Page 122 - Earth's Climate Past and Future

98 PART II • Tectonic-Scale Climate Change

Global Climate Change since 50 Myr Ago 0 First
First glaciation North American
Earth has undergone a profound cooling at both poles in the Andes and Eurasian ice
and across the lower latitudes of both hemispheres dur- 10
ing the last 50 Myr. Both ice and vegetation have left Increased ice
abundant evidence of this cooling (Figure 6-1). on Antarctica
Spread of cool
20
20 boreal spruce
6-1 Evidence from Ice and Vegetation ? forest in the
Last trees Arctic
As climate cools, two kinds of glacial ice form on land (beech family)
(companion Web site, pp. 27–30). Small mountain glac- Myr ago 30 on Antarctica Broad-leafed
?
iers and ice caps appear on the tops of high mountains, First local ice evergreen and
and large ice sheets cover much larger areas of the con- on Antarctica deciduous forest
tinents. Because average temperatures vary from region 40 in the Arctic
to region and with altitude, the conditions that permit
ice to persist year-round do not appear at the same time
in all areas. 50 Palmlike trees
In the southern hemisphere, no evidence exists for and crocodile
persistent ice on Antarctica until 35 Myr ago, when ice- ancestors north
of Arctic circle
rafted debris was first deposited in ocean sediments on Southern Northern
the nearby continental margin. Since then, the size hemisphere hemisphere
of the Antarctic ice sheet has increased irregularly
toward the present, with a major growth phase near FIGURE 6-1 Global cooling for 50 Myr Gradual cooling
13 Myr ago. Greater amounts of ice-rafted debris in during the last 50 Myr is demonstrated by the first appearance
nearby ocean sediments suggest additional increases in of mountain glaciers and continental-scale ice sheets and by a
Antarctic ice during the last 10 Myr. Today more than progressive trend toward cold-adapted vegetation in both
97% of Antarctica is buried under ice (Figure 6-2 left). hemispheres.
In the lower and middle latitudes of the southern hemi-
sphere, the earliest evidence of mountain glaciers in the
high Andes is dated to between 7 and 4 Myr ago.
In the northern hemisphere, glacial ice first devel- time (see Figure 6-1). The first evidence of glaciers in the
oped on Greenland sometime between 7 and 3 Myr high coastal mountains of southern Alaska dates to about
ago, although small mountain glaciers may have existed 5 Myr ago. The first continental ice sheets of significant
locally around the North Atlantic Ocean before that size appeared 2.75 Myr ago in North America and

FIGURE 6-2 Cooling in Antarctica (Left) Today an ice sheet up to 4 km thick covers most of
Antarctica, although mountains locally protrude through the thinner cover around the margins.
(Right) Until 30 Myr ago, Nothofagus trees, members of the beech family like those living today at
the southern tip of South America, still existed in parts of Antarctica. (Left: Ward’s Natural
Science Establishment. Right: courtesy of Calvin Heuser, Tuxedo, NY.)