Page 123 - Earth's Climate Past and Future

CHAPTER 6 • From Greenhouse to Icehouse: The Last 50 Million Years 99

FIGURE 6-3 Cooling in the Arctic (Left) Warm-adapted breadfruit trees lived above the
Arctic Circle in Canada until 60 Myr ago, but (right) land around the Arctic Ocean is now
covered by scrubby tundra vegetation grazed by caribou. (Left: Swedish Museum of Natural
History, photo by Yvonne Arremo, Stockholm. Right: Corbis.)

Eurasia. These ice sheets grew and melted in repeated thawed layers lying above permafrost, ground frozen
cycles, and their maximum size increased after 0.9 Myr each winter by intense cold (Figure 6-3 right). The
ago. Although these northern ice sheets developed more appearance of tundra and permafrost is probably linked to
than 30 Myr later than the ones in Antarctica, they are a frigid winters brought on by expanding sea ice.
response to the same overall global cooling trend. The shapes of tree leaves can be used to reconstruct
Fossil remains of vegetation also indicate a progres- past climate (Figure 6-4). Leaves of trees living today in
sive cooling over the last 50 Myr. A form of beech tree the warm tropics tend to have smoothly rounded mar-
called Nothofagus (Figure 6-2 right) lived on Antarctica gins, while leaves of trees in cooler climates generally
before 40 Myr ago, along with several types of ferns.
This vegetation disappeared as climate became more
frigid and ice spread across Antarctica. Today the only 100
vegetation on Antarctica is lichen and algae found in
summer melt water ponds in ice-free regions of a few 80
coastal valleys. 60
The same kind of long-term cooling trend is evident
in north polar regions (Figure 6-3). Palmlike and other Smooth-edged leaves (%) 40
broad-leafed evergreen vegetation existed in the Cana-
dian Arctic at 80°N from 60 to 50 Myr ago (Figure 6-3 20
left), as did the ancestors of modern alligators that
would presumably have been ill-adapted to extreme 0 10 20 30
cold. Sea ice was apparently absent, even along the Mean annual temperature (˚C)
coastal Arctic margins. FIGURE 6-4 Leaf outlines indicate temperature Trees with
Gradually the warm conditions in the Arctic gave way smooth-edged leaves flourish today in the tropics, while trees
to today’s cold. The development of conifer forests of with more jagged-edged leaves grow in colder climates.
spruce and larch by 20 Myr ago indicates cooling, and the (Adapted from S. Stanley, Earth System History, © 1999 by W. H.
ring of tundra that has encircled the Arctic Ocean in the Freeman and Company, after J. A. Wolfe, “A Paleobotanical
last few million years indicates deepening cold. Tundra is Interpretation of Tertiary Climates in the Northern Hemisphere,”
scrubby grasslike or shrublike vegetation that lives on American Scientist 66 [1978]: 994–1003.)