If the costs of global warming become large enough, we can brighten clouds to reverse global warming.
(p. A1) A little before 9 a.m. on Tuesday [April 2, 2024], an engineer named Matthew Gallelli crouched on the deck of a decommissioned aircraft carrier in San Francisco Bay, pulled on a pair of ear protectors, and flipped a switch.
A few seconds later, a device resembling a snow maker began to rumble, then produced a great and deafening hiss. A fine mist of tiny aerosol particles shot from its mouth, traveling hundreds of feet through the air.
It was the first outdoor test in the United States of technology designed to brighten clouds and bounce some of the sun’s rays back into space, a way of temporarily cooling a planet that is now dangerously overheating. The scientists wanted to see whether the machine that took years to create could consistently spray the right size salt aerosols through the open air, outside of a lab.
If it works, the next stage would be to aim at the heavens and try to change the composition of clouds above the Earth’s oceans.
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(p. A14) Brightening clouds is one of several ideas to push solar energy back into space — sometimes called solar radiation modification, solar geoengineering, or climate intervention. Compared with other options, such as injecting aerosols into the stratosphere, marine cloud brightening would be localized and use relatively benign sea salt aerosols as opposed to other chemicals.
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“I hope, and I think all my colleagues hope, that we never use these things, that we never have to,” said Sarah Doherty, an atmospheric scientist at the University of Washington and the manager of its marine cloud brightening program.
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But it’s vital to find out whether and how such technologies could work, Dr. Doherty said, in case society needs them. And no one can say when the world might reach that point.
In 1990, a British physicist named John Latham published a letter in the journal Nature, under the heading “Control of Global Warming?,” in which he introduced the idea that injecting tiny particles into clouds could offset rising temperatures.
Dr. Latham later attributed his idea to a hike with his son in Wales, where they paused to look at clouds over the Irish Sea.
“He asked why clouds were shiny at the top but dark at the bottom,” Dr. Latham told the BBC in 2007. “I explained how they were mirrors for incoming sunlight.”
Dr. Latham had a proposal that may have seemed bizarre: create a fleet of 1,000 unmanned, sail-powered vessels to traverse the world’s oceans and continuously spray tiny droplets of seawater into the air to deflect solar heat away from Earth.
The idea is built on a scientific concept (p. A15) called the Twomey effect: Large numbers of small droplets reflect more sunlight than small numbers of large droplets. Injecting vast quantities of minuscule aerosols, in turn forming many small droplets, could change the composition of clouds.
“If we can increase the reflectivity by about 3 percent, the cooling will balance the global warming caused by increased C02 in the atmosphere,” Dr. Latham, who died in 2021, told the BBC. “Our scheme offers the possibility that we could buy time.”
A version of marine cloud brightening already happens every day, according to Dr. Doherty.
As ships travel the seas, particles from their exhaust can brighten clouds, creating “ship tracks,” behind them. In fact, until recently, the cloud brightening associated with ship tracks offset about 5 percent of climate warming from greenhouse gases, Dr. Doherty said.
Ironically, as better technology and environmental regulations have reduced the pollution emitted by ships, that inadvertent cloud brightening is fading, as well as the cooling that goes along with it.
A deliberate program of marine cloud brightening could be done with sea salts, rather than pollution, Dr. Doherty said.
For the full story see:
(Note: ellipses, and bracketed date, added.)
(Note: the online version of the story has the date April 2, 2024, and has the title “Warming Is Getting Worse. So They Just Tested a Way to Deflect the Sun.”)
The article by the physicist John Latham, published in the one of the top two journals in science, and mentioned above, is:
Latham, John. “Control of Global Warming?” Nature 347, no. 6291 (Sept. 27, 1990): 339-40.