The improved lives we enjoy individually are in direct proportion to how much carbon-based fuel we burn. Carbon dioxide (CO2) occurs naturally in the atmosphere, and since the end of the ice age it has hovered around 280 parts per million (ppm). But while it occurs naturally, additional CO2 is a byproduct of the burning of fossil fuels like coal, oil, and gasoline. It is also released when forests are cleared and burned, and even when soil is plowed.

In the middle of the nineteenth century, these levels began to rise, reaching 315 ppm in 1953, the year that I was born. Last year the readings hit 390 ppm, and this spring the monthly averages have been 393 and 394 ppm. Similarly, levels of the more potent gas methane have more than doubled, from a background in the eighteenth century of 700 parts per billion to over 1,750 parts per billion today.

Higher levels of CO2 and methane in the atmosphere can cause global warming by trapping heat and solar energy, in what is known as the greenhouse effect. [See Figure 2 and Figure 3]

The link between human activity and the levels of these gases in the atmosphere is readily apparent. The rising levels of greenhouse gases in the atmosphere precisely track with the growth of population and gross domestic product over the past century. They also track with the estimates of a rise in industrial emissions from the world economy, which in frightening detail echoes the pulse of expansion, war, depression, expansion, and recession over the recent past. [See Figure 2]

Despite a drum-beat of denial from powerful vested interests, climate change is real and it is upon us. Scientists and policy-makers now debate whether it would be safe to allow atmospheric CO2 to rise to roughly 450 ppm, a level not seen since before the Oligocene (34 million to 23 million years ago), just as the Antarctic ice sheet began to form. We read daily in the newspaper about the effects of these greenhouse gases: the earth is warming up and changing its systemic behavior.

The oceans, which absorb CO2 naturally, have reached their limits and are already acidifying, threatening marine food chains already damaged from industrial fishing. Sea levels are rising, putting coastal populations around the world into serious peril from storm surges, and these storms may become more frequent. [See Figure 3]

We are bombarded with news of strange weather patterns: persistent droughts in the American south, flooding in Pakistan, Thailand, the Philippines, and China. These patterns are the signature of the "La Niña" climatic pattern that is suspected in the intensification of Atlantic hurricanes, Pacific cyclones, and Midwestern tornadoes.

And the scientists' greatest fear seems to be unfolding: having predicted that Arctic sea-ice would melt on a gradual curve down to the end of the century, they are finding that it has been melting since the 1970s much faster than their models predicted. Here the concern is the CO2 and methane frozen in the seafloor and in permafrost under the tundra; already bubbling to the surface, these gases double the greenhouse pressures in the atmosphere in short order.

The actual hard dollar costs of inaction or business as usual are now becoming apparent. Destructive climatic events (compounded by the inevitable tectonic events) already require massive expenditures for cleanup and reconstruction, and these will only get worse as sea levels rise and storms intensify.

Warming temperatures are moving ecological zones northward, and with this shift, tropical diseases will intensify and advance into temperate regions. The breakdown of oceanic food chains is already undermining important food resources, and the shifting and strengthening patterns of drought will seriously affect grain-growing regions around the world. Drought and the melt-off of tropical mountain glaciers are threatening the supply of fresh water to huge populations in several regions around the globe.

If some interest groups deny the evidence for climate change, other major institutions do not: the escalating cost of managing crises is a dire threat to the insurance industry, which has had to rethink its business models based on now-unstable predictions of natural disasters.

Meanwhile, the military—charged with maintaining a stable environment for American and indeed global economies—is actively planning for a wave of resource wars, water wars, and subsistence wars in the coming decades.

The Malthusian Model of Environmental History

The Anthropocene constitutes a great human-made rupture with the past—and this rupture has caused historians and scientists to rethink the relationships between nature, climate, and human communities.

Since the 1960s, a new discipline of environmental history began to address big questions of the changing configurations of nature, population, and economies. From the beginning, environmental historians engaged with the formative thinking of a famous English minister, the Reverend Thomas Malthus.

Looking around him in the 1790s, Malthus was alarmed by a rapidly growing English population, and wrote his Essay on Population to sound the warning. Fueled by the natural attraction between the sexes, population could grow geometrically, while the products of the farm could only grow arithmetically, if that.

So Malthus, the good moralist, issued his warning: unless the "preventive checks" of social virtue were applied, fast, the "positive checks" of famine, pestilence, and war would cut human numbers down to size.

It turns out, I would argue, that Malthus was wrong about his past, though very right to be concerned about his present and future.

According to the Malthusian paradigm, ancient and medieval societies lived on the razor's edge of crisis, and were frequently driven into crisis by overpopulation. With the Industrial Revolution, so the story goes, we escaped Malthus's grim reaper through technological advance.

Starting in the 1960s, environmental historians told a different story. Alarmed by the degradation of the natural environment, they began to sound the Malthusian alarm.

