A new paper just published in the Journal of Climate finds that global cloudiness has decreased over the past 39 years from between 0.9 to 2.8% by continent as shown in the figure below:
DJF. Continental seasonal anomalies are based on seasonal station anomalies averaged within 10˚ grid
boxes, which are then averaged over the continent weighted by land fraction and box size. Interannual
variation (IAV) is the standard deviation of the time series. Trends are determined using the median of
pairwise slopes method.
The period of the study is from 1971 to 2009. The authors say that:
“Global average trends of cloud cover suggest a small decline in total cloud cover, on the order of 0.4% per decade.”
Taken together, global cloud cover decreased and average of 1.56% over this 39 year period. WUWT readers may recall that Dr. Roy Spencer points out the issue of a slight change in cloud cover in his 2010 book intro of The Great Global Warming Blunder: How Mother Nature Fooled the World’s Top Climate Scientists. He writes:
“The most obvious way for warming to be caused naturally is for small, natural fluctuations in the circulation patterns of the atmosphere and ocean to result in a 1% or 2% decrease in global cloud cover. Clouds are the Earth’s sunshade, and if cloud cover changes for any reason, you have global warming — or global cooling.”
So there you have it, by the work of independent scientists, it is suggested that Dr. Spencer’s hypothesis of just a small change (1-2%) of cloud cover has been observed in their study. This can account for the global warming changes observed. Cloud cover has decreased over the past 39 years globally, and temperatures have risen during that time. This global decrease in cloud cover alone could account for all surface warming observed since the 1970’s. Interestingly, some types of clouds have been on the increase, while others have been on the decrease. Figure 2 shows this:
40˚-80˚N, and 20˚-180˚ E. Anomalies are calculated for individual stations, then averaged within 10˚
equal-area grid boxes, box values are averaged over the entire area, weighted by box size and land
fraction in each box.
Now, a cause needs to be identified as to why some clouds increase and others decrease. One of the obvious ones to examine is Svensmark’s cosmic ray hypothesis, which says that as solar (magnetic) activity decreases, cosmic ray insolation intensity increases, and cloud cover increases due to more cosmic ray seeding. Aerosols and ENSO may also figure greatly in cloud formation changes. It will be a tough puzzle to fully disentangle given that there have been a number of stations lost that record cloud cover type and the move has been towards automated systems (like ASOS) which only record cloud height and not type. The data in this study is mostly from human observers noting cloud type and height for aviation purposes. Perhaps there will be a way to get this information as the number of observers decrease from satellite image processing.
Here’s the paper:
Ryan Eastman and Stephen G. Warren
Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195 Journal of Climate 2012: http://dx.doi.org/10.1175/JCLI-D-12-00280.1
Abstract
An archive of land-based, surface-observed cloud reports has been updated and now spans 39 years from 1971 through 2009. Cloud-type information at weather stations is available in individual reports or in long-term, seasonal, and monthly averages. A shift to a new data source and the automation of cloud reporting in some countries has reduced the number of available stations; however this dataset still represents most of the global land area.
Global average trends of cloud cover suggest a small decline in total cloud cover, on the order of 0.4% per decade. Declining clouds in middle latitudes at high and middle levels appear responsible for this trend. An analysis of zonal cloud cover changes suggests poleward shifts of the jet streams in both hemispheres. The observed displacement agrees with other studies.
Changes seen in cloud types associated with the Indian monsoon are consistent with previous work suggesting that increased pollution (black carbon) may be affecting monsoonal precipitation, causing drought in North India. A similar analysis over northern China does not show an obvious aerosol connection.
Past reports claiming a shift from stratiform to cumuliform cloud types over Russia were apparently partially based on spurious data. When the faulty stations are removed, a tradeoff of stratiform and cumuliform cloud cover is still observed, but muted, over much of northern Eurasia.
====================================
The full paper is available on the author’s website:
http://www.atmos.washington.edu/~rmeast/Full_Text_D1.pdf
Dr. Spencer’s book is available from Amazon:
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Leif Svalgaard says:
August 24, 2012 at 7:50 am
“I guess that is always in the eye of the believing beholder…”
Actually with the aid of the edge of a sheet of paper parallel to the y-axis on the graphs, in each year, season by season.
Do clouds migrate upward as they age? I think they do. Daily convection cycles in humid air produce clouds at low levels, 5000 ft or less. Where do the mid-level clouds come from? At the top of the cloud sunlight is being scattered, which means little heat is being absorbed. At the bottom of the cloud “clear window” IR from the surface is being absorbed so the bottom of the cloud warms, the water droplets evaporate, and the warm humid air migrates to the top of the cloud where the water droplets re-condense, and the whole cloud moves upward in the process. I speculate that the increase in mid-level clouds and decrease in low-level clouds simply indicates that clouds are more persistent now than in the past. This would be consistent with reduced large-scale convective activity.
“””””…..docrichard says:
August 23, 2012 at 2:40 pm
@ur momisugly george e smith
Thanks for the heads-up about Frank Wentz. I could not see his original paper, but he gave an interview about his work here: http://earthsky.org/earth/frank-wentz-will-global-warming-bring-more-rainfall.
