Tropical "Runaway Greenhouse" Provides Insight To Venus

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 temperature.

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.