Let’s consider the most basic of facts.
1804, the world human population met a milestone of 1 billion people.
It didn’t double again until 1927, yet doubled again to 4 billion in
1974. By 2012, the world human population surpassed the 7 billion mark
with over half of the world’s population living in urban areas.
rapid growth has corresponded with amazing technological achievements
and dramatic changes in the Earth’s environment. For instance, it is
estimated there are now over 1 billion automobiles, most of which have
internal combustion engines, in operation on roads worldwide; each day
there are an estimated 90,000 commercial flights; the amount of land
currently in agriculture production is 1.5 billion hectares (about 12
percent of the land surface); there are some 50,000 power plants
worldwide, of which 2,300 (consisting of 7,000 individual units) are
coal-fired; and during the first decade of this century, deforestation —
largely due to land conversion for agriculture — occurred at an
estimated rate of 13 million hectares per year.
these anthropogenic activities, as well as many more, contribute to
changes in biogeochemical cycles. In particular, they have contributed
to changes in atmospheric chemistry, from the local (e.g., photochemical
smog) to the regional (e.g., acid precipitation) to the global (e.g.,
Antarctic ozone depletion).
What discussions of
Medieval Warm Period, Milankovitch cycles, solar variations, and/or
past glaciation ultimately show is something quite elementary. Climates
are naturally variable and have fluctuated between cool and warm
periods during the Earth’s long history. However, this simple fact
doesn’t preclude anthropogenic contributions to climate change.
the mechanisms responsible for the heating of the lower troposphere
have been understood for some 188 years. Certain atmospheric
constituents are transparent to short-wave incoming solar radiation,
yet opaque to outgoing long wave terrestrial radiation. This
differential transmissivity, known as the greenhouse effect, results in
higher surface temperatures than would occur absent these gases.
is incontrovertible that anthropogenic activity during the last couple
of centuries has contributed to increased concentrations of various
gases responsible for this differential transmissivity.
a lifelong smoker was diagnosed with lung cancer, an oncologist may be
interested in the patient’s genetic predisposition to cancer. However,
simple parsimony would likely lead the oncologist to consider the
smoking history as the contributing factor to the disease. Similarly, we
know that a multitude of human activities, from the burning of fossil
fuels to the clearance of forests, alter atmospheric concentrations of
so called greenhouse gases. We also know that variations in
concentrations of these atmospheric constituents result in changes in
At some point, both logic
and indisputable empirical evidence can lead us to only one conclusion:
We, as a species, are drivers of environmental change, including but
certainly not limited to climate change.
—Todd Fagin, Oklahoma City