Earlier this month, the South Coast Air Quality Management District approved a $4-million program to put a mustache on the Mona Lisa — at least that’s how it seems to me. What the agency actually did was approve spending millions to take 35 or so of the greenest, most energy-efficient sedans ever made — the hybrid gasoline-electric Toyota Prius — and turn them all into dirty energy guzzlers.

It is going to achieve this giant leap backward by converting the hybrids to run on hydrogen, the most overhyped alternative fuel since methyl tertiary-butyl ether, or MTBE.

Hybrids are already extremely efficient. The Prius, for example, generates only about 210 grams of carbon dioxide — the principal heat-trapping gas that causes global warming — per mile. The car is also a partial zero-emission vehicle, which means that when it uses California’s low-sulfur gasoline, it produces very little of the smog-forming pollutants, like nitrogen oxides.

Hydrogen is not a primary fuel, like oil, that we can drill for. It is bound up tightly in molecules of water, or hydrocarbons like natural gas. A great deal of energy must be used to unbind it — something the AQMD plans to do by electrolyzing water into its constituents: hydrogen and oxygen. And because the resulting hydrogen is a gas, additional energy must be used to compress it to very high pressures to put it in the tank of your car.

With all the energy needed to create and compress that hydrogen — even with the relatively clean electric grid of California — a Prius running on hydrogen would result in twice as much greenhouse gas emissions per mile as an unmodified car. It would result in more than four times as much nitrogen oxides per mile.

I own a Prius, so that’s the hybrid I am most familiar with. But Honda also makes a hybrid vehicle, and thanks to California’s leadership in vehicle emissions regulations, many other car companies plan to introduce them soon. These cars will get even greener over time as technology improves.

Sadly, two of the features I love most about my car would be wiped out by the AQMD’s expensive “upgrade.” First, the hybrid has cut my annual fuel bill by half. Hydrogen is so expensive to make that even with California’s high gasoline prices, the hydrogen hybrid will have more than four times the annual fuel bill of a gasoline hybrid. Second, my car can go twice as far on a tank of gas as my old Saturn, so I have to make those unpleasant trips to the gas station only half as often. The hydrogen hybrid would have less than half the range of my car. With hydrogen fueling stations so scarce, hydrogen hybrid drivers will constantly be scampering back to the fueling stations before the tanks get too low.

Why is the AQMD spending millions of dollars to increase pollution and destroy all the desirable features of one of the greenest, most efficient cars ever made? It has bought into the hype about hydrogen, the myth that this miracle fuel will somehow solve all of our energy and environmental problems.

When I was helping to oversee clean-energy programs at the U.S. Department of Energy in the mid-1990s, I too was intrigued by hydrogen, mainly because of recent advances in fuel cells. Fuel cells are electrochemical devices that take in hydrogen and oxygen and generate electricity and heat with high efficiency. The only “emission” is water. They have been an elusive technological goal since the first fuel cell was invented in 1839. During the 1990s, we increased funding for hydrogen tenfold and for transportation fuel cells threefold.

I began to change my mind about hydrogen while researching a book over the last 12 months. After speaking to dozens of experts and reviewing the extensive literature, I came to realize that hydrogen cars still needed several major breakthroughs and a clean-energy revolution to be both practical and desirable.

A recent Energy Department report noted that transportation fuel cells were 100 times more expensive than internal combustion engines. Historically, even the most aggressively promoted energy technologies, such as wind and solar power, have taken 20 years just to see a tenfold decline in prices.

The most mature onboard hydrogen storage systems — using ultrahigh pressure — contain 10 times less energy per unit volume than gasoline, in addition to requiring a significant amount of compression energy. A National Academy of Sciences panel concluded in February that such storage had “little promise of long-term practicality for light-duty vehicles” and urged the Department of Energy to halt research in this area. Yet this kind of storage is precisely what the AQMD plans to put in its hydrogen hybrids.

Another problem with hydrogen is in how it is made. Although people seem to view hydrogen as a pollution-free elixir, hydrogen is just an energy carrier, like electricity. And, like electricity, it is no cleaner than the fuels used to make it. For the next several decades, the National Academy panel concluded, “it is highly likely that fossil fuels will be the principal sources of hydrogen.” Making hydrogen from fossil fuels won’t solve our major environmental problems.

It’s possible, of course, to make hydrogen with renewable electricity, such as solar and wind power, but that is a lousy use for renewables, since they can directly displace more than four times as much carbon dioxide from coal power compared with using that renewable power to make hydrogen for vehicles. And these savings can all be achieved without spending hundreds of billions of dollars on a new hydrogen infrastructure and hydrogen vehicles.

As one 2002 British study concluded, “Until there is a surplus of renewable electricity, it is not beneficial in terms of carbon reduction to use renewable electricity to produce hydrogen — for use in vehicles, or elsewhere.” That surplus is, sadly, a long way off, given that Congress hasn’t been willing to pass legislation requiring that even 10% of U.S. electricity in 2020 be from renewables like wind and solar.

Finally, delivering renewable hydrogen to a car in usable form is prohibitively expensive today — equal to gasoline at $7 to $10 a gallon — and likely to remain so for decades in the absence of major technology advances.

For at least several decades, hydrogen cars are exceedingly unlikely to be a cost-effective solution for global warming. Until we achieve major breakthroughs in vehicle technology, hydrogen storage, hydrogen infrastructure and renewable hydrogen production, hydrogen cars will remain inferior to the best hybrids in cost, range, annual fueling bill, convenience, roominess, safety and greenhouse gas emissions.

While we wait, California should continue to lead the way in building renewable-power generation and in advancing the most environmentally responsible cars in the world — hybrid partial zero-emission vehicles.