Over at MR, Alex Tabarrok highlights a very interesting memo written to President Obama by Larry Summers in 2010 about the green energy loan guarantee program. While Prof Tabarrok highlights the economic aspects of the memo, this program was intended to improve the environment, so it is more interesting to hear it evaluated in those terms:

Carbon reduction benefits: If this wind power displaced power generated from sources with the average California carbon intensity, it would result in about 18 million fewer tons of CO2 emissions through 2033. Carbon reductions would have to be valued at nearly $130 per ton CO2 for the climate benefits to equal the subsidies (more than 6 times the primary estimate used by the government in evaluating rules).

In other words, the memo is pointing out that subsidizing this wind farm involves spending $1.2 trillion in order to prevent emissions that will do $200 million in damage.