On ancient Earth, it never rained but it poured
Extreme heat led to episodic deluges on hothouse Earth

Date:
November 3, 2021
Source:
Harvard John A. Paulson School of Engineering and Applied Sciences
Summary:
In a new study, researchers found that during epochs of extreme
heat - - 20 to 30 degrees Fahrenheit hotter than today -- 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.

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.



FULL STORY ========================================================================== 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.

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.

The research is published in Nature.



==========================================================================
In an atmospheric model, Seeley and Wordsworth cranked up Earth's sea
surface temperature to a scalding 130 degrees Fahrenheit, either by adding
more CO2 - - about 64-times the amount currently in the atmosphere --
or by increasing the brightness of the sun by about 10 percent.

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.

"Our research goes to show that there are still a lot of surprises in
the climate system," said Seeley. "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 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." ========================================================================== Story Source: Materials provided by Harvard_John_A._Paulson_School_of_Engineering_and_Applied
Sciences. Original written by Leah Burrows. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Jacob T. Seeley, Robin D. Wordsworth. Episodic deluges in simulated
hothouse climates. Nature, 2021; 599 (7883): 74 DOI:
10.1038/s41586-021- 03919-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211103200428.htm

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