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Contrails and Aviation-cirrus
Clouds (03)

Clouds could clear way to saving planet

By bouncing more incoming sunlight back into space we could buy time to sort out global warming, 
writes Kate Ravilious in The Guardian on Thursday February 10, 2005

Earth looks as if it is about to overheat. Temperatures are rising, ice sheets are melting and all the evidence points towards a greenhouse future. But what if we could reduce the planet's temperature? Would that give us some time to wean ourselves off fossil fuels and find alternative sources of energy?

This is what a group of eminent atmospheric physicists and an engineer are proposing, and they have come up with an idea to halt the Earth's warming. Using nothing more than salt water and wind power, they have designed a device that will increase the reflectivity of some of the Earth's clouds, bouncing more incoming sunlight back into space. They argue that this natural heat shield could be turned on and off at will, giving us a vital extra few decades to sort out the mess we are in. 

John Latham, an atmospheric physicist based at the National Center for Atmospheric Research in Colorado, first came up with the idea about 15 years ago. "I outlined my idea in Nature, but at that time there wasn't a strong awareness of the global warming problem and so there wasn't a big response," he says. But more recently, the idea of a greenhouse world has become a dinner-party conversation topic and suddenly everyone is interested in ways of preventing the Earth from turning into a sauna. Together with colleagues, Latham has resurrected the idea and this time people are starting to take it seriously.

Clouds come in different colours, shapes and sizes and occur at various altitudes; not just any old cloud will do. An increase in the high-level, wispy, cirrus clouds would actually have the opposite of the desired effect: making the Earth warmer as they trap more heat in. It turns out that the low-level, lumpy grey clouds, known as stratocumulus, are the best for the job, bouncing sunlight back into space, off their bright, shiny tops. Which is all very well, but how do you go about making stratocumulus cloud more reflective?

Stephen Salter, the innovative Edinburgh University engineer, (known best for his invention of Salter's duck - the 300-tonne floating canister designed to drive a generator from the motion of bobbing up and down on waves) thinks he has the key. "We need to atomise seawater and throw tiny droplets into the air," he says. The idea is that this fine mist of sea-spray evaporates, leaving tiny particles of sea salt that get sucked up into marine stratocumulus clouds on rising currents of air. These little particles act as centres for extra droplets to form. "Clouds become more reflective if you increase the number of droplets in them," explains Latham. A bonus of filling the clouds with smaller droplets is that they tend to last for longer, reflecting more sunlight back into space, before they disperse.

To produce this fine mist of sea spray artificially, Salter envisages thousands of unmanned yachts zigzagging across the sea, carrying equipment to make very choppy waves, known as Faraday waves. A high-frequency ultrasonic generator would spin seawater around inside a grooved drum, producing tiny waves that are thinner than a human hair. "It looks a bit like a cup of coffee on a rattling train, but it would be nearly vertical," says Salter. Once the waves are steep enough, drops of water are thrown up from their crests. "All we need to do is try and get these fine droplets into the first few metres of air, and meteorology will do the rest," says Latham.

To remain truly environmentally friendly, the yachts would be driven by wind acting on the spinning drum, like a sail. Movement of the boat through the water would drive propellers acting as turbines, to produce the electrical power for spinning the drums and driving the ultrasonics. Meanwhile, satellites would direct their movements, placing the yachts in the areas of ocean where the most effective stratocumulus clouds could be modified.

But would it really work? If calculations and computer models are to be believed, then yes, the physics of this idea is sound. Working together with Tom Choularton, of Manchester University, and Mike Smith, of Leeds University, Latham has done extensive calculations to make sure he has got his sums right. In addition, they have tested the idea using the Meteorological Office's Global Climate Model and shown that increasing the droplet numbers in marine stratocumulus clouds could have a significant effect. "Modifying an area covering around 3% of the Earth's surface produced a cooling that more or less balances the warming from doubled carbon dioxide levels," says Latham. 

Now the scientists are looking for funding to take their idea to the next stage, testing it with a small-scale pilot project over the oceans, using natural stratocumulus clouds. If they find that the extra cloud brightness would do its job and that there would be no adverse effect to rainfall on land, then Latham's original idea may begin to make good sense. Nonetheless, all the scientists stress that increasing cloud brightness is not a long-term solution to global warming, but a stopgap. "Our endeavours are directed towards buying time," says Latham. If recent reports of the melting of the Antarctic ice cap are anything to go by, we may need to start buying time very soon. 

What did you think of this article? Mail your responses to and include your name and address.

Amstelveen, February 13th, 2005

Dear Kate Ravilious,

Who guarantees that the 'extra time' will be spent wisely, and not just on developing bigger SUV's and more remote spots on Earth to fly to for a long weekend at an exotic swimming-pool?
     An ever bigger part of stratocumulus already is not purely natural at all, but a secondary result of aviation. 

     Just a part of the Sun's radiation (light and warmth) will be reflected by the stratocumulus back into space. A part will be trapped between layer of (partly man-made) man-made stratocumulus and the almost permanent layer of (also man-made) aviation-cirrus on a higher altitude. This high region of our troposphere will be warmed and will be able to contain more water. This kind of messing with our troposhere will make our man-made problems only worse.
      By seeding lower clouds with massive quantities of condensation-nuclei, aviation-cirrus is becoming a major cause of too much rainfall in some regions, and too little in others. This effect will be worsened by even more messing with our originaly quite well-balanced climatic system. Nobody can accurately enough predict where the extra man-made rain will fall.
      This method adds insult to injury and drives out the devil with Be�lzebub. The 'good news', apparently, is only that we can continue to burn fossile fuels for a while longer. Who would finance this enterprise, I wonder.
      We must, instead, 'just' start NOW with measures and rulings to effectively diminish the burning of fossile energy for all kinds of idiotic purposes. And everybody can think of a few of these highly unnecessary purposes for himself..

Kind regards,
Robert C. Van Waning
Amstelveen, The Netherlands

Mixed Layer Analysis of Stratocumulus. 
Simulations and Observations

By: Douglas Lilly, University of Oklahoma

KNMI Colloquium, May 12th, 2005


Although meteorologically benign, stratocumulus clouds are now seen to be an important element in global and regional climate dynamics, and the focus of several recent observational and numerical simulation programs.

A critical factor in stratocumulus analysis is the rate of cloud-top entrainment, which determines whether the cloud layer will grow, shrink, or evaporate. This presentation is in 4 parts, the first 3 of which include mostly completed work, with the fourth more speculative.

a) Rationalization of sharp-edged (zeroth order) cloud-top assumptions
b) Entrainment prediction model development

c) Application to DYCOMS2 data
d) Energy equation analysis of entrainment
prediction model

I show that the entrainment prediction model verifies well against the Faloona, et al analysis of DYCOMS2 data. I regard it, however, as interim, to be tested and probably improved or replaced by careful consideration of the complete kinetic energy equation.

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