If humanity has any hope of stopping the civilization-threatening climate change crisis, gas-powered cars have to go.

But how are people supposed to get around without them?

Public transit is one answer, as are electric cars. And then there are biofuels, which present the tempting halfway prospect of carbon-neutral automobiles that can use technology very similar to that which already exists. Perhaps we can get some more use out of the huge economic complex built up around gas engines ...

New York Times columnist Bret Stephens scoffed at biofuels in a recent column. But let's take stock of the state of the research and the industry. Is this a promising area for climate policy, or some kind of muddle-headed green boondoggle, as Stephens suggests?

The basic idea behind biofuels is to take agricultural products — or byproducts of other processes — and turn them into liquid fuels, generally either ethanol or diesel. Some plant or another grows using sunlight and carbon dioxide, you harvest and transform it into some workable fuel, and burn it in a modified internal combustion engine.

The idea is theoretically sound. If done right, producing and burning biofuels should release only a little more carbon dioxide than was pulled out of the air by the plant in the first place. But as in so many other areas of policy, effectiveness depends on details and implementation.

First, let's take corn ethanol, which is produced in the United States in vast quantities, backstopped by federal regulations and subsidies. Stephens is basically right to say that corn ethanol is no good. Given ideal conditions, it should be somewhat better than gasoline on carbon emissions. But if conditions are non-ideal — like if farmers plow up new land to plant more corn to grow ethanol, thus releasing lots of carbon dioxide — then it can easily become worse than traditional gasoline, and increase food prices. This has indeed happened.

All this is no mystery to climate hawks. That's basically what the EPA said back in 2011. I argued the corn ethanol mandate should be scrapped back in 2013.

However, sugarcane ethanol is a different story. In Brazil, they have developed a whole industrial complex around sugar ethanol, with efficient production systems using the sugarcane byproducts to fuel the processing plants, and price structuring to support sustained production. They have modified Brazilian cars to accept ethanol mixed with gas at any ratio, from pure gas to pure ethanol. While it has some problems (like any fuel system does), it is a functional system, cutting greenhouse gas emissions relative to gasoline by over 60 percent.

In theory, other ethanol feedstocks are even more promising. Switchgrass ought to be better than sugarcane, while giant miscanthus might even be a net negative for carbon — that is, it still might end up pulling more carbon dioxide out of the air than is released after harvesting, processing, and burning for fuel.

There's a similar story with biodiesel. Done right — like out of fryer grease, or with best-practices farming — it should be a large net positive for emissions. Done wrong, as with palm oil from clear-cut rainforest, it's a disaster.

Biofuels are obviously not without their problems. As this study explains in detail, a greater percentage of ethanol in a fuel mixture does increase acetylaldehyde emissions (since it is a natural partial combustion product of ethanol burning), as well as methane emissions. We don't want that. However, greater ethanol fractions also reduces the output of nitric oxide and nitrogen dioxide, as well as non-methane hydrocarbons — things you don't want to breathe either.

Moreover, it ought to be possible to further tune the emissions control systems of flex-fuel cars to further cut emissions. The above study and others note that the bulk of the bad emissions come during the warm-up phase of the car's operation, when the engine and the catalytic converter are below optimum operating temperature. Once the car is warmed up, emissions fall by a lot. With better technology (perhaps a catalyst warmer) or better chemistry, or both, we ought to be able to improve results. In general, more complete and efficient combustion is an achievable goal.

The reality of biofuel cars is similar to that of gas-powered cars back around the 1910s or so. It takes a lot of detailed, back-breaking industrial development to get from a paper theory to a functioning prototype to a viable large industry. Americans in those days had to set up thousands of ore mines, smelters, parts and assembly factories, as well as vast oil wells and refineries. There were side effects too, like back in the 1960s, when our cities were blanketed in suffocating, cancer-causing smog from the pollution-spewing cars of the day.

But factories were built, and side effects can be fixed. In the 1970s Congress mandated catalytic converters, gas engines got more efficient, the skies cleared up, and fewer people died of lung disease.

Standing up a huge new industry from scratch is not easy. Brazil has reached basic workability with sugarcane ethanol, which took decades of persistence, and it's not all the way there yet. Following in their footsteps with, say, giant miscanthus would require vast changes in farming, gigantic processing plants, alterations to car engines, and many other changes. No doubt there would be unforeseen difficulties, some of which might even be insurmountable — any new industrial paradigm is necessarily something of a crapshoot.

Only one thing is for sure: If we listen to ignorant, duplicitous climate trolls, we will never develop the advanced transportation technologies of the future, much less save future generations from climate apocalypse.