Their basic concern is manifested in new calculations of human pressure on the earth's carrying capacity—HANPP, or "human appropriation of net primary productivity": at present humans consume roughly a quarter of the total annual biological activity on the entire globe. These calculations are the essence of the Malthusian paradigm. Human numbers might well overwhelm the biological capacity of the earth to sustain them.

If the first generation of environmental historians attacked the common understanding that humanity had escaped Malthus, they also perpetuated the common understanding that pre-modern societies stood on the razor's edge of Malthusian crisis.

In their telling, the environmental history of humanity was a long series of inevitable crises of population overwhelming local resources. The standard list includes the degradation of soil in Mesopotamia, collapse of Mayan civilization and Easter Island, the Black Death in the Middle age, and the crises that struck across Eurasia in the seventeenth century. These events were moral examples of, in Jared Diamond's words, "societies that chose to fail" by not altering their impact on the natural environment.

I have been teaching environmental history since 1994. For almost a decade I followed the Malthusian orthodoxy of the founding generation of environmental historians—the human past was filled with Malthusian crises—in which the geometric rise of population overran the ability of limited technology to feed everyone.

Starting in the late 1990s the evidence for abrupt climate change—in climate history, archaeology, and economic history—began to complicate this story. What if these ancient societies were not doing too badly, but the playing field and goalposts changed fundamentally, on scales that they simply could not anticipate?

The Reverend Malthus was right about the world in which he lived: populations in late-eighteenth-century England were indeed growing at an unprecedented rate, as were populations in much of Europe. But China, England, and the United States led the way, England and China with annual rates of 1.3% per year—dwarfed by the surging population advance in eighteenth and nineteenth century America—but well above the healthy global average of approximately 0.6%. Nothing like this had happened before, and Malthus had a reason to be nervous about the press of population.

England escaped the Malthusian calculus through industrialization and imperial reach, and the United States escaped through the conquest of "free" land to the west. China did not escape, suffering a devastating crisis in the middle of the nineteenth century when the Taiping Civil War of 1850-1864 killed 20 million people, perhaps as many as 70 million.

The new climate science—and the insights of a generation of archaeologists and economic historians—suggests that this crisis in China was the first true civilizational Malthusian crisis humanity ever suffered. In fact, it was literally the first in human history that was not, in great measure, the result of abrupt climate change.

Human Prospects on a Volatile Earth

With the new results of climate science, historians know quite a lot about how climates have changed in the past, and the result is a renewed appreciation of the global earth system as an autonomous, exogenous actor in human history. The ways in which naturally induced climate change radically transformed human existence in past millennia is a wake-up call to the prospects of similar convulsions as a result of the contemporary, human-induced climate transformations.

At the same time, the long perspective of climate history allows us to reconsider the Malthusian calculus, and it highlights the burgeoning, unprecedented crisis of population we now face.

Ancient populations suffered poor individual life outcomes, with poor health and low life expectancy; conversely, they imposed relatively low environmental impact and enjoyed long-term societal sustainability. Whatever their flaws, ancient societies should not be condemned for failings of environmental consciousness.

Modern populations, by contrast, enjoy excellent individual outcomes, with amazingly good health and high life expectancy (even if varying geographically), but are causing systemic changes to the entire global ecology. Whether these changes are sustainable is very much an open question.

In the life spans of the past five to six generations, the double-edged sword of science and industry has carried humanity through a great and paradoxical transition to prosperity and peril. After two decades of sounding the warning, most in the scientific community are exhausted and depressed.

Throughout the world people attuned to the perils upon us are sobered by how much it will cost and how long it will take to construct a new energy system to fill the place that fossil fuels now occupy in the delivery of essential services to massive new populations around the world. Close to home, despite the fact that Americans per capita consume roughly twice the hydrocarbons of anyone else in the world, there is no sign that we are willing to voluntarily restrain ourselves.

The fertility transition that has begun to slow the rate of global population growth is a hopeful sign. Certainly much of the reduction of fertility has come by government fiat in China, but it is notable how in many countries—India, Iran, Bangladesh stand out as examples—the classic transition of educating women and raising aspirations has reduced the pressure of new births to the point that experts see population growth rates stabilizing and reversing in the coming decades.

On the climate front, the Montreal protocols to control ozone-depleting CFC emissions have been a great success since they went into effect in 1989, and this experience stands as a model for the control of carbon dioxide and other greenhouse gases.

Attention is now being focused on strictly controlling methane and black soot—the most extreme and volatile elements of the greenhouse gases emitted by the global economies. Getting these under control might make significant differences, buying time for longer-range shifts to take effect.

Whatever the future holds, it is coming at us with breakneck speed. And just as a long historical horizon allows us to look back at the circumstances of previous generations, it demands that we think forward to the fortunes of generations to come.

Check out a lesson plan based on this article: World Population