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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Search for Clouds provide too much uncertainty

Comments matching the search Clouds provide too much uncertainty:

The Big Picture (2010 version)

JasonB at 15:39 PM on 27 March, 2013

tcflood,

This is actually a better resource for finding peer-reviewed "skeptical" papers than those I gave before, sorted by subject matter. Click on the subject and SkS will list all the peer-reviewed papers known on that topic, categorised as pro-AGW, neutral, and "skeptical". If you know of any that haven't been included, you can just click on the buttons provided to add them to the database.

Regarding clouds, I think an important point to make about the uncertainty surrounding them is that the uncertainty is likely because they don't seem to be a strongly positive or a strongly negative feedback. Clouds certainly do have a big impact, but it's the change in that impact in response to AGW that's uncertain, and the change so far has been minimal (very slightly positive feedback, if anything).

In terms of physical principles, we have the direct warming caused by the increase in CO2 concentrations and the amplification of that warming by the increase in water vapour, which roughly doubles the CO2 impact. I have seen "skeptics" try to conflate water vapour feedback (which is both theoretically and empirically demonstrated) with the uncertainty surrounding cloud feedbacks (which is very hard to predict from first principles and must therefore be determined empirically) as a way of dismissing the water vapour feedback, but that is wrong.

Beyond that we have a range of feedbacks with different levels of certainty. Just off the top of my head:

Reduction in snow and ice is obviously a positive feedback due to lower albedo, and to make things worse, to date the rate of reduction seems to be grossly underestimated by models.
Increased desertification is presumately a negative feedback (since deserts have higher albedo than vegetation), but I don't think it's enough to counter the loss of snow and ice (and doesn't seem like something to hope for anyway).
Release of methane from continental shelfs and permafrost is a positive feedback.
Reduction in the ability of the ocean to absorb CO2 with increasing temperature (and, eventually, outgassing of CO2 from the oceans when the temperature gets high enough) is a positive feedback.
Changes in clouds, both in coverage and mix of types, is an unknown feedback but due to the lack of any real change to date should probably be assumed to be pretty much a wash (i.e. neither strongly positive or negative).

One thing you'll find is that people who have an a-priori belief that the climate cannot possibly change too much (for religious reasons, in the case of one prominent "skeptical" scientist) go searching for possible negative feedbacks that might counteract all of those positive feedbacks, and clouds often feature high on their list. (Deserts, not so much.) Not because the evidence tells them that clouds must be a net negative feedback, but because the uncertainty surrounding clouds allows them to believe that they might be — wishful thinking, in other words.

Presumably these same people don't take out insurance for much the same reason. Personally I prefer the Arab proverb, "Trust in God but tie up your horse".

Clouds provide negative feedback

Bob Lacatena at 23:39 PM on 29 April, 2011

157, RW1,

I do not see where the issues I've raised has been addressed or answered.

That's because you ignore the statements that do address them.

1. Your theory is inconsistent with all of the lines of evidence which point to a climate sensitivity of 3˚C or greater.

You have seen this presented to you now at least 6 times, and you keep dodging it. How does your theory account for this? Until you answer that question, your theory fails.

2. Your theory is inconsistent with the observational evidence (Dessler 2010) that demonstrates a positive, not negative, feedback in response to short-term warming. While this cannot necessarily directly support a long-term positive effect, it directly refutes your "was negative before, so must be negative in the future" theory.

3. Your argument that models "assume" and require a positive cloud feedback is wrong. While clouds do represent a large area of uncertainty in the models, it is incorrect to think that the scientists who have done the modeling have not thought things through a little more carefully, and in more detail, than you have. You cast aspersions by generalizing their work into the word "assume," and yet provide no direct evidence (other than the general positive/negative thing) that their work is not well considered.

4. Your argument that current warming "depends" on the positive cloud feedback is exaggerated, as has been demonstrated. It reduces potential warming from 3˚C to 2.5˚C or 2˚C, which while helpful is not an inconsequential amount of warming. Like many deniers, you exaggerate one point to try to make it the single, decisive, "AGW killing" argument.

5. Your argument that cloud effect is negative and would not "switch" to positive is at its root flawed and too simplistic. It's rather like saying that May was warmer than April, and June was warmer than May, so every month from now on will get warmer and warmer forever. It takes a too simple premise, and draws an invalid conclusion, because it avoids the complexities of the system.

Instead of Occam' razor, your theory uses Occam's guillotine.

It is based on a very broad, general, simplistic approach to the problem. It does not consider any details in the issue, such as why the current effect would be negative, or how clouds might or will change. It assumes that all effects are linear and additive. It simply takes the childishly simple view that if current net effect is negative, then a warmer climate must mean more clouds, which must mean more negative.

