Collectively we are a stunningly large species. A way of illustrating the global nature of our species comes from calculations of our own ecological footprint; Invariably we exceed the amount of productive land available to us. For example, the population of the Chicago Metropolitan Statistical Area contains around 9 million people. The amount of land required to sustain each person (the physical footprint of our buildings, land for agricultural productivity and so forth) is about 20 acres. Collectively this is 180,000,000 acres (281,250 square miles). Illinois's land area is 55,593 square miles, making the ecological footprint of Chicagoans five times larger than the state of Illinois. In fact Chicagoans do not live in Illinois - they live wherever their environmental shadow is cast. In turn, the US population is larger than the country which contains us, and the global population footprint is now larger than the globe. We can overshoot on the global scale only by drawing down on global environmental capital. And the planet may prove to be a rather taciturn banker when accounts come due.
The resource gluttony that got us to this point has had extravagant consequences on a global scale: despoliation of the biosphere, vast eutrophication of the hydrosphere, depletion of soils, and atmospheric changes resulting in climate disruption.
We are in the seemingly paradoxical position of not knowing the number of species on Earth to the nearest order of magnitude (are there 5 or 50 million?) but knowing that we have accelerated species loss to rates comparable to that of a mass extinction event. That extinction is the fate of all species is beside the point, since ultimately the loss is measured in repercussions to us. Not the least part of this is the implication for our ethical self-conception. Is it good to be asteroid-like, comparable to the one that took out the dinosaurs?
There are three principal ways in which we accomplished a task that formerly required an intergalactic event. Of the three, the human transformation of natural habitat into human habitat has been most consequential. One study of the amount of land converted to cropland concluded that it increased globally from 3-4 million km2 in 1700 to 15-18 million km2 in 1990. This mainly occurred at the expense of forests. Meanwhile the amount of grazing land area expanded from 5 million km2 to 31 million km2 during this period. Consider Grand Prairie in Illinois: in the 1830s its area was about 150 miles by 60 miles, though because it was connected to other Midwestern prairies one could walk in a southeasterly direction away from the newly founded city of Chicago and remain on unbroken prairie for over 300 miles. There is only a fraction of 1% of original Illinois prairie remaining, and a walk across grassland is the work of an afternoon. In addition to habitat transformation, the direct over-exploitation of individual species and the global mixing of biota have added their toll to species loss.
Accompanying the conversion of wild habitat to farms has been the accelerated rate of soil erosion. This has arisen, in part, from the simplification of habitat associated with agriculture. Nature abhors a monoculture, but farms are monoculture by strenuous design. Additionally, we have reached the dramatic point where about half of all nitrogen taken out of the atmosphere and transferred to the soil — a process formerly performed by lightning and soil microbes — is now industrially accomplished. Agricultural systems are increasingly dependent on fertilizers to compensate for losses due to erosion and to keep pace with productivity demands, but the result has been an intensified transfer of excess nutrients into waterways. This process of loading nutrients into water is called eutrophication. It stimulates excess plant growth; often toxic blue-green algae. When such plants die the amount of oxygen demanded by microorganisms responsible for their decay is so great that other life in these systems cannot tolerate it. Fish die.
The artificial fixation of nitrogen is energetically expensive. After all, we have to replicate the power of a lightning strike to accomplish it. The energy for the process comes from the burning of fossil fuels. The complicity of energy and food production is such that some have suggested that we are essentially eating oil. But of course, we have increased our energy demands almost immeasurably for a variety of other purposes. By one calculation per capita energy use has increased by a factor of 8 since preindustrial times — this means that each one of us is now eight times the size, energetically speaking, of a person living in the 1800s. From the early 1800s to now the global population size has increased about 7 times (from 1 to 7 billion) so the total energy demands have increase over 50 times preindustrial levels. When we flick on a switch we call to order vast processes that ripple unseen away from our fingers, processes that plunge deep into the pools of oil and gas and that rummage among endless fields of coal. This is how contemporary work gets done.
Since the dawn of the industrial age we have reunited enormous quantities of ancient plants and zooplankton with their long postponed fate of decomposition, by cremating their remains. However, that which took millions of years to accumulate is being burned in a matter of decades, and the resulting elevation of CO2 is creating havoc with the atmosphere. To deny this is to deny chemistry, physics, and biology. That the burning fossil fuels volatilizes carbon is chemistry, that carbon dioxide, a greenhouse gas, alters atmospheric temperature is physics; and that elevated temperatures modify the ecology of systems is biology. Pure and simple. To obfuscate against the conclusion that we are raising planetary temperatures is to argue with every National Academy on Earth that has pronounced on it.
More than anything else the elevation of carbon dioxide in the atmosphere is the signature of our times. Elevated CO2 is to the Anthropocene, a term geologists informally use to designate the epoch in which we live, what the mushroom cloud was to the Nuclear age. In fact the Anthropocene is primarily defined by atmospheric changes that we have wrought. It signifies that we have accomplished the unimaginably difficult in domesticating planet Earth — we have left the Quaternary period and irreversibly entered a new phase of Earth history.