Here comes the rain again. The skies darken and the winds howl; storms of a kind that should show up only once every half-millennium now arrive every 10 years. The world is warming up. The hotter the oceans get, the faster they evaporate, and the hotter the air gets, the more moisture it holds, with one devastating result: When the clouds let go of their watery load, it now pounds the earth for days at a time, washing away hillsides and flooding valleys and plains. Even when it isn't raining — the terrible storms alternate with equally terrible droughts — the glaciers go on melting, relentlessly raising sea levels and pushing waves farther and farther inland.
This isn't a bad description of the events of the past month. Hurricane Harvey has killed at least 50 people in Texas and one in Guyana, a death toll that will likely rise; torrential downpours have set off massive mudslides in Sierra Leone, killing more than 1,000; and violent monsoons have flooded Bangladesh, India and Nepal, killing nearly 1,300. But these tragedies aren't what I intended to describe when I originally wrote the paragraph above. Instead, it was an account of what happened 15,000 years ago, when the world was warming up at the end of the last ice age. In many ways, the wild swings in weather, ferocious storms and coastal inundation were not so different from what the 21st century looks likely to bring, and we can learn a lot — much of it alarming, but some of it reassuring — from humanity's previous encounters with the floodwaters. After all, we have been through this before.
'The Long Summer' Comes to an End
Twenty thousand years ago, much of the world was more than 10 degrees Celsius colder than it is now (although, climate being the complicated thing it is, a few places were actually warmer than they are in our own age). Sea levels were 125 meters (410 feet) lower than today's, leaving vast stretches of dry land where there is now ocean. And, with trillions of tons of water locked up in ice sheets as much as 1.5 kilometers (0.9 miles) thick, the intensely arid atmosphere carried 20 to 25 times as much dust as today. Far fewer plants could grow in that climate, meaning far fewer animals could exist either. In all likelihood, there were barely 500,000 humans in the world — just half as many as currently live in Austin, Texas.
The main culprit was Earth's orbit, which, over a roughly 200,000-year cycle, shifts from being roughly circular (as it is now) to being noticeably more elliptical, taking our planet closer to, then farther away from, the sun. Combined with a separate 22,000-year rhythm of tilts in the axis on which the Earth rotates plus a 41,000-year cycle of wobbles around the axis, this pattern makes the Earth sometimes into an icebox and other times into a hothouse. Until 3 million years ago, the world was so warm that there was no year-round ice cap at the North Pole, but by 18,000 B.C. — the coldest point of the last ice age — ice permanently blanketed most of Northern Europe and North America. After that, a long-term warming trend began, punctuated by shorter periods when the climate went haywire, producing sudden and extreme warming or cooling temperatures.
Around 12,700 B.C., temperatures apparently jumped by 3 degrees Celsius in a single generation. Massive glacial runoff raised the sea level by 10 meters over the next century or two, and humidity leaped upward. Around 10,800 B.C., though, another episode of rapid warming had the opposite effect: Ice ridges in North America melted and collapsed, allowing what geologists call Lake Agassiz — a body of frigid water four times the size of the modern Lake Superior — to drain into the North Atlantic. So much icy water was involved that it made the Atlantic Ocean too cold for the Gulf Stream (which carries warm tropical waters from the Caribbean to the coast of Norway) to work, plunging the Northern Hemisphere back into a mini-ice age that lasted another 1,200 years. When the effects of this period wore off around 9600 B.C., the world once again began warming, and by 5700 B.C., temperatures may have been higher in some places than they are today. Archaeologists often call the period between 9500 B.C. and A.D. 1950 "the long summer" because of its warmth and relative stability.
To be sure, the changes of the past half-century differ in important ways from what happened 15,000 years ago. Unlike the end of the ice age, the end of the long summer owes more to humans burning fossil fuels than to tilts and wobbles in the Earth's orbit.
This label was popularized before it became obvious that we are now entering a new age of warming and flooding, and that the long era of relative climatic stability following the ice age might now be ending. To be sure, the changes of the past half-century differ in important ways from what happened 15,000 years ago. Unlike the end of the ice age, the end of the long summer owes more to humans burning fossil fuels than to tilts and wobbles in the Earth's orbit. Global warming is also starting from a higher base temperature than it did 15,000 years ago, and it is acting on a world of megacities and advanced technologies rather than one of Stone Age hunter-gatherers.
For all the differences, though, looking at the past is the only way to know how humans respond to warming and flooding. And the evidence, drawn not only from the end of the ice age but also from milder episodes known as the Roman and Medieval warm periods, teaches us four lessons.
The Good, the Bad and the Unfair
The first is that much of the current thinking about climate change is simplistic, because it ignores history. Previous episodes of global warming certainly produced mass mortality and disruptions that, measured against the scale of the societies of the day, were even more terrible than the events we are now seeing. Although this will be cold comfort to those enduring disaster in Houston or Freetown, global warming perhaps has even been a net positive for humanity in the long run, and it certainly has been better than global cooling.
