James Hansen and colleagues have a new paper out. I notice that the press release that accompanies the paper includes the following claim:

[Hansen et al.] also dismiss the notion of “geo-engineering” solutions, noting that the price of artificially removing 50 ppm of CO2 from the air would be about $20 trillion.

Well, how does 50 ppm of CO2 for $20 trillion compare to other ways of reducing CO2?

One ppm of CO2 contains about 2.1 billion tonnes of carbon (a gigatonne, abbreviated GtC). So 50 ppm is about 105 GtC. At $20 trillion this equates to about $190 per ton of carbon, or about $52 per ton of carbon dioxide (you need about 3.7 tonnes of carbon dioxide to get one tonne of carbon). To 2030, $20 trillion represents about 1.5% of cumulative global GDP. How do these values compare to, say Stern, IEA, or IPCC estimates?

Stern suggests that we should be ready to pay between 1% and 5% of GDP to 2050. The IPCC presents a range of costs up to $200 per tonne of CO2 to 2030 (Figure TS.9 here in PDF). The IEA suggests the costs of stabilizing emissions at current levels at $50 per tonne of CO2, and the costs of cutting them by half have marginal costs of $200 to $500 per tonne (see this report).

So the costs of air capture that Hansen et al. so readily dismiss as too costly are about the same as, or even less than, those costs for other forms of mitigation proposed by Stern, IEA and IPCC. If one accepts the costs presented by Hansen et al. (drawn from the work of David Keith), then one must come to one of the following conclusions:

1. We should be pursuing direct capture of carbon dioxide from the air with great vigor because the costs are in the same ballpark as approaches proposed by Stern, IEA and IPCC, or,

2. If air capture can be dismissed as too expensive, then so too are the approaches proposed by the Stern, IEA and IPCC.

What you should not conclude is that air capture can be summarily dismissed as too expensive. There may be other reasons not to like air capture beyond costs (and which Hansen et al. do not raise). But $20 trillion for 50 ppm seems like a bargain, particularly for those, like Hansen et al., who think that carbon dioxide levels above 350 ppm represent the end of planet Earth as we know it. I am amazed that these guys can argue that the end of the world looms, and we must spend a small fraction of our global GDP over the next half century to reduce the risks, yet so readily dismiss a solution that they accept costs a small fraction of GDP. What gives?