Percy Bysshe Shelley begins his poem THE CLOUD speaking for all the clouds
I bring fresh showers for the thirsting flowers,
From the seas and the streams;
How do we know how much cloud cover we have? Well, before satellites, you had to get station data where cloud cover (in tenths or eighths) was estimated and in some cases recorded. Somehow you had to add it all up. In the old days when people were trying to calculate the planetary albedo they had to estimate cloud cover for the earth.
If you look at planetary albedo measurements from the first estimate to the recent satellite measurements, it gets more and more interesting. In the first method for albedo estimation, we measure the sunlight that bounced off earth and hit the moon (You can see the moon even when the earth's shadow is on it!) and then bounced back to earth. Then we estimate the albedo of the moon which was difficult given the blue cheese theory of lunar composition. Pencil-work at this point gives the intrepid calculators about 50% for the albedo of earth. With each new estimating technique, the magnitude of our albedo got smaller and smaller until Stephen Schneide put the value of 28% in his book on global COOLING and the coming ice age.
The regression of albedo against year is fantastically high. Perhaps it is due to cloud-climate change? No. It is just due to more and more sophistication at getting at this global number.
Well, global cloudiness is around 50%. If you randomly walked the globe for a millennium and looked up every time your random watch beeper sounded off, you would see clouds directly above about half the time.
The Swedish chemist S. Arrhenius estimated 57% in his famous 1896 paper on on halving, doubling, and quadrupling carbon dioxide on forcing global temperature change. Arrhenius was pretty good with the yellow no. 2 pencil, as computers for such work were more than a century ahead at your local app-store. Planetary albedo is usually assigned a value of 30%.
Cloudiness At Or Near LTER Sites
For decades, the measurement of cloudiness has been by estimate of the fraction of the sky covered by clouds. Two standards have emerged. 1/10 (tenths) and 1/8 (oktas). It would seem to be a no-brainer, hands-down 10 fingers up. Well the mavens test just this kind of estimation: Which does a better job tenths or octals Hands down for the oktas. Both systems are in use by weather stations around the globe. So we will pick the second best (tenths) system of estimation of cloudiness. Records of this go back to around 1900. So our question is How have things changed. Has cloudiness increased, decreased or remain unchanged. For the 10th time we will count our fingers until we get it correct. Our focus is on the LTER sites and nearby weather stations.
|Henderson-Sellers, A. (1989) Increasing Cloud in a Warmer World. Climate Change. 9:267-309
Well the numbers are in for 17 of our LTER sites with 84 years of record beginning in 1900. On average cloudiness has increased by 12.4 %.
In all, there are 77 U. S. stations with cloud cover data for the period 1900 – 1984. On average these 77 stations have exhibited about a 10% increase over the time period studied by Henderson-Sellers. It might be noted cloudiness increased most rapidly between 1930 to 1950 and more of a plateau before and after. The cause of these decadal variations is not known.
Cloud-Climate Change Over Land
What really worries me and gives me great joy is cloud-climate change. The GCMs tell us clouds will change with 2XCO2. There is no doubt about it in my mind. Cloud type, height and amount will change. So I think cloudiness will change in some way! Is cloudiness already changing? Even without the be-all and end-all of global warming? You bet. Most climate variables have had systematic changes during the last 100 years or so. Why not clouds!
One of the problems we have had in knowing much about cloud climate is that most of the cloud observations are penciled in on paper data sheets or in paper books. They are not archived in clouds of electrons. The minions who keypunched themselves to tunnel-carpel syndrome in the 1960s and 1970s did not do clouds like cleaning persons don't do windows. In steps, Anne Henderson-Sellers, a go-getter of the first order, went to the dusty old ledger sheets to harvest the numbers: decade upon decade and continent upon continent. What a fantastic job! She could have kept it all to herself but she fessed-up and published her findings promptly. Her time series of cloudiness for North America are for the period 1900 to 1984. Sounds like the stuff long-term ecologists would salivate over. She did it for just about everywhere in North America that is anywhere! Perhaps even a station near you. In addition, she did the same for Europe, Asia and Australia. Herculean stuff. If you feel the need to walk up to her and shake her hand and say well done, you best go to Macquarie University in the burbs of Sydney, Australia.
It was chance, just chance that the VCR had the biggest change in cloudiness. In 1900, about 22% of the sky was cloud covered. By 1984 it had increased to 45.6%. In the case of the Norfolk record, much of the change happened before jets and jet contrails. Lots of people have suggested that the jets are doing it. Jet contrails were studied in the1960 over Iowa. Lots of jets crossed over the central Iowa navigation beacon. Calories of solar energy on a surface were measured for contrail days and non-contrail days. There was about an 8% reduction in solar radiation reaching the ground. Most places don't get that much contrail cloud in the sky. While contrails cannot be weeded out of Henderson-Sellers records, these big increases in cloudiness must be important plant-wise!
Henderson-Sellers, A.(1989). Increasing cloud in a warmer world. Climate Change. 9:267-309.
Henderson-Sellers, A.(1989). North American Total Cloud Amount Variation this Century. Paleogeography, Paleoclimatology, Paleoecology (Global and Planetary Change Section), 75:175-194.