There are many ways to measure the impact of climate on human welfare, but I will take just the crudest: the number of people alive. The global population has been increasing at an accelerating rate for most of the past 100,000 years. But, although the relationship is messy and complicated by multiple factors besides heat and humidity, growth generally has been fastest in times of warming. During the ice age (roughly 100,000-10,000 B.C.), the world's population probably doubled once per 10,000 years, but during the great warming period from 10,000 to 3000 B.C., the time required for doubling fell to roughly 1,600 years. Between 3000 and 500 B.C., when temperatures overall remained rather steady, the doubling time continued to fall to 1,000 years; but during what climatologists call the Roman Warm Period (about 200 B.C.-A.D. 300) it fell much more dramatically, to just 500 years. In the subsequent Dark Age Cold Period (roughly 300-950), global cooling set in and the world's population actually shrank slightly; then, in the succeeding Medieval Warm Period (950-1300), it once again exploded, with the doubling time falling to just 250 years. The Little Ice Age (1300-1800) again saw cooling, but while population growth did continue, the doubling time slowed to 500 years. It then fell sharply to just 100 years in the period of relatively stable climate between 1800 and 1950, but since 1950, as the world has rapidly warmed, the doubling time has fallen even more spectacularly to 40 years. By 2050, the United Nations predicts, population growth will once again start slowing down, though there is every reason to expect temperatures to keep rising.
There is a lot of noise in this story, and other proxies for well-being such as living standards or the stock of knowledge tell different — though not wildly different — tales. If there really is a positive long-term causal connection between global warming and human well-being, it is clearly complicated by many other factors. But it is also clear that previous episodes of global warming have not automatically been long-term disasters for humanity.
The moment we start burrowing into the data, we learn a second lesson: Part of the reason for the messiness of the patterns is that global warming is unfair. It always affects different places in different ways, producing both winners and losers. Climate change essentially changes the meaning of geography, turning places that in cooler times were good to live in into ones that are bad or even uninhabitable. Sunbelts, for instance, become flood zones or disappear underwater altogether.
In the last few millenniums of the ice age and the first few millenniums following it, some of the richest hunting grounds in the world were broad, low-lying plains that now lie under the Persian Gulf and the South China, Black and North seas. Geologists have mapped the prehistoric North Sea Plain in some detail (they call it Doggerland, after the Dogger Bank shallows that were once an imposing range of hills), and trawlers have recovered hundreds of artifacts from what was, around 8000 B.C., one of the most densely settled parts of Europe. By 6000 B.C., though, huge storms and tsunamis and an underlying rise in sea levels had conspired to drown these prehistoric Gardens of Eden. When the land bridges connecting what are now the European and Asian parts of Turkey were finally breached and the Mediterranean Sea rushed into the Black Sea Plain (much of which was, by then, a huge freshwater lake), the waters may have risen by as much as 15 centimeters per day. Each time the sun came up, the waters had moved more than a kilometer inland. No wonder some geologists think this episode inspired the stories of Noah's flood.
But while global warming was dealing a deathblow to the world's richest hunting grounds, it was also doling out benefits to other places, most notably the borderlands of modern Iran, Iraq, Turkey, Syria and Israel, plus Pakistan, China, Peru and Mexico. Here, warmer and wetter weather made it possible for humans to domesticate the wild precursors of today's wheat, barley, rice, corn, quinoa, cattle, sheep, goats, pigs and llamas, in the process turning themselves into farmers. And in all these places, populations boomed, and people began to find more sophisticated ways to organize themselves. Over the next few thousand years, farmers expanded outward from their original homelands, killing, enslaving or absorbing nearly all the foragers in the world and driving the few miserable survivors into jungles, deserts and tundras that were either too hot or too cold for the farmers themselves to want.
This depressing story points toward a third lesson. Global warming is not just unfair; it also turns the geostrategic order on its head. While 21st-century storms and floods will be less devastating than those that accompanied the end of the ice age, geologists assure us that the sea is likely to swallow up much of the coastal plains of India, Bangladesh, Northern Europe, eastern China and both coasts of North America. Just like Doggerland and Sundaland (now under the South China Sea) 10,000 years ago, these are some of the most densely populated and richest parts of the world. By contrast, the areas likely to benefit most from global warming — the frozen wastes of Alaska, Canada, northern Scandinavia and Russia — are virtually uninhabited. Billions of people will thus migrate from south to north in a process that will produce new superpowers and unprecedented geostrategic instability. More alarming still, the regions where climate change is already having the greatest impact contain a disproportionate number of the world's poorest societies, unstable governments and nuclear proliferators. If the worst-case scenarios come to pass, the 21st century might definitively disprove any notion that global warming has been a net positive for humanity after all.
That said, the fourth lesson of the history of global warming is more upbeat. Humanity is astonishingly resilient: For all the traumas it inflicts, global warming has not wiped us out. (Global cooling has actually come much closer; by some calculations, things got so bad around 100,000 B.C. that there were only 20,000 humans left alive, all clustered close to the equator in Africa.) We have always risen to the challenge, by migrating, by figuring out ways to buffer the impact of climate change, or even by finding unexpected benefits in it. These responses regularly required people to change how they lived, often in ways they must have found unappealing. But they did it anyway, because the alternative was extinction.
The Wrong Side of History
Of course, there are no guarantees that we will repeat this feat in the coming century. On the upside, the past decade has already seen astonishing shifts toward new, cleaner energy sources, new ways of producing food, new ways to protect coastal cities and new forms of cooperation among governments. We have even learned to mitigate the worst floods. The cyclone that hit Bangladesh in 1970 killed 300,000-400,000 people, but the comparable disaster in 2007 killed only 4,234. In much the same way, Hurricane Katrina killed 30 times as many Americans in 2005 as Hurricane Harvey has so far this year.
On the downside, the past decade has also brought mounting resistance to change in some quarters. In the United States, the current administration has withdrawn from the Paris climate accord, disbanded its advisory panel on climate change by failing to renew appointments, and defunded the National Flood Insurance Program as part of its effort to pay for a wall along the U.S.-Mexico border. If global warming carries on as it has in the past, hanging onto our old ways while the floodwaters rise around us seems a surefire way to end up on the wrong side of the next geostrategic order.