July 20, 2007

Lost in Space

Filed under: Climate Politics

As Al Gore continues his global warming crusade, he apparently is finding it necessary to invent scarier and scarier scenarios (no doubt to keep the press and his audience interested) and in doing so, does not feel limited to reality.

For instance, Al Gore’s latest theme is that anthropogenically-induced climate change will threaten the very existence of the human species. Wow. What can be more frightful than that!? (Maybe Gore is going after his next Academy Award in the horror category).

His point of emphasis seems to be an ill-formed analogy between conditions on Earth and conditions on Venus. It goes something like this (as excerpted from his New York Times editorial on July 1, 2007):

We — all of us — now face a universal threat. Though it is not from outside this world, it is nevertheless cosmic in scale.

Consider this tale of two planets. Earth and Venus are almost exactly the same size, and have almost exactly the same amount of carbon. The difference is that most of the carbon on Earth is in the ground — having been deposited there by various forms of life over the last 600 million years — and most of the carbon on Venus is in the atmosphere.

As a result, while the average temperature on Earth is a pleasant 59 degrees, the average temperature on Venus is 867 degrees. True, Venus is closer to the Sun than we are, but the fault is not in our star; Venus is three times hotter on average than Mercury, which is right next to the Sun. It’s the carbon dioxide.

He apparently repeated this nonsense in Aspen this week. According to an article that appeared in the Wednesday, July 18 2007 edition of the Aspen Daily News:

Gore advised the audience to compare the blue orb of the Earth to Venus, where daytime temperatures reach 867 degrees Fahrenheit and it rains sulphuric acid. The two planets have the same amount of carbon, Gore explained, but Venus’ just happens to be in the atmosphere, while most of the Earth’s is still locked underground. “The habitability of this planet for human beings really is at risk,” he said.

Is it at all reasonable to bring up conditions on Venus when discussing the current goings on on Earth? No.

For a good description of what the situation really is, we’ll turn to the folks over at Real Climate (www.realclimate.org) and an article they posted in April 2006 entitled “Lessons from Venus.” Recall that RealClimate’s purpose is to be “a commentary site on climate science by working climate scientists for the interested public and journalists. We aim to provide a quick response to developing stories and provide the context sometimes missing in mainstream commentary.”

They start,

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.

Hmm, dividing Venus’s average temperature by Mercury’s, we get 740K/440K= 1.68. That is not quite the quite the 3 times hotter than Mercury number that Gore uses, but hey, 3 times sound better than 1.68 times doesn’t it? (Perhaps Gore was confused by temperature scales, i.e. he did the calculation using the relative scale of Celsius or Fahrenheit instead of the absolute scale of Kelvin—but in doing do so, you get the wrong answer.)

And what about the idea that somehow, presumably through our use of fossil fuels, that we humans will cook ourselves out of existence by creating a Venus-like environment on Earth. It seems like this is what Gore is trying scare us with, why else bring up the conditions on Venus?

Again let’s turn to RealClimate.org:

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. (emphasis added)

In the past, RealClimate, has, on occasion, taken the time to praise Gore and defend many of his ideas (see, for example, here or here ). It’ll be interesting to see if whether they recall their “Lessons from Venus” article and suggest that Gore has gone off a little half-cocked this time. Which, indeed he has.




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