The Cost of Geoengineering

I wanted to get an idea of how much it would cost to undo global warming by engineering the atmosphere. A friend stated emphatically that it was cheap, but I wasn’t so sure. So, I decided to do a back-of-the-envelope calculation. You can find more reliable numbers elsewhere, but I’m posting this anyway, in case someone might find it fun to see what one can do with some numbers found with Google, some arithmetic, and an hour or two of spare time.

The simplest thing one could do to change the climate (aside from burning fossil fuels) is to eject a chemical into the atmosphere that reflects sunlight back into space. The obvious candidate chemical is sulfur dioxide. In 1991, Mount Pinatubo erupted, and put somewhere between 17 million and 20 million tons of sulfur dioxide into the stratosphere. As a result, the Earth cooled by about 0.5 degree Celsius for a few years. To temporarily alleviate global warming of a few degrees, we would need to put a comparable amount of sulfur dioxide (within factors of a few) into the atmosphere each year.

How much would it cost to inject this much stuff into the upper atmosphere? As a rough estimate of the cost, I assumed that it would cost about the same amount as getting things into the air by airplane. A 747 freighter uses about 10,000 kg of fuel to take off to a height of 10,000 feet, for a gross take-off weight of 360,000 tons, about 140 tons of which are cargo. Fuel costs $0.70 per kg ($2/gallon, density of 2.8 kg/gallon). So, I get a cost of $50 per ton to get things into the air, based on the fuel alone. That estimate doesn’t get the material into the stratosphere, but most of the air resistance is near the ground, so this shouldn’t be too far off (feel free to correct me if I’m wrong).

So, I estimate that getting 20 million tons of sulfur dioxide into the stratosphere will cost at least $1 billion dollars a year. My number should be accurate to within an order of magnitude. I don’t know whether it is expensive to make sulfur dioxide (probably not), and the distribution system might have different costs than a simple airliner (these don’t typically fly in the stratosphere, after all).

Nonetheless, my estimate isn’t too different from one reported in ArsTechnica (quoting an
article in Geophysical Research Letters Alan Robock and collaborators): they estimate $4 billion dollars if one uses F-15Cs, and $375 million dollars if one uses KC-135 Stratotankers. However, from the description at ArsTechnica (I’d have to use a computer at work to get the actual paper), the authors of the proper study seem to think that we only need to add enough sulfur dioxide to counter the warming trend, so that Mount Pinatubo only needs to be reproduced once every four to eight years. This sounds to me like an underestimate, but either way, the numbers are within my order-of-magnitude tolerance.

My friend suggested that a single developing country might undertake geoengineering on its own. Setting aside the political problems, I also wanted to know, is this amount of money reasonable? I think so. For comparison, the budget of India’s space agency is about $750 million dollars a year (assuming an exchange rate of $1 to 46.7 Rupees, where a Crores Rupees is 10 million Rupees). Space agencies are a bit of a luxury, so $1 billion does not seem out of reach.

This scheme is also inexpensive compared to the cap-and-trade legislation that is heading through congress, which may cost on order $150 billion per year, and that’s for the U.S. alone.

So, in terms of cost, my friend is probably right: putting sulfur in the stratosphere is relatively cheap and accessible, especially when compared to changing our economy to use less carbon.

However, I still don’t think that anyone will do it, because fears of the potential side effects will probably dissuade us. The sulfur dioxide from Mount Pinatubo exacerbated the hole in the ozone layer, for instance. However, who am I to divine the heart of man? If you want politics, FiveThirtyEight probably has a better discussion of those issues.