He says “I certainly wouldn’t claim that we have any definitive answers. I think, though, we’ve been able to leave behind the upper and lower rates of the expected increase in rain. But to determine what the exact increase is going to be and where the increased rain is going to fall, those are questions that still need to be answered”……”””””
Well doc, you and I both know that although “what gose up must come down” , there is no assurance that it will come down where you want it to.
The whole Monsoon phenomenon, is indicative of the problem. The location of the evaporation, and the subsequent precipitation is of course all in the fine print of the global weather system and ultimately climate, and we should not expect it to be static over longer time intervals.
I believe this is why some people mistake global warming being associated with droughts. Globally, it must result in greater precipitation, but not surprisingly that may result in drought conditions moving in on some locations. The Oklahoma dust bowls of the 1930s would seem to be such an event, since that was also a period of notable warming; and might actually be the prototype of what is currently being experienced in the USA.
Now someone somewhere is reaping the benefit of the recent warming prior to the current plateau.
In this I plead total ignorance of where it goes to; that’s above my pay grade.
The Wentz paper in SCIENCE is in my opinion one of the seminal papers in weather/climate peer reviewed literature, and I urge you to beg, borrow or steal a copy of it from a local or university library, if the cloud subject tickles your fancy which I sense is the case. The point is that it puts the cloud issue into the ” Duh !! ” category, once you understand it.
The other cloud issue is of course the high low cloudy warm night issue.
I’d like a dollar for every time some poster here mentions the 6PM weather report ,mantra; “We have high clouds this evening, so it will be a warm balmy night. ”
I’ll give back a dollar in the remote case that someone says; “We’ve had a hot muggy day, so expect high clouds to form this evening as it cools down, and a warm balmy night. ”
It’s simply a question of which is the “cause”, and which is the “effect”. The high evening clouds are the result of the hot muggy day before; which also is the direct cause of the warm balmy night. The Temperature WILL go down at night; NOT up !
george
”
docrichard says:
”
There’s a matter of causality when it comes to less clouds. You are assuming that higher T causes fewer clouds. The alternative causality is fewer clouds cause higher T. We already know that that this alternative is true (for low clouds) due to albedo and that it is easily understood conceptually and that it is an extremely strong effect. Consequently, both cannot be true as one would have total runaway feedback with not even unstable equilibrium possible. Also, the flip side of a cooling T causing an increase in cloud cover would result in a snowball Earth if true and the T decreased slight due to massive unstable positive feedback. As for a conceptual mechanism for T causing less cloud cover, you are conveniently forgetting about the existence of a lapse rate. A warmer surface and atmosphere still has a lapse rate so that at some altitude, there will be a given T or at some altitude there will be a T differential from the surface. Moist air will be lighter in density and will absorb more IR – thus becoming warmer and even lower in density than surrounding dry air and hence will rise until that is no longer true regardless of some small difference in T as compared to a slightly lower T example where clouds would form. The result might be that the warmer T cloud forms at a slightly higher altitude versus a slightly lower altitude but that doesn’t mean there has to be somewhat less cloud cover.
Lower cloud cover means more energy arriving at the surface – most of which is water and so you will have higher rates of evaporation going on. As stated earlier, that higher evaporation rate must be countered by added rain fall.
cba
Let us agree on some facts first. I am looking Here:
http://www.climate4you.com/images/TotalCloudCoverVersusGlobalSurfaceAirTemperature.gif
where we can see that high global surface air temperature is associated with less cloud, and lower surface air temperature is associated with more cloud.
Specifically, a GST of 15.4*C is associated with cloud cover of 63.5%, and 15.1*C is associated with cloud cover of 69%. I make that 8.7% decrease in cloud for a 0.3*C increase in T, or 2.9% decrease in cloud for each 0.1*C increase.
Are we agreed on these observations, before we plunge off into the thickets of causality?
@george e smith says:
August 24, 2012 at 9:22 pm
A key issue is whether more Cirrus clouds actually increase day time surface temperatures as claimed. The majority of Cirrus clouds are in the tropics, and I would think a short term increase would inhibit daytime surface temperatures, but not show much of a trend as the tropics have less warming as a region. Changes in low cloud must have very different impacts depending on latitude and the season.
You might try to go through my previous post above ” cba says:
August 22, 2012 at 8:10 am ” to get some details of the overall concepts involved. I did look at the ref. you provided but the only thing I recall was R^2 was under 0.3 which puts the results in the realm of being indistinguishable from noise. At best, it offers less than a 30% explanation of any relationship between the two. Looking at the numbers you put forth, gut feel says they are about 1/2 to 1/3 of what they should be but I don’t have time to verify this at present. A couple of years ago I did a simple cloud cover / T estimate that provided a little better than R^2 >= 0.6 with a 1-3 month shift in data but I’m not sure where my notes are that have any of the details.
While this stuff is good for gaining a common sense understanding of things and can rule out some blatant BS, one is still faced with the fact that Earth is a rotating body and that time is involved on several levels.
For the moment I’ll accept your estimates as possibly being reasonable for T over oceans.
Now, what’s your point?
docrichard, due to work loads that are starting out, I may have trouble responding quickly.