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.
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