I’ve become so accustomed to the fact that climate change increases both droughts and floods, that when I saw research on the precipitation patterns during “hothouse earth” eras, I immediately assumed that it was about an extreme version of that pattern. It turns out I was both forgetting what “hothouse” really means, and underestimating how strange weather patterns can get at high planetary temperatures. While I think it’s possible we could reach these temperatures again, it wouldn’t be any time soon, even in the worst-case scenarios scientists look into. At the moment, I think it’s looking like we’ll see warming of around 5-6°F (I’m using Fahrenheit because this research report does) by 2100, whereas this research was looking into conditions far beyond that.

 Today, we are experiencing the dramatic impacts that even a small increase in global temperatures can have on a planet’s climate. Now, imagine an Earth 20 to 30 degrees Fahrenheit hotter than today. Earth likely experienced these temperatures at various times in the distant past and will experience them again hundreds of millions of years from now as the sun continues to brighten.

Little is known about how the atmosphere and climate behaved during these so-called hothouse periods. In a new study, researchers from Harvard University found that during these epochs of extreme heat, Earth may have experienced cycles of dryness followed by massive rain storms hundreds of miles wide that could dump more than a foot of rain in a matter of hours.

“If you were to look at a large patch of the deep tropics today, it’s always raining somewhere,” said Jacob Seeley, a Postdoctoral Fellow in Environmental Science and Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Department of Earth and Planetary Science at Harvard and first author of the paper. “But we found that in extremely warm climates, there could be multiple days with no rain anywhere over a huge part of the ocean. Then, suddenly, a massive rainstorm would erupt over almost the entire domain, dumping a tremendous amount of rain. Then it would be quiet for a couple of days and repeat.”

“This episodic cycle of deluges is a new and completely unexpected atmospheric state” said Robin Wordsworth, the Gordon McKay Professor of Environmental Science and Engineering at SEAS and senior author of the study.

There’s always the caution that these results are from a climate model, but the reality is that these models were good enough to predict the cooling effect of the 1991 Mt. Pinatubo eruption, and as you are no doubt aware, computers and our ability to use them have both improved somewhat in the last 30 years. Climate models these days can do a pretty good job of mimicking realty. So back to the original article – they wanted to see how the atmosphere and water cycle would respond to a 64x increase in atmospheric CO₂, leading to sea surface temperatures of 130°F

At those temperatures, surprising things start happening in the atmosphere. When the air near the surface becomes extremely warm, absorption of sunlight by atmospheric water vapor heats the air above the surface and forms what’s known as an “inhibition layer,” a barrier that prevents convective clouds from rising into the upper atmosphere and forming rain clouds.

Instead, all that evaporation gets stuck in the near-surface atmosphere.

At the same time, clouds form in the upper atmosphere, above the inhibition layer, as heat is lost to space. The rain produced in those upper-level clouds evaporates before reaching the surface, returning all that water to the system.

“It’s like charging a massive battery,” said Seeley. “You have a ton of cooling high in the atmosphere and a ton of evaporation and heating near the surface, separated by this barrier. If something can break through that barrier and allow the surface heat and humidity to break into the cool upper atmosphere, it’s going to cause an enormous rainstorm.”

That’s exactly what happens. After several days, the evaporative cooling from the upper atmosphere’s rainstorms erodes the barrier, triggering an hours-long deluge. In one simulation, the researchers observed more rainfall in a six-hour period than some tropical cyclones drop in the U.S. across several days.

After the storm, the clouds dissipate, and precipitation stops for several days as the atmospheric battery recharges and the cycle continues.

The researchers are clear that the temperature increase they looked at far exceeds anything scientists are now predicting, but it’s fascinating to think about what life would be like – assuming humans could survive anywhere on such a planet – a cloudburst cycle like that. If we do enough in our lifetimes, we should be able to prevent those temperatures from occurring within the next billion years or so (yes, I’m ridiculously optimistic about humanity’s capacity to survive), but it’s sobering to think how radically different this familiar planet can become.

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