Geoscience Reference
In-Depth Information
arctic Ice Sheet. It is on average 2,000 meters (1.2 miles) thick. If the
entire volume of the ice sheet were to melt—all 2,900,000 cubic kilo-
meters or 750,000,000,000,000,000 gallons—it would cause global sea
level to rise by 7 meters (or 23 feet).
12
Measurements of the GIS indicate that it was stable for most of the
twentieth century but began to shrink during the last two decades. The
current melt rate is estimated to be 0.75 mm (0.03 inch) per year of sea-
level rise equivalent (SLRe). Recent estimates suggest that the GIS will
contribute a small amount to SLRe over the next century; a central
estimate is 7 centimeters (3 inches) in the case of rapid temperature
increases. More detailed models indicate that very high temperature
increases, such as those associated with the baseline runs in Figure 9,
would lead to a SLRe of 1.5 meters (5 feet) in three centuries, and on
the order of 3 meters (10 feet) over the next millennium, just from the
melting of the GIS.
13
Now we can see the tipping element. Global warming would cause
the GIS to warm, melt, shrink, and drop in elevation. Temperatures in-
crease with lower altitude, so a smaller ice sheet will be warmer at the
top than the current ice sheet, and this higher temperature will further
accelerate the melting. The ice sheet would also tend to be darker as it
warms, absorbing more solar radiation, and warming even further.
Once the ice sheet passes some threshold in a warmer world, most of its
ice might melt away.
While this seems far in the future, some scientists worry that the GIS
is an unstable system, like the bowl shown in Figure 11. There may be
two distinct equilibria—one a cold, white, high-altitude ice sheet and the
Why might there be multiple equilibria for a given temperature?
Suppose after centuries of warming, the remaining ice sheet is in the
green, low-elevation equilibrium. Then the earth begins to warm up
again. However, since the ice sheet is warmer and darker, it remains
stuck in the low-elevation equilibrium. If there is a tipping point of this
kind, a climate that is warmed for a suffi cient period of time would
lead to an irreversible melting of the GIS and an inevitable large sea-
level rise.
