by Rasmus Benestad and Ray Pierrehumbert
A special report in The Observer on Sunday (April 9) titled ‘Venus – The Hot Spot’, provides a well-written account on a mission called the Venus Express. The Venus express is an European Space Agency (ESA) mission to probe the the atmosphere of Venus and address questions regarding the differences between the climates on Venus and Earth. According to the plans, the probe will enter the final orbit around Venus in May 2006, i.e. within about a month.
What relevance does a mission to Venus have for a blog like RealClimate? Primarily, Venus offers scientists the chance to see how the same basic physics used to study Earth’s climate operates under a very different set of circumstances. In one sense, Venus is rather similar to Earth: it has nearly the same mass as Earth, and while its orbit is somewhat closer to the Sun, that effect is more than made up for by the sunlight reflected from Venus’ thick cloud cover. Because of the cloud cover, the surface temperature of Venus would be a chilly -42C if were not for the greenhouse effect of its atmosphere. In reality, the surface of Venus, at 740K (467C) is even hotter than the surface of Mercury, which is a (relatively!) pleasant 440K. Per unit of surface area, the atmosphere of Venus has as much mass as about 100 Earth atmospheres, and it is almost pure CO2. This accounts for its very strong greenhouse effect. In contrast, the CO2 in the Earth’s atmosphere accounts for a mere .00056 of the full mass of one Earth atmosphere.
Despite the fact that Venus has vastly more CO2 in its atmosphere than Earth, the same basic principles govern the operation of the greenhouse effect for both planets: the fact that air cools by expansion as it rises means that the upper parts of the atmosphere are colder than the surface, while the opacity of greenhouse gases to infrared means that infrared radiation can only escape from the upper portions of the atmosphere. Since the rate of radiation goes down with temperature, the net effect allows the planet to lose energy at a rate much lower than it would if the radiation from the surface escaped directly to space. Although most of the warm surface temperature of Venus is accounted for by its CO2 greenhouse effect, there are suggestions that it is warmer than it should be on the basis of CO2 alone. There are various theories that have been proposed for the source of the additional greenhouse effect, and sorting this out will be one of the major objectives of Venus Express.
Another interesting difference between Venus and Earth is that Venus has a very slow rotation rate, taking 243 Earth days to complete one rotation. This is actually somewhat longer than its year. Despite the low rotation rate of the surface, the upper atmosphere is whizzing along at a rate of about one rotation every four Earth days. Pinning down the mechanisms responsible for this super-rotation will teach us much about atmospheric dynamics in general. In particular, it would be interesting to know if there are circumstances in which the Earth’s atmosphere could kick over into super-rotation.
Although the atmospheres and climates of Venus and Earth differ very greatly today, it is generally believed that the two planets started out in a rather similar state, but subsequently evolved along divergent paths. Venus succumbed early to a "runaway water vapor greenhouse," in which the increased water vapor content arising from increased temperature reached an end state with much of the ocean evaporated into the atmosphere. Once this happens, it is easy for the water vapor to decompose in the upper atmosphere, whereafter the light hydrogen escapes and oxygen either escapes or reacts with rocks. One hypothesis is that the weak magnetic field at Venus, which otherwise would protect the planet from the solar wind, is one reason for why the oxygen and hydrogen escaped faster into space. Once water is lost, the reaction that turns carbon dioxide into limestone can no longer take place, so CO2 outgassing from volcanoes accumulates in the atmosphere instead of staying bound up in the rocks. The end state of this process is the current atmosphere of Venus, with essentially no water in the atmosphere and essentially the planet’s whole inventory of carbon in the form of atmospheric CO2. Earth, in contrast, kept its water, which allowed the planet to keep most of its carbon inventory safely bound up in the crust. The amount of CO2 in the atmosphere of Venus is approximately the same as the amount of CO2 bound up in the form of carbonate rocks on Earth today.
The runaway greenhouse that presumably led to the present Venus is an extreme form of the water vapor feedback that amplifies the effect of CO2 increases on Earth. Is there a risk that anthropogenic global warming could kick the Earth into a runaway greenhouse state? Almost certainly not. For an atmosphere saturated with water vapor, but with no CO2 in it, the threshold absorbed solar radiation for triggering a runaway greenhouse is about 350 Watts/m2 (see Kasting Icarus 74 (1988)). The addition of up to 8 times present CO2 might bring this threshold down to around 325 Watts/m2 , but the fact that the Earth’s atmosphere is substantially undersaturated with respect to water vapor probably brings the threshold back up to the neighborhood of 375 Watts/m2. Allowing for a 20% albedo (considerably less than the actual albedo of Earth), our present absorbed solar radiation is only about 275 Watts/m2, comfortably below the threshold. The Earth may well succumb to a runaway greenhouse as the Sun continues to brighten over the next billion years or so, but the amount of CO2 we could add to the atmosphere by burning all available fossil fuel reserves would not move us significantly closer to the runaway greenhouse threshold. There are plenty of nightmares lurking in anthropogenic global warming, but the runaway greenhouse is not among them.
The applicability to Venus of concepts originating in the study of Earth climate is a testament to the beauty and generality of the physical underpinnings of climate science. In turn, testing the resilience of these ideas against radically different climate encountered on other planets and in the distant past of Earth serves a valuable role in helping to shake loose new ideas. We wish the Venus Express team the best of fortune for a successful mission.