Geoscience Reference
In-Depth Information
gases for the latter half of the Quaternary Period as the global
climate fluctuated between glacial and interglacial cycles. It
is believed that warming at the end of glacial episodes was
initially caused by increased insolation associated with Mila-
nkovitch cycles. Such warming was then amplified by rising
atmospheric CO
2
levels as carbon was released from warming
oceans.
After viewing these data, let's focus more closely on the
past millennia, which encompass much of recent human his-
tory. Look at Figure 9.29, which shows surface temperature
anomalies relative to the 1961-1990 average, in the Northern
Hemisphere. This graph reflects two sets of data. The blue
area from 1000 years ago to 1850 is the reconstructed re-
cord (e.g., from ice cores and pollen studies) that contains a
range of uncertainty. The red line from 1850 to the present, in
contrast, is based on actual measurements. If we assume that
the average temperature during the reconstructed record was
somewhere in the middle of the range, then it was about 0.2°C
(0.4°F) cooler than the 30-year mean about 1000 years ago.
Subsequently, between 1000 and 1900, the average tempera-
ture
further
cooled
about another 0.2°C (0.4°F). This cooling
was most intensive during the
Little Ice Age
, which occurred
from the 14th to 19th centuries.
Now, look at the change that occurred around 1900. At this
time the temperature began to warm quickly to its present state
of being 0.7°C (1.3°F) warmer than the past 30-year average. In
other words, the natural climate progression for the 850 years
prior to the Industrial Revolution was one of
gradual cooling
,
with a rapid warming in the past 100 years. Given the appar-
ent link between temperature and greenhouse gases for the past
800,000 years (Figure 9.28), coupled with the fact that the cli-
mate had been
cooling
between 1000 and 1850 a.d., the rapid
warming of the past 100 years seems to be best explained by
human-induced greenhouse gas emissions. At least, that is what
the vast majority of climatologists believe. The effort to explain
this relationship is led by the Intergovernmental Panel on Cli-
mate Change (IPCC), which is an international body of scien-
tific experts established in 1988 by the World Meteorological
Organization (WMO) and the United Nations Environmental
Programme (UNEP).
Predicting Future Climate Change
If the one-million-dollar question is whether or not human-in-
duced climate change is occurring, then the two-million-dollar
question is “How much future warming will occur?” Remem-
ber that the current proportion of atmospheric carbon dioxide is
about 400 per million parts of the atmosphere, and that this pro-
portion exists with the developed world producing 73% of the
excessive carbon dioxide. The big unknown is how climate will
respond when the developing countries increase their carbon di-
oxide emissions by the predicted 25% in the next few decades.
In the context of understanding future change in an enhanced
greenhouse world, the scientific challenge is somehow to predict
how the climate system will respond. This complex task is best
accomplished with general circulation models (GCMs), which
are mathematical models that incorporate known climate vari-
ables, such as cloud cover, insolation, latitude, and proximity to
oceans, to forecast future climate events. Using these variables,
these models run simulations involving scenarios of low or high
carbon dioxide levels. Models that run moderate-case scenarios
use a doubling of atmospheric carbon dioxide from preindustrial
levels (~300 ppm) to ~ 600 ppm by 2050 as a plausible scenario.
Figure 9.30 illustrates the geography of warming by the 2050s
using this scenario. Note that above-average levels of warming
are predicted over much of Earth, with much greater than average
warming expected at high latitudes of the Northern Hemisphere.
This model further projects that warming will increase on the
order of 2°-4° C (4°F-7°F) by 2100.
Although further warming is projected with high confidence
for the rest of this century, the actual amount and geographical
variability remain uncertain. Much depends on the nature of cli-
mate feedbacks, which are indirect or secondary changes that
take place within the overall climate system in response to the
initial forcing mechanism—in this case, increased atmospheric
carbon dioxide. For example, it is possible that increased warm-
ing will cause more water to evaporate from oceans, resulting in
more atmospheric vapor. This increase, in turn, can have a larger
effect on the global energy balance, both as a
positive climate
feedback
that further increases warming or potentially as a
nega-
tive climate feedback
that causes cooling. Increased atmospheric
water vapor could cause a positive feedback because it traps so-
lar energy, which would further warm Earth. On the other hand,
increased atmospheric vapor will increase the number of clouds,
which could increase the amount of sunlight that reflects back
into space, thus resulting in a net cooling effect.
The ultimate positive feedback is a runaway greenhouse
effect where significant warming causes the polar ice caps to
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
-1.2
Historic record
Reconstructed prehistoric record with range uncertainty
Average surface temperature (1961-1990)
1000
1100 1200 1300 1400 1500
Year A.D.
1600
1700 1800 1900 2000
Figure 9.29 Global surface temperature anomalies in the
Northern Hemisphere, relative to the 1961-1990 mean, in the
past 1000 years.
Temperature cooled slightly between 1000
years ago and approximately 1850. Since that time, temperature
has been warming dramatically. (
Source
: M. E. Mann, Z. Zhang,
M. K. Hughes, R. S. Bradley, S. K. Miller, S. Rutherford, and F. Ni.
Proxy-based reconstructions of hemispheric and global surface
temperature variations over the past two millennia. Proceedings
of the National Academy of Sciences 2008; 105: 13252-13257.
Copyright 2008 National Academy of Sciences, U.S.A.)
