|
Items in cart [1] |
Questions? Call 888-624-8373 |
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 50
6
HISTORICAL PERSPECTIVES: CLIMATIC CHANGES
THROUGHOUT THE MILLENNIA
John E. Kutzbach
We are here to consider the prospects of global change and our common
future. Our aim is to look forward. I want to share with you some per-
spectives about global change that we can gain from first looking back-
ward to our common past. Why is it important to look backward first?
The first reason is that the global changes that may occur in the next
century may be larger than any changes that have occurred in recent
centuries. We need to look to more distant times for examples of large
global change. We will see that large global climatic changes have had
large effects on plants, animals, and humans.
The second reason for looking to the past is that the past is a
laboratory in which we can study these global processes and develop our
predictive capabilities. If we can identify the factors that have caused
global change in the past, and if we can successfully estimate past
climates and climatic changes using our computer models, then we will
gain confidence in our ability to estimate and anticipate future changes.
I will describe five examples of large global change from the past:
one from about a billion years ago, another from several hundred million
years ago, and others from one million years, ten thousand years, and a
few centuries ago. Some common themes in all of these examples are that
climate and life have been intertwined since the dawn of earth history,
that relatively small causes have had large and often unexpected con-
sequences, and that the magnitude of some of the possible global changes
of the next century rival the magnitude of some of the biggest changes
from our past.
Of course, the global changes of the past were not caused by humans.
Nor could the animals of the earth, or the humans, do anything about
these changes. Animals, plants, and humans moved, adapted, or died. In
contrast, the global changes of the present and near future may be caused
by humans, and perhaps we can do something about them.
The first example of global change is taken from more than a billion
years ago. Early, innovative forms of life, blue-green algae, used the
energy from the sun to split molecules of water and carbon dioxide and
then recombined them differently to form organic compounds and oxygen--a
process we call photosynthesis. Fossilized deposits from this period are
called stromatolites; present-day structures that resemble stromatolites
grow today in warm, shallow seas. With the burial of the organic matter
produced by photosynthesis, and with other geochemical changes, the
50
OCR for page 51
51
FIGURE 6.1 Locations of continents for various times during the past
several hundred million years. (Adapted from N. Calder. 1983. Time-
scale: An Atlas of the Fourth Dimension. Viking Press, New York.)
amount of oxygen in our ancient atmosphere began to increase and the
amount of carbon dioxide began to decrease. High in the atmosphere
oxygen was split apart by ultraviolet radiation from the sun and then
recombined to form ozone. This marked the birth of our ozone shield.
That shield is now about a billion years old. It has protected the
earth's surface from harmful ultraviolet radiation and has permitted life
to flourish on the continents ever since. It has been an old friend, and
of course we need to know why it is becoming thinner now.
A second example of global change is taken from several hundred
million years ago. The drifting and rifting of our restless continents
created a grand sequence of global changes over hundreds of millions of
years. Figure 6.1 shows the continents' locations from 600 million to
100 million years ago. During a particularly fascinating period between
300 million and 200 million years ago, the continents came together to
form one supercontinent, called Pangaea, and then parted again.
We are using climate models, the same sort of models used to study
possible future climates, to calculate the climate of an idealized
Pangaean world (Figure 6.2~. This one-continent world existed at the
start of the age of the dinosaur. North America, Greenland, and parts of
Eurasia were in the northern hemisphere. South America, Africa, India,
Australia, and Antarctica were in the southern hemisphere. There was one
ocean and one large sea, the Tethys Sea, on the eastern tropical shores.
The huge land mass, according to the climate model, experienced huge
seasonal swings of temperature. The continental interior was hot and
dry, especially in summer, with temperatures exceeding 35°C, or 100°F.
OCR for page 52
90
60
30
of
-30
-60
-an
90
So
-30
-en
52
l l l l l I ~ r I I ~ l
. j , ~ l.
v~ A
_ LAURASIA
,''_'
PANTHAtASSIC ',' i___
_'' " TETHYS SEA
GONDWANALAND ~
OCEA N
- 9 0
-180 -120 -60
10 m/s 5 mls
) ~
o 60 120 180
FIGURE 6.2 (Top) The idealized Pangaean continent with Laurasia in the
northern hemisphere and Gondwanaland in the southern hemisphere.
