Tropical ‘runaway greenhouse’ provides insight to Venus

Tropical ‘runaway greenhouse’ provides insight to Venus

Subject :Environment

Section: Climate change

Runaway Greenhouse:

• 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.
• Scientist 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.
• What is limiting this effect over the warm pool of the Pacific?. 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.
• Water vapor above 5 kilometers (3 miles) altitude in the atmosphere contributes significantly to the runaway greenhouse signature. Scientist 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.
• 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.