Alan Robock, a professor of environmental sciences at Rutgers, has published a list of 27 risks and concerns raised by the technology, including its potential to deplete the ozone layer and to decrease rainfall in Africa and Asia.
Ultimately, Robock worries that geoengineering may simply be too risky to ever try. “We don’t know what we don’t know,” he says. “Should we trust the only planet known to have intelligent life to this complicated technical system?”
Robock ignores the fact that the case for geoengineering is that the world is already being unintentionally geoengineered by the side effect of industrial and transportation emissions over a couple of centuries.
Harvard’s Schrag argued the opposite: that the scariest version of the future may be one where geoengineering is never developed or deployed. “I don’t think people understand just what we’re up against with climate,” he said. “The most likely scenarios for climate over longer time scales are devastating to future generations, absolutely devastating.”
As he flashed slides highlighting the dramatic loss of sea ice in the Arctic and Antarctic in recent months, Schrag stressed that climate change is already causing visible impacts faster than anyone expected. He added that it’s difficult to foresee any scenario where we can cut greenhouse-gas levels fast enough to avoid far worse dangers: the amount we’ve already released is likely to lock in another degree of warming even if we halt emissions tomorrow, he said.
Mitchell was opposed to geoengineering for most of his career. The idea that humankind should tinker with the finely tuned climate system struck him as impossibly arrogant. But like other researchers who spent decades staring at increasingly frightening projections while the world ignored the loudest warnings scientists knew how to sound, he reluctantly changed his view.
It could take decades to learn which geoengineering methods might work, whether environmental side effects can be minimized, and whether it’s ultimately too dangerous to try. The longer we wait to begin serious research, the greater the risk we’ll deploy an unsafe tool in the face of sudden climate shocks, or not have one in hand when we need it. And no one really knows when that might be.
Says Mitchell, “The need for climate engineering could be coming faster than we realize.”
The cost to construct a Stratospheric Shield with a pumping capacity of 100,000 tons a year of sulfur dioxide would be roughly $24 million, including transportation and assembly. Annual operating costs would run approximately $10 million. The system would use only technologies and materials that already exist—although some improvements may be needed to existing atomizer technology in order to achieve wide sprays of nanometer-scale sulfur dioxide particles and to prevent the particles from coalescing into larger droplets. Even if these cost estimates are off by a factor of 10 (and we think that is unlikely), this work appears to remove cost as an obstacle to cooling an overheated planet by technological means.
HIGH-FLYING BLIMPS, based on existing protoypes, could support a hose no thicker than a fire hose (above) to carry sulfur dioxide as a clear liquid up to the stratosphere, where one or more nozzles (below) would atomize it into a fine mist of nanometer-scale aerosol particles.
The stratosphere is the weather-free portion of the atmosphere at altitudes between about 10 kilometers and 50 kilometers, or 33,000 to 165,000 feet.) The attractiveness of this approach stems largely from the fact that it happens naturally during large volcanic eruptions, such as the eruption of Mount Pinatubo in the Philippines in 1991. Intensive scientific study of the Pinatubo eruption showed that sulfur dioxide aerosols injected high in the atmosphere cooled the planet by reflecting more incoming sunlight back into space. An even larger eruption in 1815 of Mount Tambora in Indonesia led to the second-coldest
year in the northern hemisphere in four centuries, the “year without a summer”.
Preliminary modeling studies suggest that two million to five million metric tons of sulfur dioxide aerosols (carrying one million to 2.5 million tons of sulfur), injected into the stratosphere each year, would reverse global warming due to a doubling of CO₂, if the aerosol particles are sufficiently small and well dispersed. Two million tons may sound like a lot, but it equates to roughly 2% of the SO₂ that now rises into the atmosphere each year, about half of it from manmade
sources, and far less than the 20 million tons of sulfur dioxide released over the course of a few days by the 1991 eruption of Mount Pinatubo. Scientific studies published so far conclude that any increase in the acidity of rain and snow as several million additional tons a year of SO₂ precipitate out of the atmosphere would be minuscule and would not disrupt ecosystems.
A rough first-order estimate is that injection of as little as 200,000 metric tons a year of sulfur dioxide aerosol into the stratosphere above this region could offset warming within the Arctic.
Although 100,000 tons a year sounds like a lot of liquid, when pumped continuously through a hose, that amounts to just 3.2 kilograms per second and, at a liquid SO₂ density of 1.46 grams per cubic centimeter, a mere 34 gallons (150 liters) per minute. A garden hose with a ¾-inch inner diameter can deliver liquid that fast.
It takes quite a bit of energy to lift material into the stratosphere: about 30 trillion Joules of potential energy, in fact, to lift 100,000 tons to a height of 30 kilometers. If the work is spread out over the course of a year, however, that energy translates to a required power of just 1,000 kilowatts. Inefficiencies and other practical considerations will increase this amount, possibly by several times; nonetheless, the power levels are not daunting by industrial standards.
To pump 34 gallons a minute up a 30-kilometer-long hose, the system must overcome both the gravitational head and the flow resistance. The gravitational head, which is simply another way of talking about the potential energy considered previously, would amount to a pressure of 4,300 bar (62,000 p.s.i.) if the liquid has a constant density of 1.46 g/cm³—not taking into account the small attenuation in the strength of gravity with increasing altitude.