Panthalassa, the world ocean, and the Tethys Sea are indicated. Fine
dashed lines indicate very approximately the outlines of modern land
masses, but these outlines are only schematic. (Bottom) Surface winds
(arrows) and features of surface temperature (warmer than 30°C, stipple;
colder than 0°C, hatch) for June-July-August based on a climate model
simulation.
Polar regions were cold in winter, with temperatures below freezing. It
was humid along the coasts of the Tethys Sea ? where monsoon winds were
strong.
Geologic evidence of the location of plant and animal fossils,
ancient sand dunes, and mineral deposits supports much of this computer-
estimated climate scenario. This agreement of the simulated climate with
geologic data is important. It shows that we are beginning to under-
stand, and model, the processes of early global change. Although that
world was vastly different from ours in many respects, the climate model
calculates that the average temperature of the Pangaean world was only
about 5°C, or 9°F, higher than earth's present temperature. That is, the
Pangaean world was only marginally warmer than some projections of global
temperature for the next century.
Throughout this period of drifting continents there was a general
rise in the diversity and abundance of life on our planet. Figure 6.3
OCR for page 53
53
800 _ ?
In _ ~
~ _ ~
~ _ ~ 5
1°' ~ ~
~/ 34
o
500 Myr O
FIGURE 6.3 Over the past 500 million years, the general rise in the
number of families of marine life has been interrupted at least five
times by major extinctions. (Adapted from T. H. van Andel. 1985.
New Views on an Old Planet: Continental Drift and the History of Earth,
p. 282. Cambridge University Press, New York.)
illustrates the increasing number of families of marine life over the
period from 500 million years ago to the present. However, the general
rise has been interrupted at least five times by major extinctions of
life. Around the time of Pangaea, the greatest extinction of all time
occurred. By some estimates, about one-half of all families and
three-fourths of all species became extinct. This great dying may have
been related to the drastic changes in climate that accompanied the
formation of Pangaea. Perhaps the Pangaean world had too few unique
habitats, perhaps the heat and aridity were too extreme? perhaps ocean
currents were different, or perhaps the amounts of oxygen and carbon
dioxide were different. We do not know what caused the great extinction,
but we know that it happened.
Can we learn, from such unexplained catastrophic extinctions, any
lessons about the delicate balances that govern life on our planet? I
hope so. Because some ecologists estimate that the destruction of
tropical rain forests, along with other habitats elsewhere, may produce
extinction rates over the next century that will rival the five great
extinctions of the past.
Another great dying, the most recent, occurred around 65 million
years ago. The dinosaurs died. Perhaps changes internal to our globe
caused this event, too; or perhaps, as some have argued, an asteroid hit
the earth. According to one theory, a giant dust cloud may have been
thrown into the atmosphere by the impact. This cloud could have screened
out the sunlight and caused a brief but deadly cold? dark winter over the
face of the earth. For whatever reason, and there are many theories, the
OCR for page 54
54
10xCO~
CO-\
T'
100 Myr
I2XCO2 ?
O Myr
FIGURE 6.4 Over the past 100 million years, earth's climate has
experienced a long cooling trend, as indicated by the schematic arrow
labeled T for temperature. Over the same period, the atmospheric
concentration of carbon dioxide (C02) may also have declined from levels
10 times the present. For comparison, the estimated abrupt doubling of
CO2 concentration is shown with a dashed line.
dinosaurs are gone! If this great dying had not occurred, dinosaurs
might still be the dominant species. But it did occur, and soon
thereafter, mammals became the dominant species.
A third example of global change encompasses a long slide from a warm
to a cool climate (Figure 6.4~. It began around 100 million years ago,
when the climate was still much warmer than it is at present. Why was it
warmer? One strong possibility is that the carbon dioxide content of the
atmosphere may have been about 10 times the present level. This high
level could have been caused by great volcanic eruptions associated with
rapid spreading of the seafloor and rifting of the continents at that
time. However, as the continents moved toward their modern locations,
the rate of seafloor spreading slowed, and volcanic activity and
outgas sing of carbon dioxide decreased. This, according to one theory,
caused the atmospheric concentration of carbon dioxide to fall. And as
the greenhouse effect diminished, the earth cooled. In other words, this
ancient global change may also have linked changes of carbon dioxide and
climate, but with falling levels of greenhouse gases and temperature. If
the amount of carbon dioxide doubles in the next century, it will
possibly mark a return to the higher carbon dioxide levels, and higher
temperature levels, of several million years ago. This is a graphic
illustration of the potential magnitude of the experiment that we
embarked on with the rapid burning of fossil fuels--fossil fuels that
were formed, in many cases, about 100 million years ago.
