Global Climate Change:
by Dale H. Easley
Department of Applied and Natural Sciences
University of Dubuque
New Orleans, LA 70148
Note (2/8/2006): Since I first wrote this essay, the evidence supporting
human-induced global warming has become overwhelming. I think the time-prespective
a geologist brings to the issue is valuable, but I also think it's past time we made some hard
decisions about our lifestyle---driving less, using less heat and air conditioning,
being more efficient with energy in every aspect.
Note (6/10/2006): I was recently in my old hometown of New Orleans, visiting
favorite places, some of which destroyed by Hurricane Katrina. While there, I saw Al Gore's
An Inconvenient Truth. From a scientist's point of view, it was outstanding.
The case for doing something about human contributions to global warming is overwhelming.
There has been much talk about global climate change. Movies such as Water
World predict global catastrophe due to rising sea level. The ice caps
will melt, submerging coastal cities such as New Orleans. Though most
scientists are now in agreement that we are in a period of global warming, one
still hears the occasional doubter asking, ``Can we really be sure?'' After all,
the historical record for which we have temperature records is very short, and
some of that might be distorted by the effects of urbanization. The
perspective of a geologist can, perhaps, help clarify some thinking.
If you asked a geologist about global climate change, he or she would probably
ask you about the time scale you were interested in. On average, the world has
been warming since the end of the last ice age, about 10,000 years ago. And it's
likely that another ice age will come along in the future. On the other hand,
it's probably cooler now that it was when the dinosaurs were at their prime, so,
on average, it has cooled off since then. We can't seem to avoid averages---average
daily high temperatures, monthly average temperature, yearly averages, etc.
Those averages are always averaged over some time period, and averaging
inherently smooths out fluctuations. Often we may be interested in average
behaviour, but we also need to ask which fluctuations are important and what
caused those fluctuations.
An obvious factor in the climate for a specific continent is its latitude.
Throughout geologic history, lithospheric plates have been rearranging
themselves. At some times, most of the continents have been near the equator.
During such times, continental glaciation doesn't occur. At other times, some
of the continents have been near the poles and have been in connection with
other continents. This orientation allows snow to accumulate at high latitudes
and then spread laterally to lower latitudes. The advance of these continental
ice sheets has been recorded geologically by the rocks they have carried, the
scars they have left behind, and the alteration to river systems that resulted.
Rocks also record other evidence of past climates. For example, in Antarctica,
now found at the South Pole, are coal beds, indicators of warmer conditions.
Also influencing global climate is the orbit of the Earth around the Sun. That
orbit changes in predictable, periodic way, termed Milankovitch cycles.
1 The orbit of the Earth around the Sun is elliptical, but not
always in the same amount. It gradually becomes more circular and then again
more elliptical every 100,000 years. Meanwhile, we have seasons because of the
tilt of the axis of the Earth. The tilt becomes more extreme and less extreme
regularly over a 41,000 year cycle. Finally, the Earth's axis changes its
orientation relative to the sun on a 23,000 year cycle, wobbling like a top
about to fall down. Suppose that the orientation is such that winter occurs
when the Earth happens to be at its farthest from the Sun and is tilted most on
its axis. That winter will be particularly severe. Such times are when major
ice ages tend to occur. And the resulting heating and cooling of global
temperature has nothing to do with the pollution coming from my mini-van.
Another factor that has an important role in global climate is the concentration
of various gases and particulates in the atmosphere. And it is here that human
effects upon global warming become interesting. Carbon dioxide (CO2),
added to the atmosphere by burning of oil, coal, gas, and wood, is a major
greenhouse gas. A greenhouse gas acts like a sheet of glass on a greenhouse---It
allows light in but keeps heat from leaving. When we put CO2 into the
atmosphere, it is like thickening the glass on the greenhouse. Global warming
results. Methane, water vapor, and chloroflourocarbons (CFCs) are also
greenhouse gases. However, atmospheric particulates, such as those emitted
during a volcanic eruption, block out light and lead to global cooling. The
cooling resulting from the huge amount of particulates blasted into the air by a
bolide impact to the Yucatan Peninsula is believed to have been a major cause of
the extinction of the dinosaurs.
After looking at things from a geologist's perspective, we might decide to
rephrase our question as, ``Are humans altering the environment in ways and at a
rate that will have consequences less desirable than those that would result
from natural change without human influence?'' The problem is, of course,
predicting what that natural change would be. Even if you could, you could not
escape making a value-judgement about its desirability. If you can imagine being
a species that couldn't tolerate oxygen, then you can imagine being pretty upset
about what plants did to the atmosphere. That said, it is pretty clear now, and
most scientists agree, that humans have speeded the rate of global warming,
endangering many species of life, altering the climate system faster than many
species can adapt. We are making the future
more expensive and dangerous each day we delay in reforming our behavior.
1 They are named
for Milutin Milankovitch, a Yugoslavian astronomer, who is credited with
discovering them in the 1920s.
US Global Change Research Program
USGS Global Change Research
File translated from TEX by T
TH, version 2.65.
On 30 Aug 2000, 18:28.