In the past few months, extreme weather patterns and a staggering landmark in human population growth combined to give new urgency to the existential question of human prospects on this volatile planet.

The "winter that never was" in North America, as coined by a Canadian magazine, and the unusual cold and snow in Europe coincided with an October 2011 announcement that the global population had reached 7 billion. Taken together, the events raise deep concerns over long-term patterns in the relationship between human population and the earth's climate.

Humanity has been around for a very long time, but in only a few centuries we have grown to vast numbers and transformed our world in unprecedented ways. The very long view of human history—revised in the recent research of climate scientists and environmental historians—uncovers an interesting paradox and offers some sobering conclusions as we chart a course into the future.

Our ancestors lived short, difficult lives, hemmed in by environmental constraints. Whole societies were frequently overturned by sudden, unpredictable, and naturally occurring climatic shifts. In their efforts to survive, our predecessors did little to threaten the earth system that supported them.

We, by contrast, have arrived at the opposite situation: our individual lives are far more healthy and stable, but we have begun to seriously degrade the earth system that supports us. Until recently, we thought we had transcended the environmental constraints that so bounded our forebears. But efforts to break free from the bonds of nature have tended to transform the earth's climate in unpredictable ways.

Across most of human history, overpopulation didn't pose a problem to the survival of the species. Rather, humans stood under the recurring threat of natural disasters, climatic change, drought and famine, and epidemic disease on a scale that we cannot imagine. Until about 300 years ago, every large-scale reversal of human fortunes was driven by such natural forces.

The contemporary world, forged in the revolutionary changes of industry and science that began in the eighteenth century, is fundamentally different. Our vast numbers—7 billion and climbing—have begun to interact with the natural workings of the earth system in complex, unprecedented ways and in a markedly short timeframe.

Geologists are currently debating whether the last 200 to 300 years should be given a new label: the "Anthropocene," the contemporary geological period during which human action significantly reshapes global ecosystems.

We now face the dual danger of an unsustainably large global population that has set in motion a series of changes to climate that—like the many naturally occurring shifts in climate over human history—threaten our civilizations and our existence. Environmental priorities have become far more desperate imperatives in the past 20 years, as we realize that the problem facing humanity is maintaining the essential platform for human life. Life as we know it is changing fast before our eyes.

Deep History: Climate and Human Life before the Great Transition

Let us take a quick look at this sweep of human history and the climate-human relationship on this ever changing planet. Our deep origins lie in the evolutionary history of advanced primates 5 to 10 million years ago. Modern humanity—the species Homo sapiens sapiens—was born in the stresses of the glacial cycles of the Pleistocene epoch (which ran from approximately 2.6 million to 12,000 years ago).

Truly modern humans began to emerge around 250,000 years ago, as evidenced by significant shifts in stone tool technology and the "modernization" of fossil skeletons. Genetics suggests that the earliest modern humans at first comprised a small breeding population, perhaps in the thousands. But they began to grow in numbers and colonize the earth, a process of increase and migration that did not end until Polynesians arrived in Hawaii less than 2,000 years ago.

Over the past three decades, as scientists have sought to establish a baseline from which to measure human-induced climate change, our knowledge about the climate inhabited by our ancestors has advanced dramatically in scope and precision.

Most importantly, scientists and historians have come to realize that, from the great warm-up of the early Holocene (approximately 12,000 years ago) until the eighteenth century virtually every significant transition in the human condition was in some measure shaped by shifting climatic conditions, at times interwoven with the onslaught of disease and epidemics.

Evidence indicates that that there has been a broad coherence in global climatic patterns during the 12,000 years of the Holocene. A warm North Hemisphere has been associated with La Niña conditions in the Pacific and the Americas, and strong monsoons bringing summer precipitation to Asia and much of Africa. Conversely, a cold North Hemisphere has been associated with El Niño conditions bringing storms and flooding to the west coasts of the Americas, and weakening the Afro-Asiatic monsoons.

However, the current warm Holocene period, which is actually an interglacial era, has been punctuated with sharp warmups and coolings that are caused by two distinct climate cycles. The first are "Bond events," discovered in the early 1990s. Marked by great bursts of ice-rafting in the North Atlantic, Bond event sequences appear approximately every 1,470 years and are associated with broadly cooler global climates.

More importantly, the planet also experiences cooling cycles as a result of 2,300-year cycles of grand solar minima, called the Hallstatt cycle (when the strength of the sun on the planet is at a minimum). Three times during the Holocene, this cycle brought centuries of cold, almost glacial climates, most recently in the Little Ice Age of approximately 1300 to 1700 A.D. [See Figure 1]

These climate events molded the fate of the human societies that lived in them.

A major cold event in the 6000s B.C., peaking at 6200 B.C., was a post-glacial episode known as a "meltwater" crisis. It drove the collapse of early agricultural societies in the Fertile Crescent, and their subsequent intensification, as well as the beginning of domestication in the tropics.

The first major Hallstatt grand minimum, in the fourth millennium B.C., ended in intense droughts at 3200-3000 B.C. that launched the first states in Mesopotamia, Egypt, and the Indus Valley. A burst of El Niño likely launched the first city-states in coastal Peru.

