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.