This is a discussion of clouds precisely because they are a very important part of the climate puzzle.
“Clearing Clouds of Uncertainty by Mark Zelinka, David Randall, Mark Webb and Steven Klein. Their commentary is really a summary of our recent understanding and – as illustrated by the figure on the right – they conclude that the evidence is converging on the cloud feedback likely being positive.”
From FAR 1990 positive but cloud feedback represented the largest source of uncertainty in climate sensitivity among atmospheric models.
To SAR Second Assessment Report; 1995), more climate models were predicting the mass of cloud liquid and ice, and generally finding negative cloud opacity feedbacks, albeit of widely differing strengths. The report concluded that it was not possible at that time to judge the sign of the net cloud feedback.
TAR 2001 …the sign of [the cloud] feedback remains unknown.
AR4 2007 it is not yet possible to assess which of the model estimates of cloud feedback is the most reliable
AR5 “The sign of the net radiative feedback due to all cloud types is…likely positive”
BUT Cloud opacity feedback “is highly uncertain”
“I don’t really see why I should be expected to post comments that are certain, but wrong.”
I repeated a fact, “Averaged globally and annually, clouds cause cooling ” from the article overview. I said in view of this,
“any increase in cloud cover should have a stronger negative than a positive effect.”
The current point of view says this is likely wrong but admit to high uncertainty still in areas like cloud opacity. There is a threefold variation in the global sensitivity parameter FAR 1990 due to
differences in cloud feedback.
This is a most important discussion which is not yet settled and needs open discussion.
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Just a quick note of thanks. Not nearly well versed enough to comment on the material, but enjoy reading and learning bits and pieces.
On Sat, Oct 7, 2017 at 3:02 PM, …and Then There’s Physics wrote:
> …and Then There’s Physics posted: “A few years ago I posted a video by > Andrew Dessler that was discussing whether or not Equilibrium Climate > Sensitivity could be less than 3oC. The bottom line was that the best > estimate for ECS is about 3oC. Given that we’re quite confident about water > v” >
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The article overview states
“Averaged globally and annually, clouds cause 18W/m2 of cooling relative to a hypothetical
cloud-free Earth. This is the net result of a 46 W/m2 cooling from reflecting sunlight back to space (an albedo effect) partly offset by a 28 W/m2 heating due to reduced terrestrial radiation emitted to space (a greenhouse effect). The net planetary cooling provided by clouds is roughly five times as large as the planetary heating from a doubling of CO2.”
So even though the albedo effect usually works only half the time [there is a minuscule effect for moonlight etc] it produces much more cooling in that 12 hour period than a whole 24 hours of
cloud GH positive effect.
“Why would the albedo negative effect be larger than the cloud GH positive effect if they increase”
Because it is positive with more clouds. As stated above in a cloud free earth it would be 18W/M energy warmer. So if there is no effect with no cloud and 18 W/m2 cooler with current cloud it stands to reason that with more cloud from global warming there should be an increase in the cooling effect.
The authors state,
“The overall cloud feedback is actually the aggregate effect of several individual cloud feedbacks, commonly separated into three components: cloud amount, cloud altitude,and cloud opacity feedbacks” “Nearly all current climate models simulate an overall positive cloud feedback ”
Despite Victor’s assertion he is referring to a decreasing amount of ice and here we are referring to an increasing amount of water vapor. Which up to this point in the climate causes cooling of a known degree. The models simulate and argue for positive feedbacks from clouds from this point on without acknowledgment of how the clouds caused cooling in the first place.
It appears to be models all the way down with “weak further support from observations”