A fourth example of global change comes from the glacial cycles of
the past million years (Figure 6.5~. The diminished greenhouse effect
mentioned above, perhaps aided by mountain uplift of the Rockies and in
Tibet, seemed to help set the stage for the growth of glaciers and huge
ice sheets. Geologic data for the most recent glacial age, which
occurred about 18,000 years ago, indicate that glacial ice covered much
OCR for page 55
5s
DATA
, . . . . . . . . .. . . . . . . . .
,3 ~ ,N,~
SURFACE PRESENT
TYPE PRECIPITATION
O ICE SHEET ~ DATA ANNUAL
f~ YEAR ROUND ~ I`IOOEL ANNUAL
~ SEA ICE _ < 1000 mm
pit WINTER ONLY
Ed S" 1"
O LAND
O OCEAN
The Earths Orbit
June 2:
POLLEN
OAK
SPRUCE
FORAMS ATLANTIC
I `: r``har ? 20%
N. AMER. EUROPE
20Yo > 10%
> 20°h > 5%
INDIAN
OCEAN
> 20q~o
G bu/Io~des — > 30%
: G pachyderms > 75Yo
March 21
-.'.'
· - ,I~
~ \
it_
2 3 1/2.
I_'_ ~
December 21
~J
1'~: )
September 23 '/
FIGURE 6.5 (Left) Changes in the earth's climate and vegetation that
accompanied the transition from glacial conditions (18 ka, around 18,000
years ago) to interglacial conditions (present), as illustrated by
geologic and paleoecologic evidence. (Right) Changes in the earth's
orbit, shown here for modern conditions, are thought to pace the timing
of glacials and interglacials. The minimum earth-sun distance occurs now
in northern winter but cycles through the calendar year with a period of
about 20,000 years. The axial tilt, now 23~°, varies between about 22°
and 25° with a period of about 40,000 years. (Left: Reprinted, by
permission, from CORMAP Members, 1988. Copyright (c) 1988 by the AAAS.
Right: Reprinted, by permission, from N. G. Pisias and J. Imbrie.
1986/1987. Orbital geometry, CO2, and Pleistocene climate, Oceanus
29~4~:43. Copyright (c) 1986 by Woods Hole Oceanographic Institution.)
of North America and Europe and reached south to the locations of
modern-day cities such as Chicago, New York, London, Berlin, and Moscow.
For comparison, current maps show glacial ice only on Greenland.
In the past decade, great progress has been made in understanding the
cause of these huge swings from glacial to nonglacial climates. Almost
certainly, the swings are triggered, or paced, by relatively small
changes in the earth's orbit, small changes that alter the amount of
sunlight reaching the earth. These small changes in amount of sunlight
equal only a few percent, but the consequences are large. What are these
OCR for page 56
56
GLOBAL TEMPERATURE
CHANGE
5 'C
18 ka 9 ka 0 ka
GLACIAL AGE PRESENT
FIGURE 6.6 The simulated gradual increase in surface temperature,
averaged for the globe, from the glacial age (18,000 years ago) to
present is about 5°C (9°F). For comparison, the estimated abrupt
increase in temperature of about 3°C over the next century due to
greenhouse warming is shown with a dashed line.
Orbital changes? The earth's axis wobbles, like the axis of a spinning
top, completing one wobble cycle in about 20,000 years. Also, the tilt
of the earth's axis changes slightly, taking about 4O, 000 years for one
cycle. And the orbit itself alternates between being almost circular and
slightly egg-shaped, with a cycle time of about 100,000 years. When the
earth's orbit favors less sunlight in summer, the climate cools,
mile-high mountains of ice rise, and the sea level falls by several
hundred feet. When the orbit favors more sunlight in summer, the climate
warms, the ice melts, and the sea level rises.
What lessons can we learn from our glacial past? One lesson is that
small causes (such as small changes in the earth's orbit) may have large
consequences. There are also potential amplifiers in the global system.
In the case of glacial cycles, we now know that the amount of carbon
dioxide in the earth's atmosphere drops during the glacial swings, making
them even colder, and climbs as the ice melts. Thus changes in
greenhouse gases may amplify the consequences of a relatively small
initial change in the amount of sunlight.
Our roots, and our common past, have links to these glacial ages.
