Humans have begun an international project to move water around the world, far more ambitious than any network of aqueducts or hydroelectric dams ever constructed or conceived. The drivers of this global system are billowing vapors, which trap heat and propel the world’s water faster and farther around the globe. The first results of this project may already be seen in the outrageous rainfall totals of storms like Hurricane Harvey, or in landslides on remote mountain hillsides, and even in the changing saltiness of the oceans.
The Earth system is getting warmer. Water is evaporating faster. There’s more of it in the air. It’s moving through the system faster. As a result, the coming centuries will play out under a new atmospheric regime, one with more extreme rain, falling in patterns unfamiliar to those around which civilization has grown.
“Basically the idea is that as the climate warms there’s more energy in the atmosphere,” says Gabriel Bowen, a geochemist at the University of Utah. “That drives a more vigorous water cycle: Evaporation rates go up, precipitation rates go up—there’s just more water moving through that cycle faster and more intensely.”
For each degree Celsius of warming the atmosphere is able to hold 6 percent more water. For a planet that’s expected to warm by 4 degrees by the end of the century, that means a transition to a profoundly different climate.
“Rainfall extremes have increased in intensity I think at every latitude in the northern hemisphere,” says Massachusetts Institute of Technology climate scientist Paul O’Gorman.
In 2012, a study led by Lawrence Livermore National Laboratory oceanographer Paul Durack found that the global water cycle was actually speeding up at twice the rate predicted by climate models, likely intensifying by 16 to 24 percent by the end of the century.
Amazingly this same signal of a turbocharged water cycle has been pulled out of fossils from deep in Earth’s history during the alien greenhouse climates of the ancient past. These were much different worlds, with antarctic forests and warmth-loving reptiles at the poles—balmy interludes driven by naturally higher carbon dioxide in the atmosphere. Researchers, using proxies as inventive as fossilized sea-cow teeth, have deciphered the chemistry of these ancient oceans, finding in them seas similarly swinging between intensely fresh and salty regions, the signature of the more intense water cycle of a greenhouse planet. We’re a long way from the ancient greenhouse of 50 million years ago, but Durack’s work implies that in the coming decades, in places where evaporation on the planet is strongest, there may be ever wilder extremes in drought and aridity. And where that water eventually falls out of the sky, episodes of unprecedented precipitation.
Hurricane Harvey was a truly one-off storm, one whose unusual path—strangely stalling out over East Texas for days—can explain much of its destructiveness. But imagine if, in its sojourn over Houston, Harvey had dumped out 25 percent more rain, as O’Gorman and colleagues have forecast is possible by the end of the century. In the tropics, rainstorms may well become even wilder.
But perhaps even more important than how fast water is moving through the system, is where it may move thanks to human intervention. The Intertropical Convergence Zone is a band of precipitation that flirts with the equator—where the trade winds meet in the tropics and hot air rises, dumping out prodigious amounts of rain. The ITCZ feeds monsoons and, throughout history, entire civilizations. Its position on the planet has meant life or, in the case of Classic Mayan civilization, perhaps death. Their collapse is linked to a subtly shifting ITCZ and the droughts that followed. And the movement of the ITCZ under a warming climate, and the waters it carries, is one of the most urgent and unresolved questions in climate modeling today, with some forecasts calling for dramatic shifts and expansions.
Eleven thousand years ago in China strong monsoon rains filled a gigantic Lake Dali and northern China greened, as documented in a recent study led by Columbia university researcher Yonaton Goldsmith. Six thousand years later the monsoon shifted hundreds of miles to the south, the rains halved, and the lake shriveled up. Ancient Chinese societies, the Hongshan and Yangshao, scattered and collapsed, while politically complex cultures in central China blossomed. The response of this monsoon to warming, one that holds the fate of Southeast Asian agriculture in the balance, is a similarly unresolved question in climate modeling. Some models show the monsoon strengthening, others show it migrating far from its familiar haunts.
Around the same time as northern China was blossoming in the early Holocene, so too was Earth’s most desolate desert. The African Humid Period brought rains to the Sahara, perhaps the result of more sunlight in the northern hemisphere as the Earth carried on its celestial wobble. Today, by warming the northern hemisphere faster than the southern hemisphere, humans may well again bring more water to this, the world’s largest desert, greening its wastes once more. If so, and perhaps quite unexpectedly, the hurricanes that hit our shores a hemisphere away could become more frequent and intense. A verdant Sahara, by reducing the amount of dust wafting out over the ocean, will allow the sun to beat down on the Atlantic more intensely, forging more powerful cyclones. The idea that shifting rains might turn deserts in Africa to green, spurring more intense hurricanes that will eventually hit North America, illuminates the Rube Goldberg connections of the climate system, and proves there may be more than a few surprises in store as the world changes.
