One of the great fears generated by global warming is that the ocean is about to rise and swallow our coasts. These concerns have been heightened by the substantial uptick in Atlantic hurricane activity that began in 1995. The frequency of really strong storms striking the U.S now resembles what it was in the 1940s and 50s, which few people (aging climatologists excepted) remember.
Those arguing that global warming is an overblown issue have been claiming for years that "consensus" forecasts of sea-level are equally overwrought. The United Nations' Intergovernmental Panel on Climate Change predicts a global average rise of from 3.5 to 34 inches by 2100, with a central estimate of 19 inches. Depending upon how you slice or dice the data, the last century saw maybe six inches.
Critics have long argued that these changes require a substantial net melting of some combination of the world's two largest masses of land-based ice, Antarctica and Greenland. In addition, they note that observed global warming is right near the low end of the U.N.'s projections, which means that realized sea level rise should be similarly modest.
Over 15 years ago, John Sansom published a paper in Journal of Climate that showed no net warming of Antarctica. While it was widely cited by critics of global warming doom, no one seemed to take notice. After all, it relied on only a handful of stations. Then, in 2002, Peter Doran published a more comprehensive analysis in Nature and found a cooling trend.
At the same time, a deluge of stories appeared, paradoxically, about Antarctic warming. These studies concentrated on the tip of the Antarctic Peninsula, the narrow strip of land that juts out towards South America. That region, which comprises less than one-half of one percent of Antarctica, is warming because the surrounding ocean has warmed.
Warmer water evaporates more moisture. The colder the land surface over which that moisture passes, the more it snows. So, Antarctica as a whole should gain snow and ice. Last year, C.H. Davis published a paper in Science about how this accumulating snowfall over East Antarctica was reducing sea level rise. This year, Duncan Wingham, at the 2005 Earth Observations summit in Brussels, demonstrated the phenomenon is observed all over Antarctica.
Greenland is more complex. In 2000, William Krabill estimated the contribution of Greenland to sea level rise of 0.13 mm per year, or a half an inch per century. That's not very much different than zero. Just last month, using satellite altimetry, O.M. Johannessen published a remarkable finding in Science that the trend in Greenland ice is a gain of 5.4 cm (two inches) per year.
Almost all of the gain in Greenland is for areas greater than 5000 feet in elevation (which is most of the place). Below that, there is glacial recession. It shouldn't be lost on anyone that because no one ventures into the hostile interior of Greenland, all we see are pictures of the receding glaciers near the coast!
The temperature situation in Greenland is more mixed than in Antarctica. Over the last 75 years, there's been cooling in the southern portion (where the recession is greatest) and some warming in the North.
The only other masses of ice on the planet that can contribute to sea level rise are the non-polar glaciers, but they are very few and far between. The biggest is the Himalayan ice cap, but it's so high that a substantial portion will always remain. Most of the rest are teeny objects tucked away in high elevation nooks and crannies, like our Glacier National Park.
If all these glaciers melted completely -- including the Himalayan ice cap -- sea level could rise no more than five to seven inches, because there's just not that much mass of ice, compared to Antarctica and Greenland.
It is simply impossible for the scientific community to ignore what is going on, even as prone to exaggeration of threats as it has grown to be. The planet is warming at the low end of projections. Antarctica is undoubtedly gaining, not losing ice. Greenland appears to either lose a little ice, or, in the recent study of Johannessen, gain dramatically. It's going to take some time for it to contribute much to rising oceans.
Meanwhile, Antarctica grows. Computer models, while still shaky, are now encountering reality, and every one of them now says that Antarctica contributes negatively to sea level rise in the next century, while almost every model now has Greenland's contribution as a few inches, at best.
It is inevitable that one of tomorrow's headlines will be that scientists have dramatically scaled back their projections of sea level rise associated with global warming. Had they paid attention to data (and snow) that began accumulating as long as fifteen years ago, they would have never made such outlandish forecasts to begin with.
Davis, C.H., et al., 2005. Snowfall-driven growth in East Antarctic ice sheet mitigates recent sea-level rise. SciencExpress, May 19, 2005.
Doran, P.T., et al., 2002. Antarctic climate cooling and terrestrial ecosystem response. Nature, 415, 517-520.
Johannessen, O.M., et al., 2005. Recent Ice-Sheet Growth in the Interior of Greenland, Sciencexpress, October 20.
Krabill, W., et al., 2005. Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning, Science, 289, 428-430.
Sansom, J., 1989. Antarctic Surface Temperature Time Series. Journal of Climate, 2, 1164-1172.