(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.
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Comments View as Flat
Zarkov Posted 7:26 pm
18 Nov 2006
Clouds = Rain
>> according to the best estimates is likely to be positive over the coming century >>
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
More rain?
Depends on where you are. This year and last, in New England, have been among the wettest on record. This year, so far, we've had a good 20 to 25 inches more rain than "normal", which is substantial! And we've also had many cloudy days. This past spring, for example, it rained on (I believe) 21 days in May and some of the days that didn't rain were cloudy. Luckily we did get some sun during the summer. But this gray, rainy spring seems to be the trend. And warmer winters, more rain, less snow. Then there are other places where rain is desperately needed.
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gzuckier Posted 7:41 am
04 Jul 2007
"No clouds" global warming theory
This might be a good place to note the "cosmic rays cause clouds; clouds reflect the sun; we are going though fewer cosmic rays now, therefore fewer clouds, therefore more warming" theories of Svensmark et al. Aside from the general haggling about the accuracy of the model and whether there is any correlation at all, and the question of how a 14 million year cycle can cause rapid warming over a couple of decades, there is this basic problem: the current measurements show a greater effect for nighttime temperatures than daytime temperatures, and a greater effect at the poles, which are more covered in snow and ice than the rest of the earth and therefore already more reflective; the opposite of what any "clouds reflect the sun, less clouds means more cooling" theory would predict.
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Louise L Posted 7:10 am
26 Feb 2008
Well
I wouldn't like the clouds to dissapear for good, I believe they are doing us some good.
Louise, Freelancer currently working on the lose 50 pounds fast project.
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rcglinsk Posted 4:56 am
24 Mar 2008
Clouds
Ummm.... aren't you basically saying here "yeah, properly modeling clouds is about as important as it gets to understanding climate, and no, it's not currently done well at all." That's not much of a response; more of a admission.
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Dan Halen Posted 7:29 am
02 Jul 2008
It's an explanation, not an admission
What he's probably getting at is this: I make a computer model, I have various measurements I take or assumptions I make that lead to uncertainty, some of the uncertainties are small, others large. I carefully propagate my error mathematically through the calculation at the end and publish my results. Let's say they boil down to, oh say something like, I dunno, a temperature. The number is published as 5C +/- 2C. Of the 2C worth of uncertainty about 80% is due to a single rather uncertain measurement or factor. Thus I say "factor A accounts for the majority of uncertainty in this value". This does NOT mean that my whole experiment or model in useless or invalid.
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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