Why Spraying Sulfur into the Sky is Not Crazy

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We primarily have two climate problems. One is global warming from carbon dioxide, and the other is global warming from tipping points.

We can solve the carbon dioxide problem by replacing coal, oil, and gas with solar, wind, hydro, and nuclear. However, this will probably not solve the tipping point problem. We seem to be tipping too fast. Therefore, we probably need to reflect a tiny percentage of sunlight back into outer space. There are several ways to do this, one of which is to inject sulfur into the atmosphere, above where airplanes typically fly.

Our society is already emitting sulfur

Sulfur is an element in the periodic table, and large amounts of sulfur are contained within coal and oil. For this reason, sulfur is typically emitted into the atmosphere when these are burned.

Sulfur is harmful to people, plants and oceans. Therefore, governments often require some sulfur to be filtered out, before or after combustion. However, even with some filtration, approximately 100 million tons of sulfur dioxide gas (SO2) are emitted globally into the atmosphere each year.

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Sulfur cools the planet

After SO2 gas is emitted into the atmosphere, it typically combines with water and oxygen to form H2SO4. It then nucleates, which means it converts to a tiny physical particle. Water sticks to this particle, and eventually grows into a physical water droplet. These are so small and so sparsely populated, they are often imperceptible to the naked eye.

Droplets with sulfur typically reflect more sunlight than droplets without. Therefore, more sulfur causes sunlight to reflect back into outer space, instead of being absorbed by the planet. In effect, sulfur cools the planet.

High-altitude sulfur is way cooler

As noted previously, sulfur is contained within coal and oil, and is therefore emitted upon combustion. In theory, we can filter more of it out before emission, move the harvested sulfur to an airplane, and emit it at a high altitude instead of at ground level.

High-altitude sulfur stays aloft for a year or two, while ground-level sulfur typically stays aloft for several days. Therefore, changing the emissions site reduces the temperature of the planet, while not increasing total sulfur emissions. The latter point is important, since sulfur is harmful, as noted previously.

How much does this cost?

To justify the expense, our society would need to compare the cost of cooling the planet with the cost of not cooling the planet. One study suggests large-scale planet cooling would cost $18 billion a year. For comparison, the total value of New York City property is $1.4 billion. And this is just one coastal city that would be lost to sea level rise.

If the U.S. paid half, planet cooling would cost each American $27 per year (50% x $18 billion/330 million).

Ozone makes high-altitude air warmer

Air temperature varies with altitude in unobvious ways, as shown in the below graph.

Air temperature versus altitude. Values shown are typical. (Source: Glenn Weinreb)

A plot of temperature versus altitude shows that it gets colder as we go up. However, this reverses at about 20km altitude since ozone at that height absorbs heat and causes temperatures to become warmer as one goes even higher. In other words, we have a layer of relatively cold air at about 20km altitude, and even warmer air between about 20km and about 50km.

Also, warm air rises, which means that material injected at about 20km will go up, and not quickly fall to earth due to gravity. Therefore, one gram of sulfur injected at about 20km will cool the planet approximately 100× more than one gram injected at ground level.

Twenty-kilometer airplanes

Most commercial airplanes can only fly to 12km (40,000 ft). However, dozens of airplanes are capable of reaching 20km (65,000 ft).

If one increases the wing surface area and engine diameter of a commercial aircraft, then maximum altitude increases, maximum horizontal velocity decreases and fuel efficiency decreases. In other words, special-built aircraft could potentially provide service to 20km, with less horizontal flight efficiency.

Atmospheric reflectivity R&D

Increasing the reflectivity of the atmosphere is a new field and there are many things we do not know. We do not know exactly what to inject, when, where and how. And we do not have an accurate assessment of costs, and adverse side effects.

To resolve unknowns, we need to do R&D. This includes developing better instrumentation for measuring atmospheric reflectivity, developing equipment that injects small amounts of material for field experiments, and developing equipment that injects large amounts of material for full-scale operations.

How much sunlight must be reflected?

The below graphic shows sources of global warming.

Sources of global warming and global cooling in units of Watts per square meter of Earth surface area. (Source: Wikipedia)

Total global warming is approximately 3 Watts per square meter of Earth surface area, on average over a 24-hour period. This is the amount of energy that enters the earth’s atmosphere from the sun, minus the amount of energy that leaves the planet due to outgoing heat radiation (i.e. earth energy imbalance). And this is proportional to the warming-rate (i.e. rate of average global temperature increase in units of degrees-Celsius-increase-per-decade).

The amount of energy that enters the atmosphere from the sun is 340 Watts per square meter of Earth surface area, on average over a 24-hour period. Therefore, reflecting approximately 1% of sunlight (1% of 340) back into outer space will fix the climate problem, in theory.

In summary, scientists and engineers need to figure out how to do this, at reasonable cost, without inflicting harm.

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