This book describes both clearly and in detail the complexity behind the deceptively simple subject of sea level rise, a topic of considerable scientific interest and increasing economic importance. The concept of sea level rise is quite straightforward. Some 97% of all the water on Earth is now in the oceans; most of the rest is found in glaciers, much of it in Antarctica and Greenland. Some 20,000 years ago at the peak of the last ice age, much more water was in ice and the sea level was more than 100 meters lower than it is today. The glaciers began to melt, the oceans began to fill, and the shorelines were pushed back as the sea level rose. The process continues, and the results are obvious. Archeologists don Aqua-lungs and explore the ancient port of Alexandria. Closer to home and more recent in time, St. Clements island in the Potomac River was a heavily wooded 160 hectares when first occupied by Virginia colonists. Today, some 350 years later it is about 16 hectares and has little in the way of vegetation. Pictures of battered beach houses and hotels eroded by waves after a particularly vicious winter storm moves up the east coast of the United States are a regular feature of our television news.
In the early 1980s when the issue of global warming first grabbed the headlines, I was in Washington as head of the National Oceanic and Atmospheric Administration. One of NOAA's tasks is to predict the tides and maintain this nation's vast array of tide gauges. What effect, I was asked, will global warming have on the change in sea level? It was embarrassing to admit that we really could not say much more than is in the above paragraph. Yes, sea level has risen in the past; we assume it still is rising, but uncertainty remains about how fast it has been rising recently, and thus we are not in a very good position to estimate how fast it might rise in the future. As this volume attests, there continue to be a number of perplexing issues. There is still uncertainty in some areas, but we do know so much more, not just about the changing volume of the ocean, but about yearly and regional variations in sea level and the reasons for them. Even more exciting, the technology now available suggests that we will soon know very much more.
We now believe that the ocean volume has been increasing since the middle of the 19th century at a rate equivalent to raising sea level almost 2 mm/yr, a rate considerably faster than that for the previous thousand years, although how much faster is subject to some uncertainty. That this increase in the rate of sea level rise began well before the rise in our mean atmospheric temperature of recent years gives pause to those who wish to assign its cause to anthropogenic-driven global warming.
Tracking the changing volume of the waters of the ocean, as distinguished from measuring local sea level, is not a simple task. Many traps lie in wait for the unwary, and not long ago many who had examined the problem were skeptical that we would ever achieve useful quantitative information. It has not been easy. Do not expect to examine a half-dozen years of local tide gauge records and derive a useful value. First, at least a 50-year record is required, because there are year-to-year changes (some of which we understand, but a number of which we do not) which are likely to bias records that are much shorter. Second, and often more difficult to resolve, the land bordering the sea also moves up and down. In much of Scandinavia the local sea level is dropping because the land is rising (several millimeters a year in places), continuing to rebound from the heavy weight of the glaciers removed several thousand years ago. But isostatic adjustment in those areas formerly under the ice requires some form of viscoelastic compensation in those areas away from the former ice sheets. For example, even if there were no change in the volume of the oceans, we now believe that the sea level would be rising along the east coast of the United States at about 1.4 mm/yr because that is the rate the earth is sinking in this part of the world. As a consequence the actual rise of sea level in this region is nearly double that caused by the change in the ocean volume.
Why is the volume of the oceans increasing? Do not expect to find an unambiguous answer in this book. Perhaps it is the melting of the last of our major ice fields. That is certainly what many believe, but we do not have sufficient information about the volume of ice on either Greenland or Antarctica, let alone its rate of change, to give an unambiguous answer. Perhaps the ocean is getting slightly warmer. If it is, then seawater will expand, and the volume of the ocean will increase although its mass will remain unchanged. An increase of the average ocean temperature (top to bottom) of only a few hundredths of a degree per year is all that is required to raise the sea level a couple of millimeters per year, but we do not have the kind of historical ocean temperature records to either prove or disprove such a possibility.
There may be better data on why humankind's activities of the last half century should be driving sea level lower. We have a good record of the number of dams built in the last half century and the amount of water they control. These dams change the historical flow of water from land to rivers and on to the ocean, and one can make educated guesses whether this should either increase or decrease the rate at which water reaches the ocean. Apparently, the largest single effect is the loss of water from behind the dam which leaches out of the bottom and back into groundwater. This water never makes it to farmland, homes, or industry, nor does it evaporate, later to fall as rain.
This water completely bypasses the ocean. A strong case can be made that the rate at which the volume of dammed water is increasing, and thus the rate at which this water is bypassing the usual cycle, is equivalent to a decrease in sea level of possibly many tenths of a millimeter per year.
The rate of change of sea level varies from year to year and place to place. Evidence of past El Ninos can clearly be seen in the long-term tidal records of San Diego and San Francisco. Year-to-year changes in the intensity of the wind-driven circulation in the North Atlantic are captured in the yearly changes in mean sea level recorded by tide gauges along the U.S. east coast. With the significant increase in tide gauge accuracy, not the least of which is the removal of the earth movement problem with the availability of GPS, one can expect tide gauges to contribute to an ever-increasing array of geophysical problems.
And finally, if sea level continues to rise, if it is indeed rising at a more rapid rate now than it was a century ago, and if, as some suggest, that rate of rise will increase as a consequence of global warming, what effect will this rising sea level have on society? To those who live in the Ganges delta of Bangladesh, on coral atolls in the Pacific, or below sea level in The Netherlands, this subject holds special interest. One estimate has some 100 million of us living within one meter of sea level. I expect they will be among those most interested in the latest news on this subject.
John A. Knauss
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