You may feel that this logic is persuasive, and for simple minded people who like to stop thinking as soon as they see an argument that supports their predetermined beliefs, this might well be persuasive.

But it doesn't persuade me in the slightest.

It matters very, very much where additional clouds form in a warming world (high latitudes, or near the equator), when they exist (during daylight, summer hours, or nighttime or winter hours), and what kind of clouds form (i.e. low, reflective clouds or high, heat trapping clouds).

Quantifying the human contribution to global warming

Chris Colose at 08:58 AM on 5 September, 2010

There's been some interesting questions and a few off-target remarks here, which touch on several topics. Hopefully I can provide some focus below.

The radiative forcing for a change in CO2 is highly dependent on the the temperature structure of the atmosphere, cloud cover, and water vapor. You can boil down the physics to a simple statement about forcing going like the logarithm of concentration, but this cannot be derived (or the 5.35 W/m2 coefficient) without spectrally resolved calculations. This is in part because the other gases/clouds "steal" some of the radiation that would otherwise. In fact, the power of a specific greenhouse gas is maximized when it is acting by itself and does not need to compete with the spectral overlap by other constituents. This is discussed in an in press paper by Schmidt et al 2010 which attempts to partition the total greenhouse effect by contribution to individual gases, and I discuss it here. What's more, the logarithmic dependence breaks down under situations not too far from modern-day Earth-like conditions, such as at the very high CO2 concentrations thought to exist at the termination of a snowball planet. To touch on RSVP's point in 9, he is correct that there is still some uncertainty in the forcing for a doubling of CO2 (The central value is 3.7 W/m2, though with a range of about +/- 0.3 W/m2, see Forster and Taylor, 2006 depending on method used) but becomes much larger for different atmospheres (like past Mars or early Earth).

The vertical temperature profile is also relevant because the greenhouse effect requires some sort of lapse rate to allow colder atmosphere aloft to radiate to space at a temperature colder than the surface. Even though the concentration of CO2 is pretty uniform over the globe, the forcing does have some variation over the planet due to changed tropopause location and lapse rate effects. The solar radiation of course does matter for the greenhouse effect to be relevant at all (and even the shortwave absorption by water vapor and CO2 is not completely zero, though much smaller than the longwave part). If the Earth had no incoming sunlight, the greenhouse effect would not support any temperature higher than the 'cosmic background' temperature, although it would take the planet a bit longer to cool off to that point than a planet where you turned off the sun and had no atmosphere at all. This is obviously quite removed from reality though and Dima is correct that the small variations in solar radiation do not matter for the CO2 greenhouse effect.

In fact, the true no-feedback sensitivity parameter (in the article, 0.27 K/(W/m2))is also dependent on the finite absorption of the atmosphere, and so becomes more on the order of about 0.3 to 0.31 K/(W/m2). This so-called Planck feedback response is pretty robust across various models; see for example Table 1 in Soden and Held, 2006 [PDF]). This table gives an estimate of the magnitude of the Planck feedback amongst various models (you need to take one divided by these numbers to be consistent with my units). Water vapor acts to enhance this sensitivity by making a plot of the outgoing longwave radiation vs. temperature more linear than the T^4 dependence that a blackbody has. This enhances the sensitivty by a factor of about two, to the extent that the upper tropospheric moisture content scales with the Clausius-Clapeyron relation. To humanity rules (#11), the vapor content should go up nearly exponential with T, but the absorptivity goes up nearly like the logarithm (though not as nicely as CO2, and some have argued more of a square root dependence), so the feedback of water vapor as T increases should be somewhat linear. See my article on feedbacks here for more. Also keep in mind that the typical sensitivity-forcing equations in this article apply only at equilibrium and the timescale to reach equilibrium depends on the climate sensitivity, so the "heating in the pipeline" becomes larger as sensitivity increases.

To HumanityRules (#11 regarding the natural oscillations)-- I don't think anyone argues that the polar regions are highly variable and exhibit significant amounts of influence from ocean circulation. In particular, windiness and advection is a large part for estimating year to year 'minimum' in sea ice extent, but the preconditioning of ice loss (not just extent but thickness) is clearly due to albedo feedback which is in part related to rising temperatures, and these temperatures are rising almost everywhere on the globe, it's not a redistribution, so this is a signature of external forcing. Just from the abstract, the Chylek paper uses detrended data to see a see-saw effect, so it's unclear to me how you can make statements about the trend from this (but I have not looked at the whole paper and cannot for some time), but the abstract itself says that natural variability in the Arctic as well as the trend are both at play here.

Finally, the canonical 2 to 4.5 C estimate of equilibrium temperature change per doubling of CO2 (and the feedback parameter lambda itself) does encompass water vapor feedbacks but also the other effects (lapse rate, clouds, ice-albedo) required to get the full range.

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