On Indulgences and Carbon Offsets

Catholics have a notion called “Indulgences”. Wikipedia summarizes it as “a way to reduce the amount of punishment one has to undergo for sins.” In the Middle Ages, indulgences were commercialized, so wealthy people could buy themselves out of punishment (or the loss of wealth might be considered the punishment, if you want to be charitable).


In the modern world, carbon emission is a great sin. Those traveling by air sin the most. Prominent environmentalists are often targeted for hypocrisy, and those hoping to avoid hypocrisy might purchase “Carbon Offsets“. These are the more equivalent of commercial indulgences. The airlines offer them to guilt-ridden passengers.

Perhaps the most obvious, ‘common sense’, solution when demand (pollution) is in excess of supply is to expand capacity.  This is what we do with most things if we can. If our house is too small, we make it bigger. If our wallet can’t hold all of our cash and ID cards, we get a bigger one. If the internet is too slow, we add capacity. In roads, this usually means adding lanes to existing roads. Perhaps we could plant more trees to absorb more carbon pollution.

Consider for instance the Boston to London round trip. It is 3255 miles each way (5237 km) and 1.1799 metric tons of Carbon roundtrip. For $14.16 or 1,888 Award Miles a United Airlines passenger can support the  Alto Mayo Conservation Initiative. Objectively this is not a lot of money in the scheme of things, and maybe it will offset your trip. I don’t have the impression most travelers purchase these indulgences.

More importantly, I don’t think this scales. Some estimates below:


A Trans-atlantic flight might require 11 trees per person per flight to do a full offset.  There are about 100 million international enplanements from the US per year.  Not all are Transatlantic of course, many are Trans-Pacific or to South America, and so longer. I will leave it to a research paper to figure out total distance. So that is on the order of 1100 million trees per year (probably more) to be planted to guiltlessly offset US international air travel.  Let’s assume 5m x 5m per tree (25 m^2). 25*1100M = 27,500 million square meters to offset international aviation from the US (excluding US domestic aviation and travel in other countries. That is 27,500 km^2, or an area of about 165 x 165 km on edge per year (for say 50 years until aviation switches to biofuels). This is the size of Massachusetts.

While that is technically feasible, since the US has lots of land (and is more than 50x the size of Massachusetts, as Massachusetts is a smaller than average state), no-one is actually doing this, and the offset is over the life of the tree, not immediate, so we would need one Massachusetts per year until the end of carbon-emitting aviation to make offsets work.

I like to think in terms of queues. The environment can clear (absorb) a certain amount of CO2 per year, basically the equivalent of net zero carbon emissions. If there is a positive amount of emissions, the CO2 queues in the atmosphere, waiting to be absorbed. (And probably doing things to the environment we wish were otherwise.)

If offsets are not employed, the alternative is that the accumulated CO2 queue from US Transatlantic enplanements will continue to grow. We could pull out Kant’s categorical imperative “Act only according to that maxim whereby you can, at the same time, will that it should become a universal law.” and argue since this doesn’t scale (can’t become a universal law), you shouldn’t do it. But that’s the sort of philosophical nonsense that we hope philosophers have recovered from.

Just because it can’t solve the entire problem and can’t become universal doesn’t mean it can’t be useful to plant more trees.  Trees are good. However, while a carbon offset indulgence may absolve you from guilt on a particular trip, it cannot absolve the industry, since it cannot scale.  Imagine the number of trees required for all aviation, not just international, and for auto travel (about 10x aviation). A more serious solution is required, one which either takes CO2 out of the air more efficiently, produces less CO2 per flight (through say biofuels or electric power), or reduces the number of CO2 emitting activities like flights (and internal combustion engine car trips) (by reducing travel).

Now to be clear, if you expand the capacity of the planet to absorb pollution (i.e. plant more trees), and people pay for their pollution, the reduced cost of per unit of pollution means that people will pollute more. Drivers will travel longer, industry will use less socially efficient means for energy generation. There might be a small amount of GDP growth associated with both the geo-engineering and resource extraction, so it is not entirely a bad thing, but it may not solve your pollution problem.