Runaway greenhouse effect

Almost 80% of the sunlight that hits Venus is reflected back into space by the thick clouds surrounding the planet before it ever reaches the surface. Even so, temperatures at the surface of Venus are much hotter than those on Earth. However, this is not because Venus is closer to the Sun than the Earth. Scientists believe that the difference in the temperatures of the two planets is due to a runaway greenhouse effect caused by the large amount of sulfur dioxide in Venus atmosphere. 

AU from the Sun, just outside the orbit of Mars, whereas the inner boundary is determined by the runaway-greenhouse effect as observed on Venus. If the surface temperature were too hot, above 373 K, this would vaporise all water on the surface of the planet. The inner boundary is around 0.85 AU so the habitable zone spans 0.85-1.7 AU for our Sun (Figure 7.7) but the current habitable zone spans 0.85 - 1.3 AU (t — 0) in Figure 7.7. The habitable zone was much larger when the Sun s luminosity was greater, and narrower when the luminosity was smaller. 

Water vapour makes a sizeable contribution, and probably the largest, to radiation trapping and as the temperature increases the water vapour concentration increases. Temperature rises as a result of increased water vapour concentration and hence a mechanism for a positive feedback in the greenhouse effect that might lead to a runaway greenhouse effect. When the vapour pressure for water reaches saturation, condensation occurs and water rains out of the atmosphere this is what happens on Earth and Mars. On Venus, however, the water vapour pressure never saturates and no precipitation occurs and the global warming continues to increase. Thus Venus suffers from extreme temperatures produced by both its proximity to the Sun and the presence of water vapour and carbon dioxide in its atmosphere. 

The combination of forces that caused these changes is still in question but it involves the stability of the climate. The climate has fluctuated between limits of plus or minus 15°C (27°F) for hundreds of millions of years. These limits are large enough to have a major influence on species extinction and evolution. A runaway greenhouse effect is thought to have changed Venus where the oceans boiled. 

There does not seem to be much chance that the Earth would be vulnerable to a runaway greenhouse effect such as Venus where the oceans would boil away. But, climate changes as great as an ice age could be disastrous if they occurred rapidly. 

In fact, the conditions on Venus today are sometimes described as the result of a "runaway greenhouse effect. The planet s surface temperature of about 730 K (430°C) cannot be explained simply on the basis of the heat it receives from the Sun. Indeed, it is warmer... 

As time evolved and the energy provided by the sun increased, the gradual removal of carbon dioxide from the atmosphere became critical to avoid a runaway greenhouse effect (with extremely high surface temperatures) as observed on Venus. This removal of CO2 was accomplished by weathering of calcium silicate (CaSiOs) minerals by acidic C02-rich rainwater, leading to the formation of limestone (CaC03). 

Runaway Greenhouse Effect Effect produced when a planet builds up an intense surface temperature like that on Venus because its atmosphere tends to trap heat rather than allow it to escape to space. 

Compare Earth to its nearest planets—Venus and Mars. The atmosphere of Venus is so thick that observers cannot see the surface of Venus from Earth. (Our only images of Venus s surface have come from probes that parachuted through Venus s atmosphere to the surface, and then promptly melted.) As a result, Venus experiences a runaway greenhouse effect that keeps Venus too hot to support liquid water or (probably) Ufe as we know it. Humans could not Uve on Venus. 

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