Ancient Earth Hit by Rain Storms 100s of Miles Wide Due to Extreme Heat

New research suggests that during times of extreme heat in Earth's ancient history, so-called "hothouse" epochs, the planet was alternately scorched by periods of intense dryness, then battered by massive rainstorms. The ancient storms could span hundreds of miles and dump more than a foot of rain in just hours.

The cycles described by the researchers in a paper published in Nature occurred billions of years ago, during periods of the planet's history when temperatures were between 11 and 17C hotter than today.

Earth is expected to return to these temperatures and similar conditions millions of years from now, as the Sun's brightness and energy output increase.

Aside from helping us to predict what the Earth of the future may look like, the research could also help us contextualize the weather impacts of global warming as driven by human activities, that we are currently experiencing.

Additionally, beyond Earth and its solar system, the findings could help us understand the environments of planets found orbiting distant stars.

Jacob Seeley, an environmental scientist from Harvard, was the first author of the paper. "If you were to look at a large patch of the deep tropics today, it's always raining somewhere," he said in a statement. "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."

Earth's recent history, the previous few million years, is characterized by cool glacial periods and warmer glacial-free, or interglacial, periods. But, further back in our planet's 4.5 billion year existence, its climate varied wildly.

Examples of this are the Hadean, and Archaean periods, ranging from 4.6 billion to 4 billion years ago and 4 billion to 2.5 billion years ago, respectively. Until now, very little was known about the hothouse epochs of Earth's history.

To crack the mystery surrounding these conditions, Seeley and his co-author Robin Wordsworth, devised an atmospheric model that saw the temperature of the sea increase to a scorching 55C.

This was achieved by either increasing the carbon dioxide in the atmosphere to over sixty times its current level, or by increasing the brightness of the sun by around 10 percent.

Doing this, the duo discovered, had a surprising effect on the planet's atmosphere. They found the air at the surface of Earth's oceans became extremely warm when atmospheric water vapor absorbs sunlight and heats it.

This forms what is known as an "inhibition layer," which acts as a barrier preventing evaporated water from rising to the upper atmosphere and becoming rain clouds. Instead, this water is held in the lower atmosphere.

As this is happening, heat is being lost to space from the upper atmosphere and clouds are forming above. Rain from these clouds evaporates before it can fall back to the surface, with the effect of immediately placing water back in 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."

Seeley said that if something can break through that barrier, it releases surface heat and humidity into the cool upper atmosphere, causing an enormous rainstorm. After several days, this causes a deluge that can last hours.

In one simulation, the team found that during a rainstorm that lasted just six hours, more water was deposited than during some tropical storms in the United States that rage for several days. Following the storm, the savage rain bombardment ceases for several days as the system "recharges" once again.

"This study has revealed rich new physics in a climate that is only a little bit different from present-day Earth from a planetary perspective," said Wordsworth. "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."

Seeley said that though the increase in sea surface temperature used in the models were more extreme than the changes being driven by climate change, that doesn't mean this research can't help shed light on Earth's current warming period.

"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," he said. "Our research goes to show that there are still a lot of surprises in the climate system."