Climate Controls

If we treated global warming as a technical problem instead of a moral outrage, we could cool the world.

Gregory Benford from the November 1997 issue

(Page 4 of 5)

Other sorts of reflectors at high altitudes are promising. Spreading dust in the stratosphere appears workable because at those heights tiny particles stay aloft for several years. This is why volcanoes spewing dust affect weather strongly. The tiny motes that redden our sundowns reflect more sunlight than they trap infrared.

Even better than dust are microscopic droplets of sulfuric acid, which reflects light more effectively. Sulfate aerosols can also raise the number of droplets that make clouds condense, further increasing overall reflectivity. This could then be a local cooling, easier to monitor than carbon dioxide's global warming. We could perform such small, controllable experiments now. The amount of droplets or dust needed is a hundredth of the amount already blown into the atmosphere by natural processes, so we would not be venturing big dislocations. And we would get some spectacular sunsets in the bargain.

As usual, there are human-centered concerns. The Environmental Protection Agency hammers away at particulate levels, blaming them for lung disorders. Luckily, high-
altitude dust would come down mostly in raindrops, not making us cough. The cheapest way of delivering dust to the stratosphere is to shoot it up, not fly it. Big naval guns fired straight up can put a one-ton shell 20 kilometers high, where it would explode and spread the dust. This costs only a hundredth as much as the space-parasol idea. But booming naval guns that rattle windows for miles around are likely to provoke more than a few Not in My Backyard reactions.

Fortunately, there is a ready alternative to dust in any form: jet fuel. Changing the fuel mixture in a jet engine to burn rich can leave a ribbon of fog behind for up to three months, though as it spreads it becomes invisible to the eye. These motes would also come down mostly in rain, not troubling the brow of the EPA. Fuel costs about 15 percent of airlines' cash operating expenses, and running rich increases costs by only a few percent. For about $10 million, this method would offset the 1990 U.S. greenhouse emissions. Adding this to the cost of an airline ticket would boost prices perhaps 1 percent. An added asset is that quietly running rich on airline fuel will attract little notice, doesn't even change sunsets, and is hard to muster a media-saturated demonstration against.

But there are, as always, side effects. Dust or sulfuric acid would heat the stratosphere, too, with unknown impact. Some scientists suspect the ozone layer could be affected. If a widespread experiment showed this, we could turn off the effect within roughly a year as the dust settled down and got rained out. (Smaller experiments should show this first, of course.)

These ideas envision doing what natural clouds do already as the major players in the total albedo picture. A 4 percent increase in stratocumulus over the oceans would offset global carbon dioxide emission. Land reflects sunlight much better than the wine-dark seas, so putting clouds far out from land, and preferably in the tropics, gets the greatest leverage.

Clouds condense around microscopic nuclei, often the kind of sulfuric acid droplets the geoengineers want to spread in the stratosphere. The oceans make such droplets as sea algae decays, and the natural production rate sets the limit on how many clouds form over the seas. Clouds cover about 31 percent of our globe already, so a 4 percent increase is not going to noticeably ruin anybody's day.

Tinkering with such a mammoth natural process is daunting, but in fact about 400 medium-sized coal-fired power plants give off enough sulfur in a year to do the job for the whole Earth. (This in itself suggests just how much we are already perturbing the planet.) There are problems with using coal: Arguing that more air pollution is good for Mother Earth sounds intuitively wrong. Coal plants sit on land, and the clouds would be most effective over the oceans. A savvy international strategy leaps to mind: Subsidize electricity-dependent industry on isolated Pacific islands, and ship them the messiest, sulfur-rich coal. The plants' plumes would stretch far downwind, and the manufactured goods could revitalize the tropical ocean states, paying them for being global good neighbors. The wealthy states would then get their mitigation carried out far from home and far from vexatious neighborhood committees, using labor purchased at low rates. And nobody has to take the plants; prices will mediate the demand.

A more boring approach, worked out by the National Academy of Sciences panel, envisions a fleet of coal-burning ships which heap sulfur directly into their furnaces. (Maybe some collaboration would work here. Freighters burning sulfur could also spread iron dust, combining the approaches, with some economies.) The ships spew great ribbons of sulfur vapor far out at sea, where nobody can complain, and cloud corridors form obediently behind. It would be best to use these sulfur clouds to augment the edges of existing overcast regions, swelling them and increasing the lifetime of natural clouds. The continuously burning sulfur freighters would follow weather patterns, guided by weather satellite data.

At first these could operate as regional experiments, to work out a good model of how the ocean's cloud system responds. This low-tech method would cost about $2 billion per year, including amortizing the ships.

The biggest political risk here lies with shifts in the weather. The entire campaign would increase the sulfur droplet content in our air by about 25 percent. Probably this would cause no significant trouble, with most of the sulfur raining out into the oceans, which have enormous buffering capacity. Keeping the freighters a week's sailing distance from land would probably save us from scare headlines about sudden acid rains on farmers' heads, since about 30 percent of the sulfur should rain out each day.

Albedo Chic

The NAS panel found that "one of the surprises of this analysis is the relatively low cost" of implementing some significant geoengineering. It might take only a few billion dollars to mitigate the U.S. emission of carbon dioxide. Compared with stopping people in China from burning coal, this is nothing.

We should not take the 1992 panel report, thick with footnotes and layers of qualifiers, to be a road map to a blissful future. The NAS estimates are simple, linear, and made with poorly known parameters. They also ignore many secondary effects. For example, forests promote clouds above them, since the water vapor they exhale condenses quickly. Those lovely cumulus puffs reflect sunlight. So growing trees to sop up carbon dioxide also increases albedo, a positive feedback bonus. But is that the end of the chain? No, because water vapor itself is a greenhouse gas. Thick clouds absorb infrared as well. If forests respire a lot, they can partially trap their own heat. Understanding this, and calculating it in detail, will take a generation of research.

But perhaps the greatest unknown is social: How will the politically aware public react--those who vote, anyway? If geoengineers are painted early and often as Dr. Strangeloves of the air, they will fail. Properly portrayed as allies of science--and true environmentalism--they could become heroes. Not letting the radical greens set the terms of discussion will matter crucially.

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