Page 374 - Earth's Climate Past and Future

350 PART V • Historical and Future Climate Change

simulate a northward shift of mid-latitude hardwood
trees like maple and beech during the next century, with
Subsea
permafrost
warm-adapted trees like oak and hickory moving north
Continuous
to replace them. By one reckoning, mid-latitude tree
permafrost
types already need to shift northward at an average rate
Discontinuous
of 10 m (>30 ft) per year to remain within their areas of
permafrost
optimal growth conditions during the current rate of
High mountain
warming. Most species can match this rate of movement
permafrost
by dispersal of pollen, seeds, and cones by winds and ani-
mals. With the larger and faster shifts expected in the
next century, however, some species may not be able to
E U R O P E
A S I A keep up with the northward displacement of their opti-
mal environment.
In the tropics and subtropics, scrub and tree vegeta-
tion were more prevalent 10 million years ago in several
arid regions: the sub-Himalayan region of India and
Pakistan, the western North American high plains, the
South American pampas region of Argentina, and parts
of sub-Saharan Africa and the East African highlands.
Higher levels of CO in the atmosphere allowed C3
2
vegetation (trees and shrubs) to live in arid regions, but
then the gradual CO lowering during the last 10 mil-
2
lion years made C3 vegetation less competitive with C4
grasses. In the next century, we will pass through the
same 2 × CO threshold but heading in the opposite
N O R T H 2
direction and at a much faster rate. As C3 shrubs and
A M E R I C A
trees replace C4 vegetation, some arid and semiarid
regions could become greener (see Figure 19–9).
The warming during the next century will also alter
regional patterns of precipitation and evaporation in
significant ways. Evaporation will increase worldwide
FIGURE 19-8 Melting permafrost Today’s large ring of
permafrost around the Arctic Ocean will become vulnerable because warmer temperatures will permit air to carry
to gradual melting in the warmth of a 2 × CO world. (From more water vapor. With more water vapor in the air,
2
F. Press and R. Siever, Understanding Earth, 2d ed., © 1998 by global average precipitation will also increase but in
W. H. Freeman and Company.) patterns that may vary from region to region. With
evaporation increasing, areas that fail to receive more
precipitation will become drier while those that receive
Because sea ice and vegetation are relatively fast- more precipitation could become wetter. Unfortu-
responding parts of the climate system, we can expect nately, because climate model simulations of regional
much larger transformations of polar sea ice, tundra, and precipitation often disagree, moisture trends are diffi-
northern forests as climate warms to the 2 × CO level cult to predict region by region.
2
(Figure 19–9). The high climatic sensitivity of this region No evidence of mountain glaciers has been found in
is obvious from the northward retreat of tundra and North or South America, Africa, or Asia before about
sea ice limits caused by summer insolation values ~5% 7 million years ago. The only place where mountain
higher than those today near 6000 years ago (Chapter glaciers may have existed was in far northern Scandi-
13). Trees will move north of the Arctic Circle, and sea navia and on Greenland, where the combined effect of
ice will retreat from the coasts. As a hint of what the high latitudes and high altitudes may have made tem-
future holds, summer sea ice limits have shrunk by more peratures cold enough for ice to persist. Almost every
than 20% in the last four decades. Melting of surface per- mountain glacier on Earth has already been retreating
mafrost (already underway) will continue at a rapid rate, for a century or more, and the rates are accelerating (see
but much of the deeper subsurface permafrost will not be Chapter 17). At a prevailing lapse rate of 6.5°C/km, a
affected. 2.5°C warming in the future should cause a vertical
At northern mid-latitudes 10 million years ago, retreat of glaciers up the sides of mountains by some
forests of deciduous trees grew much farther north than 330 m (about 1000 ft). Because mountain glaciers can
they do today. Coupled models of climate and vegetation begin to respond to climate changes within just decades,