When last we checked in on California’s plans to build a high-speed rail line from San Francisco to Los Angeles, the situation was grim. Costs had shot upward and legislators were nervous. So then Gov. Jerry Brown’s administration revised its bullet-train plan, whittling the price tag down to $68 billion.

An artist’s rendition of a gizmo that, while nifty, is not the best way to reduce carbon emissions. (Anonymous/AP)

To help fill the gap, Brown’s administration has proposed using money from California’s new climate law. Under the state’s cap-and-trade system for carbon emissions, power plants and factories will soon have to buy permits to pollute. Brown has hinted at diverting tens of billions of dollars into high-speed rail. But there are two problems here. For one, it’s not clear this is legal, as the state’s Legislative Analyst’s Office concluded on Tuesday. Second — and more broadly — high-speed rail may not be the most efficient use of money that’s supposed to be used to combat global warming.

Some rough numbers help show this: The California High Speed Rail Authority claims that by 2030, if the train ran entirely on renewable energy, then it would start reducing the state’s carbon emissions by about 5.4 million metric tons per year. That would mean the rail network would cut California’s emissions at a cost of, at the very low end, $250 per ton of carbon dioxide over the ensuing 50 years, given the system’s current price tag. (This is being extremely generous, since it ignores the energy used to build the system — by some estimates, high-speed rail would actually increase emissions in its first few decades.)

And that’s a pricey way to cut carbon. To put this in perspective, research has suggested that you could plant 100 million acres of trees and help reforest the United States for a cost of somewhere betwen $21 to $91 per ton of carbon dioxide. Alternatively, a study by Dan Kammen of UC Berkeley found that it would cost somewhere between $59 and $87 per ton of carbon dioxide to phase out coal power in the Western United States and replace it with solar, wind and geothermal. If reducing greenhouse gases is your primary goal, then there are a slew of more cost-effective ways to do it than building a bullet train.

Now, to be clear, this is not an argument against high-speed rail. There are all sorts of non-climate reasons why California might want a rail system: It might boost economic development in cities along the route, or offer residents more convenient transportation options, or allow the state to build fewer roads and airports. It’s even possible that, over the very long run, a passenger rail system could spur denser development that, in turn, reduces California’s dependency on automobiles. (That’s a more diffuse calculation and isn’t figured into the carbon numbers above.)

But as the Legislative Analyst’s Office notes, it’s harder to justify devoting scarce climate funds toward a new high-speed rail system on the grounds that it’s the most effective way for California to curb its carbon emissions. Especially since California is already in a race to get its carbon pollution back down to 1990 levels by 2020.

Update: Terrible typo in my math on carbon costs. Fixed.