A mystery drought at 2200 B.C., which registered throughout the world but the causes of which are as yet unclear, punctuated the histories of Bronze Age societies throughout the Old World, interrupting them around the Mediterranean, ending an epoch of urban civilization in the Indus and launching the first state in China.

The second major Hallstatt minimum hit around 1200 B.C., with a burst of cold combining around the eastern Mediterranean with what has been called an "earthquake storm." The result was war, famine, and epidemic disease—events that had nothing to do with the pressure of overpopulation, and everything to do with a potent change in the earth system.

This global climatic reversal brought the end of the Bronze Age in southwest Asia and Egypt, and the collapse of the Shang dynasty in China, where climate reversal would bring down dynasties regularly for the next 3,000 years.

Rainfall shifts in this epoch were involved in the establishment of the large villages and towns of "Early Formative" Mesoamerica, which would establish the basis for rise of states. A whiplash of El Niño flooding and drought is similarly seen as establishing the basis for the Early Horizon cultures and the Chavin cult in the Andes.

The ebb and flow of climate over the next 2,000 years—a warm Classical antiquity, a cold Dark Ages, a warm Medieval Regime, and a cold Hallstatt- driven Little Ice Age—interacted with war and epidemic to powerfully shape the fate of cultures and states around the world.

If the details of this history are too complex to even begin to describe here, the lessons of the new climate history seem to be plain.

First, for most of human history, major crises and ruptures in human societies came as a result of climate change, not because of too many people or human misuse of resources. Second, deep human history warns us that when climate patterns did change significantly, the result was societal collapse, war, epidemics, and fundamental restructurings of the geography and structures of human communities.

Ancient and medieval agrarian societies were threatened by under-population, not overpopulation. When populations grew during climatic optimums, they generally managed to achieve incremental improvements to agricultural productivity.

Life was not pleasant. Studies of wealth and income present a persistent pattern of hierarchy and poverty. A peasant family in the late Middle Ages, on average, had a standard of living not unlike that of a peasant family in the Bronze Age, and probably the late Neolithic. Average life expectancy at birth ranged from the low twenties to the mid-thirties at best.

But these societies were amazingly durable; they lasted for hundreds of years at a stretch, and then only "collapsed" when hammered by earth system forces. These were crises driven by external factors of the natural environment, not internal pressures of overpopulation or economic practices that degraded their surrounding environment.

The Great Transition: Into the Anthropocene

Beginning in the eighteenth century, changes in human population size and economic activity transformed the long-standing relationship between climate and people.

After the sudden warming that followed the final glaciation of the last Pleistocene Ice Age, by perhaps 9000 B.C., humanity might have numbered as many as 7 million (a guess to be sure, but based on a mountain of evidence). Since that population baseline, humanity grew slowly for thousands of years—generally at rates of less than 0.2% per year, with crises of collapsing populations counterbalanced by occasional surges.

But then, from roughly 1750 forward, the population began to grow at unprecedented annual rates: 0.46%, 0.61%, and 0.64%. Then the growth of human numbers accelerated, hitting 2 billion around 1930 and 3 billion around 1960. During the 1950s, annual growth rates climbed from 1.7% to 1.9%, and then peaked in 1962 and 1963 at a rate of 2% per year.

Since then the rate has slowed to 1.3% per year. Nonetheless, we hit an estimated 7 billion in October 2011: a total population a thousand times greater than the hunter-gather peoples who inhabited the earth after the last glaciation, a mere 12,000 years ago. [See Figure 2]

It helps to put this accelerating history in generational terms. Assuming roughly twenty-five years per generation, there have been 11,000-12,000 generations since the dawn of modern human abilities 250,000 ago, 500 since the end of the last ice ages, 200 since the founding of the first states in 3000 B.C., and perhaps 25 since 1492, the age of Columbus. But there have been only five to six generations since the launch of the modern industrial scientific revolutions around 1870.

Taking place a little more than a century ago, these revolutions brought us household electricity, the internal combustion engine, and modern medicine. These technological and scientific advances, as historian Vaclav Smil has argued, were a fundamental leap, taking humans into a new world utterly unknown to their forebears. The benefits of this revolution to the quality and length of human life have been enormous.

The team led by Roderick Floud and Robert W. Fogel has detailed this era, especially the way that life expectancy has risen dramatically. Where the average age of death around the world in 1820 was roughly 26, it is currently roughly 66, in the high seventies to low eighties for developed countries, and in the mid-fifties for the least developed countries. The increase in life expectancy—driven by our advancing control of disease mortality—is the direct cause of the rise of global population to 7 billion.

Heights and weights of mature adults have also risen dramatically from the late nineteenth century with better nutrition and medical care. An estimated one-fifth of the population of eighteenth-century France was too weak to work; conversely, today's stronger bodies and healthier lives translate into higher and higher levels of intellectual capability and more effective work lives, with progressive cumulative effects on the prospects of future generations.

Across the planet, we are all doing much better than our recent and distant ancestors (although unevenly, with people in certain regions faring better than those in others).

Human Population and the Earth System

But this great transformation has had an enormous impact on the earth system.

No longer simply worrying about a degradation of nature, scientists fear that we are pushing the limits of the envelope that stands between us and the chaos of space. In a little more than a century—in the blink of an eye in historical terms—we have begun to destabilize the workings of the biological, geological, and atmospheric system that maintains life as we know it.