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Cloudy forecast

By hrynyshyn on September 3, 2010.

Long weekend reading: Over at e360, Climate Central's Michael Lemonick sums up the latest thinking on the big question of whether clouds will alleviate or accelerate global warming.

It's no small detail. Just about everyone agrees that anthropogenic climate change will produce more cloud cover. The mystery is whether that in turn will produce a positive or negative feedback. Lemonick's take-home message is that the evidence is beginning, just, to tilt in favor of the bad-news scenario.

And although researchers are still far from certain whether an anticipated increase in cloudiness will further heat up the planet or offset the warming a bit, a growing consensus among climate modelers is that clouds will increase, rather than hold back, the warming triggered by greenhouse gases. That's largely because water vapor itself is a powerful greenhouse gas, which means that clouds should trap more heat than they are likely to reflect back into space.

Still, at this point, few climate scientists would be willing to stake their reputations on a definitive forecast of how clouds will impact the climate system in coming decades and centuries. Stanford University climate scientist Stephen Schneider, in an e-mail written just a week or so before his untimely death on July 19, said, "Cloud feedback has been uncertain by a factor of 3 since I did the first paper with that title nearly 40 years ago -- we are still no closer to an answer."

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"That's largely because water vapor itself is a powerful greenhouse gas, which means that clouds should trap more heat than they are likely to reflect back into space"

Aren't clouds either liquid or solids though? Water doplets or ice? Sure, increased water vapour should be a positive feedback, but do clouds have the same effect? or is this just another example of dumbing-down?

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I just read the article - it's great. Unfortunate that you chose the only problematic part to quote :D

IMHO he was actually avoiding the big question. Here's the comment I submitted:

I'm surprised there was nio mention of the definitive paleoclimatic evidence against a negative cloud feedback sufficient to put a damper on the current warming trend. To argue otherwise one would have to e.g. explain why such a negative feedback would operate significantly in the near future and yet have had no apparent effect on the rapid warmings associated with the Pleistocene deglaciations (~6C warmings over the course of a few thousand years).

Similarly, it's been firmly established that the last time CO2 levels were ~350 ppm (we're at 390 now) in the mid-Pliocene (~3.3 million years ago), global temperatures were 2-3C higher than present and sea levels were ~25 meters higher. Going farther back in time, we see that CO2 levels of ~500 ppm (a level we're likely to pass by 2050) are incompatible with the presence of any permanent ice on the planet (=> ~70 meters of sea level rise). Whatever the cloud feedback is, that's the sort of climate impact we're guaranteed if we leave CO2 high enough for long enough.

All of that said, determining the sign and exact magnitude of the cloud feedback is critical to establishing how quickly the climate system will respond to the increasing CO2 levels. Paleoclimatic evidence alone isn't fine-grained enough to prove that there's no short-term cloud feedback that will stretch what would otherwise be decades of warming into centuries. But as scientists focus on this critical work, let's not lose track of the larger picture.

Also, IMHO it was a little lazy of Lemonick to refer to the unqualified Freeman ("I did one paper on climate 40 years ago") Dyson and and the perennially-repudiated Dick ("forget about that paper, just you wait 'til my next one, you whippersnappers") Lindzen while not referencing the paleo evidence that knocks the pins out from under their arguments.

By Steve Bloom (not verified) on 06 Sep 2010 #permalink

Also, naught101 is correct that the quoted sentence is confusing. We know non-cloud water vapor has a positive feedback, but that doesn't determine the cloud feedback.

By Steve Bloom (not verified) on 06 Sep 2010 #permalink

Very nice explanation. I've wondered about that before myself. I always wondered why the side of the ball that was spinning into the direction of velocity wouldn't feel more air resistance and cause the ball to curve in an opposite direction to what is experimentally observed. Thanks, well done!

"That's largely because water vapor itself is a powerful greenhouse gas, which means that clouds should trap more heat than they are likely to reflect back into space"

Aren't clouds either liquid or solids though? Water doplets or ice? Sure, increased water vapour should be a positive feedback, but do clouds have the same effect? or is this just another example of dumbing-down?

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I just read the article - it's great. Unfortunate that you chose the only problematic part to quote :D