Geoscience Reference
In-Depth Information
lower-latitude subarctic climates (
Dfd
,
Dwd
). Average
annual precipitation is low.
they are indirect evidence for past climate conditions. Scientists
who are interested in this kind of prehistoric detective work are
called
paleoclimatologists
. This part of the chapter focuses on
the kind of evidence paleoclimatologists use to reconstruct past
climates.
3.
The ice cap and sheets climate (
EF
) is associated with
large glaciers at high latitudes. Thus, average annual
temperatures are very cold, and very little or no vegeta-
tion is present.
Pollen Records
One kind of clue that provides indirect evidence of past climate
conditions is ancient plant pollen, which reflects the kind of
vegetation that occurred in a region at any given time. Given the
demonstrable link between climate and vegetation, as we have
seen earlier in this chapter, it is logical to assume that changes
in the prehistoric plant assemblage of an area over time would
indirectly reflect climate change. Such change in a plant com-
munity can be reconstructed in regions (usually cool and moist)
where pollen is well preserved in the deposits that slowly accu-
mulate in lakes and marshes. Scientists who study such records
are called
palynologists
.
The premise of palynology is that plants produce micro-
scopic pollen that are unique in shape and size to specific spe-
cies. You can see this kind of variation in Figure 9.14. Each
year, plants within an area produce pollen as part of their repro-
ductive cycle, which is then spread by wind as pollen rain. If
you happen to have seasonal allergies, you are acutely aware of
airborne pollen because of the discomfort it may cause. Some
of this pollen accumulates at the bottom of lakes and marshes in
sediments that progressively thicken with time. Given that the
deposits at the base of the deposit are the oldest, and progres-
sively decrease in age toward the top of the sequence, the change
in pollen composition through time can be reconstructed. To do
4.
The highland climate (
H
) occurs at high elevations in
mountain ranges such as the Rockies, Himalayas, and
Andes. Although temperature in these areas varies an-
nually, the range is much narrower and colder than
nearby localities at lower altitudes.
As you probably know, an important environmental issue
currently receiving a lot of attention is
global warming
, which
is the belief that human industrial activity is gradually causing
the planet to warm. Although over 90% of publishing clima-
tologists believe human-induced global warming and associ-
ated climate change is occurring, the issue has become greatly
politicized because numerous special interest groups are now
involved in the debate. This part of the chapter focuses on
the widely accepted science associated with ongoing climate
change, and as a result, will provide you with the scientific
background required to consider the issue. Although the politi-
cal dimensions of the problem are fascinating in their own right,
they are beyond the scope of this chapter. A good place to begin
the discussion of ongoing and future climate change is by in-
vestigating the magnitude and frequency of climate fluctuations
in recent Earth history, particularly during the
Quaternary Pe-
riod
. This period of time encompasses the past ~2 million years
of Earth's history and is a time of frequent climate changes,
ranging from intensive ice ages to intervals similar to current
conditions. You might be thinking, why do we look at Quater-
nary climate change to understand the future? The reason is that
by understanding the nature of past climate changes, we gain
insight into the range of possibilities that might occur in the
future.
The challenge facing scientists investigating Quaternary or
other past climates is that no direct temperature and precipita-
tion data exist to reconstruct conditions at that time. After all,
humans have been keeping detailed climate records for only
about 150 years. In some parts of the United States, such as
the western states where European settlement occurred later,
these records span an even shorter amount of time. Although
150 years sounds like a long time in the context of the average
human life span, it is really quite short from a geological per-
spective. In order to reconstruct the climate conditions of any
given place, say, 20,000 years ago, it is necessary to investigate
scientific clues from that era about the character of the environ-
ment at that time. Such clues are called
proxy data
because
Figure 9.14 Pollen grains.
Different plant species produce dis-
tinctive forms of pollen that can be seen under a microscope.
Note the different forms of pollen in this image, which is magni-
fied 400 times.
Proxy data
Indirect evidence of an event. For example, fossil
pollen is a proxy indicator of climate change because vegeta-
tion reflects climate.