Some of our ancestors painted pictures of elk on the walls of the caves
of Southern France, sheltered from the cold northerly winds blowing
across the tundra of Ice Age Europe. Others took advantage of the
lowered sea level to cross the Bering Straits from the Old World to the
New World. Then, we humans could only react to nature's moves. Now we
are making the moves; we are causing global changes.
What happened when the recent glacial age ended? Starting about
18,000 years ago there was a gradual global warming trend of about 5°C,
or about 9°F (Figure 6.6~. (Of course, much greater warming occurred in
polar regions.) The warming trend extended over more than 10,000
years--until roughly 6,000 years ago. That gradual warming trend is
contrasted with a projected, abrupt, 3°C warming trend in the next
OCR for page 57
57
DATA
-
FIGURE 6.7 Changes in the earth's climate and vegetation that
accompanied the transition from glacial conditions (18 ka, around 18,000
years ago) to rapid deglaciation (9 ka, around 9,000 years ago) to the
present, as illustrated by geologic and paleoecologic evidence (see
Figure 6.5 for key). (Reprinted, by permission, from COHMAP Members,
1988. Copyright (c) 1988 by the AAAS.)
century. This is a rather startling comparison. The temperature
increase during the next century could be similar, in magnitude, to the
entire temperature increase that has occurred since the last glacial age.
How have natural communities responded to these climatic changes of
the past? One example is that whole forest communities have marched
across the land. About 18,000 years ago, spruce forests were located in
the central United States south of the ice sheet (Figure 6.7~. As the
ice melted and the climate warmed, the spruce forest moved to the Great
Lakes area 9,000 years ago and is currently located in southern and
northwestern Canada.
OCR for page 58
58
Some forests moved more than 500 miles to the north in the
10,000-year period of gradual warming, or about 5 miles per century. For
comparison, the projected midlatitude warming trend in the next century
might force our forests to try to march 250 miles per century--in other
words about 50 times faster than the most recent natural rate. The
inability of plant and animal communities to move together at such rates
might literally tear communities apart. Even at the more sedate pace of
the warming at the close of the last glacial age, there were
extinctions--of the mammoth, for example, perhaps due to its failure to
adjust to the changing environment, or to human impacts (such as hunting
pressure), or to both; we do not know.
Climate in the tropics undergoes changes, too. Tombstone-like
towers that are eroding fragments of lake sediments and that contain the
skeletons of fish and crocodile bones are standing today in North Africa
in the heart of the Sahara Desert. Around 5,000 to 10,000 years ago a
vast lake covered the region, and a whole network of lakes and Neolithic
fishermen occupied the Sahara. We have assembled geologic data to
describe the climate of that time. Some 9,000 years ago, the area of
wetter climate included much of North Africa, the Middle East, and parts
of southern and eastern Asia (Figure 6.8~. Experiments with climate
models have shown that these huge changes in precipitation were also
caused by small changes in the earth's orbit.
The important point is that, with the help of climate models, we now
know how these changes came to be. About 9,000 years ago, the earth's
orbit favored more summer sunlight. The land became hotter and the
monsoon winds blew more strongly from sea to land. Rainfall increased
and pushed farther inland. The increased rain created lakes in shallow
depressions of the desert floor and, in parts of North Africa and the
Middle East, created conditions more favorable for the agricultural
revolution then under way. The fair agreement between model results and
the geologic record of this global change gives us confidence that we are
beginning to have a crude predictive capability for understanding what
happened and why.
A fifth and last set of examples of global change comes from the
present millennium. The golden age of the Mesa Verde Indians of Colorado
may have been cut short by problems of overpopulation and overuse of the
land. These problems were perhaps accentuated by persistent drought that
began abruptly in the late thirteenth and early fourteenth centuries.
The droughts in Mesa Verde began about the time that temperatures in
Europe fell (Figure 6.9~. An important point is that these relatively
recent climatic changes can be dated very accurately. And from this we
have learned that the climate can change abruptly.
