4 ms·

Semi-offtopic: water vapour is by far the most powerful atmospheric greenhouse gas per unit volume and as a whole, many times more 'powerful' than CO2. Hardly a

• scarmig 13y ago
  That's worth some elaboration, though: the water vapor in the atmosphere is (to a very, incredibly rough approximation) a system where the average global vapor concentration oscillates tightly around an equilibrium point. More than that, the average residence time in the atmosphere is a bit over a week. So when you say we can't change it, it's very true in one sense: if we vaporized the Mediterranean into the atmosphere, well before a month had gone by it'd have precipitated back to the liquid state on the Earth's surface. On the other hand, what you said could be misunderstood to mean that human activities have no effect on the average water vapor concentrations, and that'd be inaccurate. The equilibrium point is determined in large part by global temperatures: the higher they are, the more water vapor is pulled into the atmosphere. And we do have a mechanism to increase atmospheric temperatures: CO2. CO2 by itself is fairly limited in terms of temperatures: doubling its atmospheric concentrations only increases average global temperatures by about a degree, which is simple to calculate and more or less undisputed. But what really kills us (and where uncertainties creep in) is in the positive feedback loops, the biggest of which is water vapor. A degree increase in temperature from CO2 alone results in an additional couple degrees of increase in temperature due to the effects of increased water vapor. Combine a couple of those positive feedback loops, and you've got a problem.
  • logfromblammo 13y ago
    Such as firing the methane clathrate gun.
    • scarmig 13y ago
      Yes, but that's almost a separate issue. Water vapor is a limited feedback: if you double CO2 concentration, the additional water vapor will increase temperatures further, but only enough to induce a fraction of the original amount of increased water vapor to be sucked into the atmosphere. If this weren't the case, small deviations in the amount of water vapor in the atmosphere would cause even greater deviations, leading to runaway global warming until other, negative feedbacks grow large enough to counteract it; in other words, we must be in a stable equilibrium, as an unstable equilibrium would have already collapsed into a nearby stable one. Methane clathrate guns aren't the same: the idea there is that we've not historically explored the relevant parameter landscape (oceanic acidity + temperatures). People hypothesize that, if we're not careful, we can stumble into a saddle point where a small delta propels us away from our current equilibrium and toward one with much higher mean surface temperature. All of this is to say that water vapor feedbacks are well established, while a methane clathrate gun is just a speculative hypothesis.
• schiffern 13y ago
  >it is not something we can change. On the contrary, deforestation has a huge effect on the water cycle. Most people don't realize that if you're more than ~200 km inland most of your precipitation comes from transpiration by plants upwind instead of evaporation. How do we know? Oxygen isotopes. Basically, the differential fractionation of oxygen (and hydrogen) isotopes in water during evaporation vs. transpiration allows you to trace where your rainfall comes from. http://www.nature.com/nature/journal/v496/n7445/abs/nature11983.html http://www.nature.com/nature/journal/v496/n7445/abs/nature11... Also, don't cut down forests thinking it'll fix global warming. Overall they have a net cooling effect. What you'll do instead is create deserts downwind.