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    The 'Runaway Greenhouse' Overheating Stage - Earth In 1.5 Billion Years
    By News Staff | July 30th 2013 05:11 PM | 5 comments | Print | E-mail | Track Comments

    A computer model estimate says it might be easier than previously thought for a planet to overheat into the scorchingly uninhabitable "runaway greenhouse" stage. That may mean some planets thought to be habitable right now actually are not.

    In such a runaway greenhouse stage, a planet absorbs more solar energy than it can give off to retain equilibrium. As a result, the world overheats, boiling its oceans and filling its atmosphere with steam, which leaves the planet glowing-hot and forever uninhabitable, as Venus is now. One estimate of the inner edge of a star's "habitable zone" is where the runaway greenhouse process begins. The habitable zone is that ring of space around a star that's just right for water to remain in liquid form on an orbiting rocky planet's surface, thus giving life a chance. 

    Revisiting this classic planetary science scenario with a numerical model, a group of astronomers came up with a lower thermal radiation threshold for the runaway greenhouse process, meaning that stage may be easier to initiate than had been previously thought.

    "The habitable zone becomes much narrower, in the sense that you can no longer get as close to the star as we thought before going into a runaway greenhouse," said paper co-author Tyler Robinson, a University of Washington post-doctoral researcher. 

    But don't get nervous. This "single-column, clear-sky model," a one-dimensional measure averaged around a planetary sphere that does not account for the atmospheric effect of clouds, shows that while Earth will move into the runaway greenhouse stage, it won't be for a billion and a half years or so. 


    If the model is good, a recalibration of where the habitable zone begins and ends might make sense. In doing so, some planets declared as as possible habitable worlds would have their candidacy revoked. 

    "These worlds on the very edge got 'pushed in,' from our perspective — they are now beyond the runaway greenhouse threshold," Robinson said. And, the astronomers write, "As the solar constant increases with time, Earth's future is analogous to Venus's past."

    Citation: Colin Goldblatt, Tyler D. Robinson, Kevin J. Zahnle&David Crisp, 'Low simulated radiation limit for runaway greenhouse climates', Nature Geoscience 6, 661–667 (2013) doi:10.1038/ngeo1892



    Comments

    So, a hyper-simplified model gets a lot of attention because it supports the dangers of AGW. Clouds complicate things hugely, and columns affect other columns, and there are distinctive layers, as well. This is a spherical cow - and the differences between a cow and a spherical cow are huge.

    Hank
    You need to read the article. (a) you didn't invent the term greenhouse effect, nor did anyone 20 years ago, it's been around for a long time and (b) this is about the habitable zone for planets being different than proposed in the past, not global warming.

    This does not even mention global warming.
    "easier to initiate" may have been a poor choice of words, then.

    Gerhard Adam
    Not at all, although AGW is completely misplaced.
    Mundus vult decipi
    Hank
    Right, I had to read it again to see if I was missing something. They are talking about the habitable zone for 'Goldilocks planets' and they "came up with a lower thermal radiation threshold for the runaway greenhouse process, meaning that stage may be easier to initiate than had been previously thought".

    It simply means that the habitable zone is different than believed, at least according to one numerical model - I don't think painting the entire planet Venus as a global warming alarmist really holds up. And even if this were about global warming, 1.5 billion years would seem to be plenty of time to fix it.