A region in the western tropical Pacific Ocean may help scientists
understand how Venus lost all of its water and became a 900-degree
inferno. The study of this local phenomenon by NASA scientists also
should help researchers understand what conditions on Earth might
lead to a similar fate here.

The phenomenon, called the ‘runaway greenhouse’ effect, occurs when a
planet absorbs more energy from the sun than it can radiate back to
space. Under these circumstances, the hotter the surface temperature
gets, the faster it warms up. Scientists detect the signature of a
runaway greenhouse when planetary heat loss begins to drop as surface
temperature rises. Only one area on Earth – the western Pacific ‘warm
pool’ just northeast of Australia – exhibits this signature. Because
the warm pool covers only a small fraction of the Earth’s surface,
the Earth as a whole never actually ‘runs away.’ However, scientists
believe Venus did experience a global runaway greenhouse effect about
3 billion to 4 billion years ago.

“Soon after the planets were formed 4.5 billion years ago, Earth,
Venus and Mars probably all had water. How did Earth manage to hold
onto all of its water, while Venus apparently lost all of its water?”
asked Maura Rabbette, Earth and planetary scientist at NASA Ames
Research Center in California’s Silicon Valley. “We have extensive
earth science data to help address that question.”

Rabbette and her co-investigators from NASA Ames, Christopher McKay,
Peter Pilewskie and Richard Young, used atmospheric conditions above
the Pacific Ocean, including data recorded by NASA’s Earth Observing
System of satellites, to create a computer model of the runaway
greenhouse effect. They determined that water vapor high in the
atmosphere produced the local signature of a runaway greenhouse.

At sea surface temperatures above 80 F (27 C), evaporation loads the
atmosphere with a critical amount of water vapor, one of the most
efficient greenhouse gases. Water vapor allows solar radiation from
the sun to pass through, but it absorbs a large portion of the
infrared radiation coming from the Earth. If enough water vapor
enters the troposphere, the weather layer of the atmosphere, it will
trap thermal energy coming from the Earth, increasing the sea surface
temperature even further.

The effect should result in a chain reaction loop where sea surface
temperature increases, leading to increased atmospheric water vapor
that leads to more trapped thermal energy. This would cause the
temperature increase to ‘run away,’ causing more and more water loss
through evaporation from the ocean. Luckily for Earth, sea surface
temperatures never reach more than about 87 F (30.5 C), and so the
runaway phenomenon does not occur.

“It’s very intriguing. What is limiting this effect over the warm
pool of the Pacific?” asked Young, a planetary scientist. He suggests
that cloud cover may affect how much energy reaches or escapes Earth,
or that the ocean and atmosphere may transport trapped energy away
from the local hotspot. “If we can model the outgoing energy flux,
then maybe we can begin to understand what limits sea surface
temperature on Earth,” he said. The Ames researchers are not the
first to study the phenomenon, but no consensus has been reached
regarding the energy turnover or the limitation of sea surface

Rabbette analyzed clear-sky data above the tropical Pacific from
March 2000 to July 2001. She determined that water vapor above 5
kilometers (3 miles) altitude in the atmosphere contributes
significantly to the runaway greenhouse signature. She found that at
9 kilometers (5.6 miles) above the Pacific warm pool, the relative
humidity in the atmosphere can be greater than 70 percent – more than
three times the normal range. In nearby regions of the Pacific where
the sea surface temperature is just a few degrees cooler, the
atmospheric relative humidity is only 20 percent. These drier regions
of the neighboring atmosphere may contribute to stabilizing the local
runaway greenhouse effect, Rabbette said.

It is important to note that the Ames team uses real climate
information such as relative humidity and temperature-not
hypothetical numbers-in the Moderate Resolution Atmospheric Radiative
Transfer, or MODTRAN, modeling program. The program calculates how
much energy escapes back to space from the top of Earth’s atmosphere.
The researchers plan to experiment with the model to test the runaway
greenhouse signature’s sensitivity to climate conditions. By varying
the abundance of other greenhouse gases such as carbon dioxide and by
adding clouds in the model, they will see the overall effect on the
outgoing energy.

The model may help researchers uncover why Venus experienced a
complete runaway greenhouse and lost its water over a period of
several hundred million to a billion years. The research may also
help determine which planets in the so-called ‘habitable zone’ of a
solar system might lack water, an essential ingredient for life as we
know it.