Evidence and Contributing Factors

Sea level is known to be rising, but there is no convincing evidence that the rate of rise has increased during the 20th century: the rate of sea level rise seems to be constant and has not measurably increased due to global warming of the last 90 years. There is weak evidence for an acceleration over the last two to three centuries that could be due to the warming since the last ice age.

If sea level were to change due to global warming, the contributing factors would be (1) thermal expansion of the oceans, (2) melting of glaciers and small ice masses in mountainous areas or high latitudes, (3) melting of the Greenland ice sheet, or (4) break-off of the West Antarctic ice sheet, and (5) change in the mass balance of the Antarctic ice sheet. Thermal expansion refers to the expansion of ocean water in a fixed size basin that results in a rise in sea level in coastal regions.

Measurements over the last 100 years shown in Figure 2 show a change of about 12 centimeters for 100 years, or about 1 to 2 millimeters per year. The two different data sets shown seem to agree reasonably well.

Observations of the terminus (furthest extent down a mountain valley) of a glacier provide evidence of changes in the total ice volume of the glacier. Although not a quantitative measurement of total ice volume, glacier termini offer, through old photographs, paintings, and historical accounts, a long record of changes in ice volume. Figure 3 shows the changes in terminus locations for 6 different glaciers in Norway, Iceland, France, Switzerland and Austria. Records from certain parts of Europe go back to the 1600s.

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