But that doesn’t mean that more water vapor in the air will necessarily beget a uniformly wetter, more stormy world. Strangely, while the atmosphere is growing more suffused with water vapor, it may also be getting more arid on land in much of the world. In recent years the relative humidity over land has been falling, a trend that may increase in the future, even as the atmosphere sucks up more water vapor. This isn’t as paradoxical as it sounds. Though the planet’s air as a whole is getting warmer, and warmer air can hold more water, land is warming up much faster than the ocean, and the hotter air above the land isn’t evaporating as much water as the air over the oceans. As a result, even as the atmosphere sucks up water and absolute humidity on land rises, bringing intolerable conditions to humanity in some parts of the world, relative humidity—how much water air can hold at a given temperature—isn’t keeping up over land, the air doesn’t saturate, and it doesn’t rain. Even stranger, this fall in relative humidity may be mediated by plants, which have already changed their physiology in response to our newly high-CO2 world.
It’s well-known that in rain forests plants and trees actually create their own weather, by opening their pores to take in CO2 and breathe out oxygen and, in doing so, losing vast amounts of water vapor to the air above. That water then rains out of the sky in a perpetual hydraulic dance between life and the air around it. In fossil leaves from the greenhouse climates of the geological past, and in modern laboratories, plants have been shown to reduce the number of pores on their leaves in response to high atmospheric CO2, sipping from the surfeit of carbon dioxide around them, and becoming stingier with the amount of water they give up to the atmosphere. In fact, scientists at Indiana University Bloomington and Utrecht University found that in the past 150 years plants have reduced the number of pores they use to breathe by 34 percent. This adaptive change vastly limits the amount of water plants release to the air around them, perhaps contributing to the mysterious fall in relative humidity, and the potential for future aridification.
All of this would seem to indicate that we’re headed toward a largely more arid world but also one with far more water vapor in the air for the occasional devastating storm to sweep up, bringing unprecedented bursts of floodwaters. And in parts of the tropics perhaps a coming deluge unlike any witnessed by humanity.
Committing to profound climate change outside the window of the past few million years, as it seems we’re determined as a species to do, means embracing not just a world like ours only more so, but a leap into the unknown.
Still, on a world as ancient as ours, there are precedents for even the most extreme changes if one looks far enough back into the past. 56 million years ago, at the boundary between the Paleocene period and the Eocene period, in an already very balmy world, volcanoes released amounts of carbon dioxide equivalent to all the world’s fossil-fuel reserves in a matter of a few thousand years. As a result the water cycle jackknifed, a signal starkly visible in the rocks.
“In northern Spain it looks like we’ve got your normal summer dominated by Harvey-type events,” says Utah’s Gabriel Bowen. In the Pyrenees, rocks testify to a placid coastal plain suddenly overtaken by dramatic flood deposits carrying boulders and gravel called “megafans.”
“We go from a system where you have sediment building up slowly over time to these megafan deposits which we only see in places that receive really extreme seasonal precipitation in the modern world.”
Bowen has found the similar signature of raging floods in Big Bend National Park from the same global-warming event. But elsewhere, like in Bighorn Basin in Wyoming, the same interval produces a more arid landscape sparse with vegetation. The starting states in the Eocene and in our modern chemistry experiment with the atmosphere were wildly different, a disparity that would seemingly limit the ancient event’s usefulness as a guide to the coming decades. But Bowen says there are still lessons we can take from it.
“I think you need to look at the delta, or the type of change that we see, as we go into these [past warming events] because we know that we’re forcing the system by adding carbon to it, and we can see just what the pattern of change is over that base condition,” he says. “That’s not to say that we expect that starting from where we are today is going to be identical, but it gives us really our only experimentally based look at how the system responds to that kind of push.”
For the past few thousand years we have lived on a planet unreasonably cozy for civilization. We return to these ancient worlds only foolishly.
“The worst-case scenario is that we see Harveys happen not once in a lifetime but routinely every summer in multiple places, and it’s exacerbated by the fact that sea level is rising rapidly,” says Bowen. “And then we see many of the agricultural areas of the world—sunny places with marginal water availability—become dust bowls.”
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