Henderson-Sellers, A. (1987). Climate is a cloudy issue. New Scientist 23 July.
Henderson-Sellers, A. and K. McGuffie. (1989). Sulphate aerosols and climate. Nature 340:436-438.
Angell, J.K. 1990. Variations in United States cloudiness and Sunshine Duration between 1950 and the Drought Year of 1988. J. of Climate 3:296-308.
Many of you know that cloudiness IS going to decrease WHEN a CO2 warmed world is here -- for sure. The GCMs tell us so. A group at the University of Liverpool studied historical cloudiness records and concluded that when it warmed in the past it got cloudier not less cloudy. (New Scientist, 17 April 1986 p. 24 and 15 January 1987, p. 28.) This may be a wait and see thing.
Now the NASA's Earth Radiation Budget Experiment (ERBE) uses our eyes-in-the-skies (or at least our birds-in-space) to study the impact of cloudiness on the radiation budget. They find that more clouds in a warmer world lessen the overall increase in temperature. So, when it gets warmer, you get more clouds. You get more clouds and the clouds lessen the temperature increase. A negative feedback if I have ever seen one. GCMs say fewer clouds yields more warming, yielding in turn fewer clouds, yields more warming. Positive feedback if I have ever seen one. It will be interesting to see who is right: nature based on her (I think it is still a she) history OR our forward-looking GCMs.
For Europe, the U.S and the Indian Sub-continent, Henderson-Sellers found that the historical warming from the 1901-1920 period to the 1934-1953 of 0.3 C was accompanied by an increase in cloudiness of between 3% and 10%. 195 stations were used in these tabulations. Cloudiness increased almost everywhere in the U.S. over that period.
Jim Angell has also looked at the changes in cloudiness in the US in recent years. He compared the 28 years 1950-1968 with 1970-1988 and found a 3.5% increase in cloudiness (58% cloud cover to 60% and 2/58 * 100 = 3.5%) and a 1.2% decrease in sunshine duration. The changes were largest in fall and least in spring. He also found that El Niño years were cloudier and had a lower sunshine duration than non-El Nino years.
Jim Angell is at the Air Resources Lab. ERL, NOAA, Silver Spring, Md. The stations he used that are close to LTER sites are: Hartford, CN; Norfolk, VA; Wilmington, NC; Grand Rapids, MI; Wichita, KN; Minneapolis, MN; Denver, CO; Albuquerque, NM; El Paso, NM; Portland, OR; Green Bay, WS. His was a lower 48 study! Sorry Alaska and points at sea.
Cloud-Climate Change At Sea
DOE got into the climate business some years ago. One of the fruits of their work was an atlas publication titled "Global Distribution of Total Cloud Cover and Cloud Type Amounts over the Ocean" by Warren, Hahn, London, Chervin and Jenne. It is basically about clouds at sea.
The neat thing about the document was the collection of microfiche in the back of the atlas. It had graphs of the historical variation in cloud type and amount.
Now that your tongues are watering, I can tell you that I think the atlas is still free. Ask for DOE/ER-0406, Office of Energy Research, and Office of Basic Energy Sciences, Carbon Dioxide Research Division, and Washington, D.C. 80307. You might be able to get it from NCAR as well.
In any case, the atlas part is a little on the dull side but the time series are not dull at all. They tell the story that cloudiness at sea has changed over this century! Yikes, Batman is Global Warming here? Stratus clouds off the east coast of North America have increased. It has also increased off the east coast of Asia as well. It is most likely the cloudiness resulting from the plume of sulfate riding downwind from the fuel-using-to-keep-warm, mid-latitudes. We don't know if the continental cloudiness increases reported by Angell and by Henderson-Sellers is sulfate related or due to some natural process (read: not due to people-kind).
Cloudiness & Ecosystems
Well, lets consider that we may well have had a 15% increase in cloudiness and a perhaps an 8% decrease in duration of sunshine over the last 100 years. So what!
Cloudiness would mean higher (warmer) nighttime minimum temperatures. Nights have indeed warmed in this land of ours. It would mean cooler daytime temperatures. Yes, we see some of that in the record but not as strongly as the warmer nights. Daily temperature range is getting smaller. Sounds like Seattle! Fewer heating degree-days mean you need fewer logs for your tepee. A northward shift in the hardiness zones would be expected. (We have seen a southward shift!) Cloudiness may not tell us about extreme low temperatures since they always happen on clear nights with clean Canadian air.
Well, is that good or bad for our chlorophyll plastid-friends? Less evaporative demand perhaps is also expected. That would mean better water use efficiency. Fewer hours of sunshine could mean less primary production or, just a shift in the fortunes of war between the shade-loving and sun-loving troops is the consequence. Looks like the stuff that FORET modeler types might get all computationally excited about.
Well, for you and me, it means climate change is here. It has been here for some time and it may go on some more. It may or may not be related to greenhouse warming. The GCMs say cloudiness should be getting lower and lower! It is always fun to beat up on the modelers. They do computer experiments, they pretend the products are predictions and then, eventually, they must live in the bright light of observational data as it comes in. Then they say the observational data must be wrong! Talk about playing the game from behind the 8-ball!