Let’s consider the most basic of facts.
In 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.
This 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.
All of 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.
Indeed, 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.
It is incontrovertible that anthropogenic activity during the last couple of centuries has contributed to increased concentrations of various gases responsible for this differential transmissivity.
If 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 tropospheric temperatures.
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