Facts and figures on sea level rise

by Wm. Robert Johnston
last updated 5 April 2002

Those that allege that climate change poses an imminent threat often cite rising sea level and/or its indirect effects. Rising sea level is, of course, said to result from rising global temperatures caused by man-made emissions of carbon dioxide and other gases. This article will present some facts and figures relating to the specific claims regarding sea level rise.

The scientific facts regarding climate change in general should be pointed out. The global warming hypothesis claims that man-made emissions of carbon dioxide and other gases have caused global temperature to increase in the 20th century and will cause further increase in the 21st century, with abundant negative side effects. This hypothesis is not supported by scientific observations. The 20th century temperature increases largely occurred prior to the largest increase in atmospheric carbon dioxide. The widely acclaimed temperature increases in the 1980s and 1990s most likely are flawed readings affected by urban heat-island effects: independent atmospheric readings show relatively constant global temperatures for the past 50 years. Despite claims to the contrary, a majority of scientists (both in general and in fields related to atmospheric physics) do not accept the global warming hypothesis as fact.

Even though the claims of future sea level rise hinge on this hypothesis, examination of these claims is useful to clarifying some popular misconceptions.

The United Nations International Panel on Climate Change is an international group of scientists, politicians, and others which have met several times, each time producing a "consensus" statement regarding predictions and proposed responses regarding climate change. The last few statements are tied to the Kyoto Protocol treaty, which would selectively restrict carbon dioxide emissions and other activities. The politicized nature of this "scientific" conference has been addressed by others; what follows are its claims regarding sea level rise.

The IPCC's 2001 report predicts that global average sea level will rise by 10 to 80 centimeters (median estimate 48 centimeters) by the year 2100. This will result from thermal expansion of ocean waters, net melting of glaciers, and net melting of polar icecaps. Predicted consequences include coastal flooding, incursion of salt water into coastal freshwater supplies, and a host of other effects. It might also be noted that environmental organizations have extended these predictions. For example, the UCS and ESA recently predicted sea level increases of up to 1 meter along the U.S. Gulf Coast by combining IPCC predictions with ground subsistence projections. By combining well-established effects with highly questionable predictions, they prevent straightforward testing of their predictions.

Currently there are 28,700,000 cubic kilometers of icecaps and glaciers in the world. This includes grounded ice in Antarctica and Greenland; floating ice shelves in the Arctic Ocean and seas near Antarctica; and glaciers in various mountain regions of the world. This represents the remaining unmelted ice from the last ice age, when total ice volume was about 3 times greater (and world sea levels about 120 meters lower).

Calving of icebergs from floating ice sheets is periodically cited as an indicator of climate change. Regardless of the cause, even the complete melting of the ice sheets would have no effect on sea level. This is a consequence of Archimedes' principle of buoyancy. The mass of floating ice (above and below water both) is identical to the mass of the water displaced. If the ice melts into water, its density decreases but is mass is the same, and water level is unchanged. There are potential side effects to large scale melting of ice sheets. One is a decreased reflectivity of the Earth's surface; due to clouds and low sunlight angles near the poles the consequences are minimized. Another is a change in ocean currents in the Arctic Ocean.

Those that express concern over an increase in sea level make the implicit assumption that the current stability in sea level is normal. Currently the Earth is exiting a period of glaciation. As seen in the graph, rising sea level has been the norm for the last 20,000 years, not the exception. The average rate of sea level rise in this period was 60 centimeters per century.

Fig. 1: Rate of sea level rise (in cm/century) vs. year (from 18000 BC to 2000 AD); derived from graph by Lambeck cited in IPCC's Climate Change 2001

Consider the following: in the IPCC's predictions, 20% of the expected sea level rise over the next century is due to net melting of continental ice (outside Greenland and Antarctica). This would require that 20% of the Earth's continental ice melt in the next century. This ice is the remnant of the ice cover from the last ice age; what remains is 0.4% of the ice cover at the last peak of glaciation. On one hand, for this ice to melt in the next century would involve a rate of melting only one-fourth of the average over the last 20,000 years. Probably more relevant is the fact that this ice has apparently been hard to melt.

Limited data suggests that around the mid 1800s the rate of sea level rise increased to about 15 centimeters per century. This rate has apparently remained constant for the past 150 years; various tidal gauge measurements during the last two decades give results comparable to this rate. While some suggest a link between this and current man-made carbon dioxide emissions, note the following: the observations suggest a constant rate of sea level rise for the past 150 years, while rate of man-made carbon dioxide emissions has increased over 100-fold. Additionally, most of the cumulative rise in sea level preceded the majority of cumulative carbon dioxide emissions. Global temperature change and sea level rise do correlate with each other, but not with human activities; thus it appears that both temperature and sea level are changing principally due to natural phenomena.

Note that there are uncertainties even with these modern measurements of sea level change. Tide gauge measurements for the past 150 years show rising sea level at some locations and dropping sea level at others. The primary factor is sinking and rising of the ground, respectively. The 15 cm per century sea level rise incorporates model-based adjustments for these ground motions. Parts of Europe and North America are still rising in adjustment to the removal of the ice sheets by melting over the past several thousand years. Some sources question the accuracy of these sea level rise rates because of limits in our understanding of this isostatic rebound.

The IPCC predictions heavily depend on models that have limitations. It is first necessary to model global climate change; these models make assumptions regarding future increases in atmospheric carbon dioxide and proceed to model global and regional changes in temperature, precipitation, and other climatic factors. Models of sea level rise use these results and further model mass-balance for the icecaps (considering precipitation and melting) and thermal expansion of the oceans (requiring modeling of changes in temperature-depth relations for the global oceans). These models involve a high degree of uncertainty. The models for temperature change fail miserably to predict temperature changes for the last 20 years, yet their predictions for the next 100 years are still assumed valid. Nearly all the models require "fudge" factors to correctly simulate a steady state situation. The fact that the various models cited by the IPCC give relatively consistent predictions does not reflect reliability; the models have been adjusted to conform to each other, but fail to conform to real world observations. When regional climate changes are considered, the models give inconsistent and sometimes dramatically contradictory results. Further, the models are modeling global carbon dioxide balance, which is very poorly understood at this time. Sea-level change models likewise attempt to model icecap mass balance, also poorly constrained by current observations.

© 2001, 2002 by Wm. Robert Johnston.
Last modified 5 April 2002.
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