been observing Earth's gravitational field, taking accurate measurements in order to determine by how much ice sheets and glaciers are shrinking.
Data from these satellite missions and other resources tell us that our oceans have risen by about 8 inches (203 mm) since the onset of the 20th century, and that we may be facing a rise in excess of 3 ft (0.9 m) before the century is out. A visual representation of the phenomenon is presented in the video below (starting at the 40-second mark), which draws on data from 23 years of satellite observations, the result of a collaboration between NASA and the French space agency Centre National d'Études Spatiales.
The visual representation shows us that water is not rising at a global standard rate. Instead there are significant levels of variation and in some isolated pockets, the water level is actually subsiding, as evidenced by the rare blue patches. It is however worth noting that these discrepancies are due to identifiable ocean mechanics.
"Sea level along the west coast of the United States has actually fallen over the past 20 years because long-term natural cycles there are hiding the impact of global warming," explains Willis. "However, there are signs this pattern is changing. We can expect accelerated rates of sea level rise along this coast over the next decade as the region recovers from its temporary sea level 'deficit.'"
Measurements from the TOPEX/Jason satellites displaying sea level change in the short-term (top) and longer-term (bottom)
During the teleconference, the panel of experts was keen to point out a major flaw in our ability to create accurate models of sea level rise – our lack of understanding regarding the role that the degradation of East Antarctica's leviathan ice sheet has on rising sea levels. This is due to the fact that many of the changes taking place with the ice sheets occur under the water line, making it very difficult to observe them from space.
"The prevailing view among specialists has been that East Antarctica is stable, but we don’t really know," states Eric Rignot, a glaciologist at the University of California Irvine and JPL. "Some of the signs we see in the satellite data right now are red flags that these glaciers might not be as stable as we once thought. There’s always a lot of attention on the changes we see now, but as scientists our priority needs to be on what the changes could be tomorrow."
Simply put, current equipment must reach a greater level of sophistication, with computer models needing to be run at a higher resolution than the current generation of computers can handle. This summer, in an attempt to increase our understanding of the impact that ice sheets are having, NASA initiated a six-year field campaign, Oceans Melting Greenland (OMG), to study how warmer ocean waters are affecting the Greenland ice sheet. By combining satellite imaging with terrestrial based campaigns like OMG, NASA hopes to generate more accurate forecasts.
The teleconference paints a grim picture of what can be expected in the decades and centuries to come, and the question was raised as to how the information can be acted upon now. Eleven of the world's largest cities are situated on coastal regions, and in America alone, around 160 million people live on the country's shores. The danger is not confined to seawater flooding low-lying coastal areas – rising seas could have an affect on extreme weather systems such as hurricanes, which will be able to project their influence ever farther inland.