Geoscience Reference
In-Depth Information
These simple examples from daily life are refl ected on a global scale
as well. Scientists are concerned about critical thresholds in earth sys-
tems that may be crossed because of climate change. But be warned:
These processes are much less well understood than the sequence of
events described so far. We now leave the realm of relatively well-
understood systems and enter areas that are much more complex and
poorly mapped. Although we see these systems through a clouded lens,
these phenomena encompass some of the most dangerous and frighten-
ing potential effects of climate change.
THE VARIABILITY OF PAST CLIMATES
One of the triumphs of modern earth sciences is the development
of techniques that map out the climatic history of our world. These
techniques include taking core samples from ice sheets and measur-
ing the width of tree rings. Using these proxy variables, scientists can
construct estimates of past climates, sea levels, vegetation, and atmo-
spheric gases.
The major conclusion drawn from this research is that past climates
have differed dramatically from what we live with today. Studies show
that the earth has experienced a sequence of cold and warm periods. In
some periods, ice sheets may have covered virtually the entire planet,
while at other times the earth was ice free. Many of the largest climatic
changes were caused by changes in the earth's orbit. The reasons for the
timing of short-run fl uctuations are still unclear, but we do know they
happened. Small changes in the energy balance of the earth can lead to
vast changes in the distribution of ice, vegetation, animals, and living
conditions.
A second and equally surprising fi nding is that our planet has expe-
rienced an unusually stable climate for about 7,000 years. There are
many different methods to determine this, but one method calculates
temperatures based on ice core samples from Greenland (see Figure 10).
This reconstruction uses the quantity of an isotope of hydrogen called
deuterium as a thermometer. 1
Look at the last 7,000 years as shown in Figure 10 (time runs back-
ward from right to left). Note how stable the temperature has been over
 
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