Practicing Geology Exercise

Why Is Sea Level Rising?

Over the twentieth century, sea level rose about 200 mm, and it is currently rising at a rate of about 3 mm/year. Rising sea level is a grave threat to human society because it could inundate deltas, atolls, and other coastal lowlands, and it could erode beaches, increase coastal flooding, and threaten water quality in estuaries and aquifers. Why is sea level rising, and can we predict the rate of its rise in the future?

We know that anthropogenic warming of the polar regions is reducing the amount of sea ice and causing the breakup of large ice shelves (see Figure 21.13). Because of isostasy, however, this decrease in the volume of floating ice does not contribute to sea level rise. Melting ice can cause sea level to change only if the ice is on land, not floating in water (see Earth Issues 21.1).

Most of the world’s ice is locked up in the huge continental glaciers that cover Antarctica and Greenland. Is global warming causing these ice sheets to melt faster than they can be regenerated by new snowfall? In the past, it has been difficult to answer this question because scientists had to compute the glacial budget; that is, they had to figure out the difference between accumulation and ablation, and these are hard quantities to estimate accurately over large regions. But now radar instruments mounted on Earth-orbiting satellites can directly measure changes in the ice volume of a region. The results have been surprising.

First, according to the IPCC’s most recent assessment, the East Antarctic ice sheet, the largest ice reservoir on Earth (see Figure 21.6), has been gaining ice mass at about 21 Gt/year in the period 1993–2010. Recent climate changes have evidently increased the amount of snowfall in East Antarctica so that accumulation exceeds ablation. This net accumulation is good news because it subtracts from any sea level rise. Unfortunately, the West Antarctic ice sheet is losing mass at a much higher rate, at about 118 Gt/year, and the smaller Greenland ice sheet is losing about 121 Gt/year. Most surprising of all is a net loss of 57 Gt/year in the mass of continental valley glaciers and smaller ice sheets (such as those in Iceland), which together account for less than 1 percent of the total ice volume of the cryosphere. The rates are especially high for valley glaciers in temperate and tropical regions, which are vanishing very quickly.

Summing up these numbers yields 275 Gt/year as the current rate of continental ice loss. Essentially all of this mass goes into the ocean. One gigaton of water occupies one cubic kilometer (its density is 1 g/cm³), so the increase in ocean volume is about 275 km³/year. We can convert this volume change into sea level change using the formula

From Appendix 2, we obtain an ocean area of 3.6 × 10⁸ km², so

or about 0.8 mm/year.

This figure is only a fraction of the current rate of sea level rise. The rest is coming from the warming of the ocean itself. In the twentieth century, the sea surface temperature increased by nearly 1°C, which caused the water in the upper portion of the ocean to expand a tiny fraction, about 0.01 percent. That small increase in volume can account for most of the 200 mm rise in sea level during that period.

We can conclude that melting of continental ice has thus far contributed only a small amount to sea level rise. However, the process of glacial thinning is increasing rapidly, primarily by the acceleration of glacial flow (see Figure 21.12). The satellite observations reveal that flow accelerations of 20 to 100 percent have occurred over the past decade. A key question that concerns scientists is whether these accelerations will increase in the future.

BONUS PROBLEM: If seawater expands by 0.01 percent for each 1°C of temperature increase, how deep is the layer of the ocean that must be heated by 1°C to explain the twentieth-century sea level rise of 200 mm?