Starting in the fourteenth century and continuing through the
nineteenth century, Europe was colder than it is now (Figure 6.9~. This
period, called the Little Ice Age, has been illustrated by a nineteenth-
century artist's sketch of the advance of a mountain glacier in
Switzerland. In the nineteenth century, the glacier had descended to a
valley floor, threatening a village. In the twentieth century, as a
recent photograph shows, the glacier has retreated many miles up to the
head of the valley. The climate has warmed from the nineteenth to the
OCR for page 59
59
DATA
9 ka i ~Idr88 ~ea~onalit,) |
PAST EFFECTIVE
MOISTURE
O THAN PRESENT
Fj /A LESS THAN
LO PRESENT
F I SAME AS
1~ PRESENT
O INDETERMINATE
FIGURE 6.8 (Left) Features of the earth's climate around 9,000 years
ago (9 ka) based on geologic and paleoecologic evidence (top panel) and
climate model simulations of enhanced monsoonal circulations (bottom
panel). (Right) Changes in the earth's orbit from the present
configuration, where perihelion (minimum earth-sun distance) is in
northern winter, to the configuration for 9,000 years ago, where
perihelion was in northern summer and the axial tilt was 24°, account in
climate model simulations for the enhanced monsoons. (Left: Reprinted,
by permission, from COHMAP Members, 1988. Copyright (c) 1988 by the
AAAS. Right: Reprinted, by permission, from N. G. Pisias and J. Imbri~
1986/1987. Orbital geometry, CO2, and Pleistocene climate, Oceanus
29~4~:43. Copyright (c) 1986 by Woods Hole Oceanographic Institution.)
F O R A M S oCEAAN
> 20%
G bu/loldes > 30%
,,,,,
~ G. pachyderms
The Earths Orbit
March 21
June 2~ cember hi
September 23
twentieth century, and present-day temperatures are already warmer than
they were at any time in the last millennium.
We do not know, for certain, the causes of the climatic changes of
recent centuries and decades. Perhaps there have been small changes in
the amount of sunlight, or small changes in the frequency of volcanic
eruptions, or subtle internal oscillations of atmosphere and ocean.
These recent climatic changes are smaller in magnitude than the changes
suggested by our models for the twenty-first century. Nevertheless, even
these small changes are obviously large enough to have major regional
OCR for page 60
60
I i I I I I I 1
r
1.5 °C
LITTLE ICE
AGE
r i /
~ I
l 1 1 1 1 1 1
900 1 100 1300 1500 1700 1900
YEAR A.D.
L ess
glaci o 1
More
I glacial
FIGURE 6.9 Estimates of the changes in temperature in Europe over the
past 1,000 years. (Reprinted, by permission, from J. Imbrie and K. P.
Imbrie. 1986. Ice Ages: Solving the Mystery, p. 181. Harvard
University Press, Cambridge, Mass. Copyright (c) 1988 by John Imbrie and
Katherine Palmer Imbrie.)
impacts, as indicated by the advance of glaciers in Switzerland of the
thirteenth century or by the Dust Bowl years in America of the twentieth
century. This backdrop of ongoing natural climate variability has
another very serious consequence. It complicates our task of recognizing
the initial phases of human-caused climate change--witness the
discussions of the hot, dry summer of 1988.
To sum up, I have highlighted several examples of global change from
the past: the dawn of life on our planet, the restless rifting of the
continents, the waxing and waning of ice ages and monsoons, and the
droughts and cold spells of recent centuries. There are a number of
lessons to be learned, I think, from this brief historical perspective on
global change throughout the millennia.
1. Climate and life have been intertwined since the dawn of earth's
history.
2. Relatively small causes (such as orbital changes) have had large
consequences.
3. Global change has sometimes been accompanied by the growth or
catastrophic decline of species; only five great dyings in the past may
compare in magnitude to some estimates of near-future extinctions from
the diverse global changes now under way.
4. The potential magnitude of climatic change in the next century,
caused by human activities, is comparable to that of some of the large
natural climatic changes of the past, but human-caused changes may occur
at much faster rates.
5. The global system we need to understand is complicated, but we
are making progress in understanding how it works and in constructing
predictive models.
OCR for page 61
61
Perhaps the most important lesson is that even though there is still
much that we do not know about global change, past or future, we already
know enough to begin to act now.
REFERENCES FOR ADDITIONAL READING
COHMAP Members. 1988. Climatic changes of the last 18,000 years:
Observations and model simulations. Science 241:1043-1052.
Crowley, T.J. 1983. The geological record of climatic change.
Geophys. Space Phys. 21:828-877.
Kutzbach, J.E., and R.G. Gallimore. 1989. Pangaean climates:
Megamonsoons of the megacontinent. J. Geophys. Res. 94:3341-3358
Schneider, S., and R. Lander. 1984. The Coevolution of Climate and
Life. Sierra Club Books, San Francisco, 317 pp.
Rev.
Representative terms from entire chapter:
global changes
