(Part of the How to Talk to a Global Warming Skeptic guide)
Objection: Clouds are a large negative feedback that will stop any drastic warming. The climate models don't even take cloud effects into account.
Answer: All of the atmospheric global climate models used for the kind of climate projections synthesized by the IPCC take the effects of clouds into account. You can read a discussion about cloud processes and feedbacks in the IPCC TAR.
It is true, however, that clouds are one of the largest sources of uncertainty in the GCMs. They are complicated to model because they have both positive feedbacks, preventing surface heat from escaping back into space, and negative feedbacks, reflecting incoming sunlight before it can reach the surface. The precise balance of these opposing effects depends on time of day, time of year, altitude, size of the water droplets and/or ice particles, latitude, current air temperature, and size and shape.
On top of that, different types of clouds will interact, amplifying or mitigating one another's effects as they coexist in different layers of the atmosphere. There are also latent heat considerations -- water vapor condenses during cloud formation and precipitation events, and water droplets evaporate when clouds dissipate.
The ultimate contribution of clouds to global temperature trends is highly uncertain, but according to the best estimates is likely to be positive over the coming century. There is no indication anywhere that any kind of cloud processes will stop greenhouse-gas-driven warming, and this includes observations of the past as well as modeling experiments.
Comments
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Zarkov Posted 7:26 pm
18 Nov 2006
Lets hope so, but clear sky days seem to be increasing. We could do with much more rain.
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SMLowry Posted 9:45 am
19 Nov 2006
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gzuckier Posted 7:41 am
04 Jul 2007
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Louise L Posted 7:10 am
26 Feb 2008
Louise, Freelancer currently working on the lose 50 pounds fast project.
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rcglinsk Posted 4:56 am
24 Mar 2008
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Dan Halen Posted 7:29 am
02 Jul 2008
Of course the best test of a computer model is to match it against the real world. This has been done through hindcasting. You take known historical data on CO2, pollutants, etc... and run the GCM. You then see how well the temperatures it produces matches what happened in the real world. It turns out that the current GCMs are quite good at matching with known historical data in fact. If the model clouds were behaving much differently than real ones this would not have been possible.
You also seemed to have missed the bit at the end where he told you that nothing in the past indicates that clouds will save us. We've seen massive warming lately along with massive output of GHGs. The clouds haven't saved us yet, why would anyone think they will suddenly start doing so? And where were the clouds during the paleoclimactic periods that were very warm? Napping on the job again.
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