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.