The distant past of Earth and potentially its future include extremely warm ‘hothouse’ climate states, but little is known about how the atmosphere of our planet behaves in such states.
Earth may have experienced cycles of dryness followed by massive rain storms. Image credit: Simone Marchi & Dan Durda, Southwest Research Institute.
“If you were to look at a large patch of the deep tropics today, it’s always raining somewhere,” said Dr. Jacob Seeley, a postdoctoral researcher at Harvard University.
“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,” added Harvard University’s Professor Robin Wordsworth.
The research not only sheds light on Earth’s distant past and far-flung future but may also help to understand the climates of exoplanets orbiting distant stars.
In an atmospheric model, the researchers cranked up Earth’s sea surface temperature to a scalding 54 degrees Celsius (130 degrees Fahrenheit), either by adding more carbon dioxide — about 64-times the amount currently in the atmosphere — or by increasing the brightness of the Sun by about 10%.
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. You have a ton of cooling high in the atmosphere and a ton of evaporation and heating near the surface, separated by this barrier,” Dr. Seeley said.
“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.
“Our research goes to show that there are still a lot of surprises in the climate system,” Dr. Seeley said.
“Although a 30-degree increase in sea surface temperatures is way more than is being predicted for human-caused climate change, pushing atmospheric models into unfamiliar territory can reveal glimpses of what the Earth is capable of.”
“This study revealed rich new physics in a climate that is only a little bit different from present-day Earth from a planetary perspective,” Professor Wordsworth said.
“It raises big new questions about the climate evolution of Earth and other planets that we’re going to be working through for many years to come.”
The findings were published in the journal Nature.
J.T. Seeley & R.D. Wordsworth. 2021. Episodic deluges in simulated hothouse climates. Nature 599, 74-79; doi: 10.1038/s41586-021-03919-z