If all the water currently trapped in all the glaciers across the entire world melted, the sea level would rise far more than most people imagine. Almost everyone living anywhere in the world at an elevation of below about 500 feet with a direct drainage to the sea would be directly affected; The sea level rise itself might be a bit over 300 feet, but oceans tend to migrate horizontally when they rise onto previously uninnundated land surfaces. So if you lived at 500 feet above sea level in most of Maine, you’d have a much shorter walk to the rocky shoreline, but if you lived at 500 feet across much of the Gulf Coast it would only be a matter of time until the eroding sea cliff reached you incorporated you into the offshore sediments.
Having said that, Anthropogenic Global Warming has resulted in only modest sea level rise to date, and it is at this point probably true that warming of the ocean causing thermal expansion has been at the same level of magnitude (or greater) than seas rising because of the influx of melted glacial water.
The problem is, it is very difficult to measure either sea level rise or ice loss very accurately, for a number of reasons. But there is a saving grace. Or should I say, GRACE. GRACE is a NASA project; Twin satellites measure changes in the Earth’s gravity field in such a way that it is possible to identify changes in the distribution of water. From the GRACE overview statement:
GRACE will be able to map the Earth’s gravity fields by making accurate measurements of the distance between the two satellites, using GPS and a microwave ranging system. It will provide scientists from all over the world with an efficient and cost-effective way to map the Earth’s gravity fields with unprecedented accuracy. The results from this mission will yield crucial information about the distribution and flow of mass within the Earth and it’s surroundings.
The gravity variations that GRACE will study include: changes due to surface and deep currents in the ocean; runoff and ground water storage on land masses; exchanges between ice sheets or glaciers and the oceans; and variations of mass within the Earth. Another goal of the mission is to create a better profile of the Earth’s atmosphere. The results from GRACE will make a huge contribution to the goals of NASA’s Earth Science Enterprise, Earth Observation System (EOS) and global climate change studies.
A major study of GRACE data by Thomas Jacob, John Wahr, W. Tad Pheffer and Sean Swenson was published moments ago in Nature. Jonathan Bamber supplies a summary:
Understanding, and closing, the sea-level budget (the relative contributions of mass and thermal expansion to ocean-volume change) is crucial for testing predictions of future sea-level rise. Estimates of the future response of [glaciers and ice caps] to climate change are, in general, based on what we know about how they have responded in the past. A better estimate of past behaviour, such as that obtained by Jacob and colleagues, will therefore result in better estimates of future behaviour. Discussion of the demise of the Himalayan glaciers has been mired in controversy, partly because of basic errors, but also because of the dearth of reliable data on past trends. Given their role as a water supply for so many people1, this has been a cause for concern and an outstanding issue.
The method works like this: Ice bearing region are defined as “mascons” (sampling units in fancytalk) where mass values are taken by the satellite. Factors that are not ice are subtracted from the changes in gravity in each mascon and a fancy modelling program that Global Warming Denialists will not understand and therefore claim to be a hoax is applied to the data. The result is something like this:
This shows the change in mass for numerous regions around the world.
Over the seven year period of this study, mountain glaciers and ice (outside of Greenland and Antarctica) contributed to about 0.041 cm of sea level rise a year, and the major ice sheets of Greenland and Antarctica produced about 0.106 cm of sea level rise a year, for a total of 0.147, according to this study. This is in the same order of magnitude as other studies, though the new research tends to show less melt than previously thought. There is a major discrepancy between the new GRACE study and earlier work; The Tibetan Plateau is shown in other studies to have lost much more ice than GRACE indicates. The authors are convinced that they’ve got the number right. The main problem here is that gravitational studies (and the discrepancies are between different gravitational studies) measure the movement of major land masses. You can think of the Tibetan plateau as a big pile of stuff that is currently being shoved up into a bigger pile of stuff by tectonic movements. It is dyanamically moving for that reason. Then, glaciers grow and shrink on this elevated mass. That complicates the movement of the plateau over time. It is a little like measuring the exact location of a fishing bobber on a rippling pond.
To give you an idea of the relative magnitude of 0.147 cm a year of sea level rise, consdier that the total sea level rise since the last glacial (ice age), if averaged over 12,000 years (roughly the time since intensive melting started) would be about 1 cm a year. Of course, the actual rate of sea level rise was much greater than that when the glaciers first started melting and then slowed considerably later.
Current sea level rise is not very significant, but we expect the rate of glacial melting to increase if Anthropocentric Global Warming is not curbed. In fact, this study does not help us at all in estimating the rate of future melting. What it does do is refine our ability to measure ice wasting as it happens, in a way that is probably more accurate than previous method and more comprehensive, including all the ice globally.
Jacob, T., Wahr, J., Pfeffer, W., & Swenson, S. (2012). Recent contributions of glaciers and ice caps to sea level rise Nature DOI: 10.1038/nature10847