Page 114 - Earth's Climate Past and Future

90 PART II • Tectonic-Scale Climate Change

Plateau
Larger ocean basin
Continent
Ocean crust Ocean crust
Continent
Continents
Plate motion
collide
Mantle
Mantle

Root

FIGURE 5-11 Continental collisions and sea level When continents collide, the continental crust
doubles in thickness and creates a high plateau with a thick low-density crustal “root.” This
thickening reduces the original area of the continents and increases the area of the ocean basins.
The increased area and volume of the ocean basins causes sea level to fall.

areal extent of the continents requires an equivalent waters has cooled by about 5ºC over that interval, while
increase in area of the ocean basins (see Figure 5-11). the high-latitude surface ocean and the deep ocean have
With a larger area of ocean to fill, the water level in the both cooled by 10º–15ºC. The contraction of seawater
ocean should drop. caused by this cooling has reduced global sea level by an
As noted in Chapter 4, continental collisions have estimated 7 m.
occurred only sporadically through geologic time. The The quantitative effects of several of the above fac-
only major collision that has occurred since 100 Myr tors on global sea level have to be adjusted for further
ago began when northern India first made contact with complications (Box 5-1). One problem is the fact that
southern Asia, some 55 Myr ago. This collision, still in water moving into (or out of) the ocean basins represents
progress, has increased the area of the ocean by some
2
2 million km over the last 55 Myr. As seawater has
flowed in to fill this new area of ocean basin, global sea
level has fallen an estimated 10 m below the level of
100 Myr ago, a time when no collisions were occurring. Ocean
water
Climatic Factors Ice sheet
3. Water stored in ice sheets Continent-sized ice
sheets several kilometers in thickness and thousands of Low
kilometers in lateral extent can extract enormous vol- Continental sea
margin
umes of water from the ocean and store it on land (Figure level
5-12). Because no permanent ice sheets existed between
100 and 80 Myr ago, little or no water was stored on the
land as ice. Today the Antarctic ice sheet holds the equiv- A
alent in seawater of 66 m of global sea level, and the ice
sheet on Greenland contains another 6 m. The Antarctic
ice sheet has come into existence and grown to its present Glacial meltwater
size within the last 35 Myr, and the Greenland ice sheet
has done the same within the last few million years.
Together these ice sheets have extracted a volume of High
sea
ocean water equivalent to 72 m of global sea level. level
4. Thermal contraction of seawater Ocean water has
the capacity to expand and contract with temperature
changes. The thermal expansion coefficient of water
(the fractional change in its volume per degree of
change in temperature) averages about 1 part in 7000 B
for each 1ºC of temperature change. Because of this FIGURE 5-12 Ice sheets and sea level Ice sheets covering
thermal behavior, even a constant amount of seawater large parts of continents hold volumes of water equivalent to
would have lost volume during the cooling of the last 80 tens of meters of global ocean level. Sea level (A) falls when ice
to 100 Myr. The temperature of low-latitude surface sheets are present on the land and (B) rises when they melt.