A common liberal talking point is that, based on the historical record, reducing greenhouse gases should be fairly easy. After all, acid rain was once a terrible problem, but government regulations have brought it under control in most developed countries (though it remains a scourge of the developing world). Similarly, chlorofluorocarbons (CFCs, a family of chemicals sometimes called by the trade name Freon) once threatened to destroy the ozone layer, which shields the Earth from nasty ultraviolet rays. But thanks to a series of international agreements, CFCs, too, have been phased out almost entirely.

Neither of these tasks was easy, but both were critically important, and as soon as their urgency became clear, they were largely accomplished within a couple of decades. It might seem, then, that a similar push could reduce carbon dioxide (and other greenhouse gases) to acceptable levels just as quickly. But cutting carbon dioxide emissions is a much greater task than either of these precedents would suggest. Making a significant dent in America’s output would require not just modest adjustments in a few industries, but wholesale changes in virtually every sector of the economy. It’s like the difference between getting a tune-up and buying a new car.

Consider acid rain. It has been causing trouble since the start of the Industrial Revolution — killing plant and animal life, altering soil chemistry, eating away at buildings and monuments, and harming human health. The biggest culprit is sulfur dioxide, an impurity present in coal and other fossil fuels; the other main acid-rain gases are nitrogen oxide and nitrogen dioxide, collectively known as NOx.

What makes the acid-rain story seem so appealing is that industrial emissions were reduced through a cap-and-trade scheme similar to the one proposed in the current Waxman-Markey bill. Companies were allocated permits (called “allowances”) based on the amount of sulfur dioxide they were already emitting. Over time, the tonnage corresponding to each allowance was reduced, which meant emissions had to be reduced. Companies that cut down by more than the necessary amount (by installing scrubbers on smokestacks, shutting plants, improving efficiency, using cleaner fuel such as natural gas, treating coal to remove impurities, or shifting to other forms of energy) could sell their extra allowances to companies that had not made sufficient reductions. The result: a 40 percent decrease in sulfur dioxide emissions in the United States since 1990, with further reductions to come. (A separate allowance scheme, but with no provision for trading, has achieved similar results for NOx.)

Can cap-and-trade reduce carbon dioxide emissions as successfully as it did with sulfur dioxide? The Environmental Defense Fund thinks so: “What is cap and trade? It’s the policy that stopped acid rain. Our goal is to apply the same principles to stop climate change.” Sam Napolitano, director of the EPA’s Clean Air Markets Division, told SolveClimate.com that the acid-rain regime has stimulated engineering creativity, and if Waxman-Markey passes, “our expectation is that industry will again figure out [solutions] beyond what we are already aware of what we can do.”

Unfortunately, the two problems are vastly different in scale. The key point to understand is that sulfur is an impurity, present in fossil fuels in small amounts (usually less than 1 percent). But carbon is fossil fuels; it’s what they’re made of (usually 75 to 90 percent). Cutting sulfur dioxide is not complicated: If other methods don’t work, a power plant that burns coal (which accounts for about half of America’s electricity) can bring its emissions below the cap with a scrubber. They don’t come cheap; a typical model costs about $100 million, with additional running costs. Once the scrubber is in place, though, the problem is largely solved; you might even make some of the cost back by selling your unneeded allowances.

Reducing America’s carbon dioxide output to any significant degree would be a much greater problem. Engineers are studying a technology called carbon sequestration, in which a power plant’s carbon dioxide output is collected, liquefied, and stored underground. This method is already being tested, and on a pilot-project scale it can probably be made to work. But cost would be a major obstacle: $1 billion or so to retrofit an existing plant, or a similar amount to equip a new one, plus hundreds of millions to install a pipeline between the plant and the nearest geologically suitable burial site. An even bigger obstacle would be the energy requirement: Depending on the method, as much as 40 percent of a power plant’s output would go to running the sequestration and pumping systems. It’s also far from clear that enough suitable and accessible storage sites can be found.

Until carbon sequestration becomes cheap and dependable — which will quite possibly turn out to be never — the only way to release less carbon dioxide into the atmosphere is to produce less of it in the first place. Switching from coal to cleaner-burning natural gas helps somewhat, so the trend toward building gas-fired plants may accelerate, though the future supply and price of gas are uncertain enough to make investors nervous. Outside of that, the only real alternative is to get more power from renewable sources. The problem with renewable sources is that environmentalists hate the ones that are cheapish and dependable (nuclear and hydroelectric), while the ones that environmentalists love are expensive and unreliable (wind, solar, geothermal, etc.). So if industry is forced to generate lots more power from wind and solar sources, the price of energy will get much higher and outages will be frequent.

Basically, cap-and-trade for sulfur dioxide creates an expense item in a company’s budget; cap-and-trade for carbon dioxide creates an entirely new, very expensive business model. Acid-rain reduction has turned out to cost American industry only $1 to $2 billion per year, much less than predicted, but according to a Heritage Foundation study, the cost of Waxman-Markey would be easily 100 times as great. That’s why the smooth success of acid-rain reduction is not a valid analogy for carbon dioxide. Another difference: Acid rain is a local problem; dirty air in Eastern Europe and China has little effect on America (though dirty air in Mexico is another matter). But greenhouse-gas emissions anywhere in the world increase the atmospheric concentration everywhere, so whatever reductions America makes will be overwhelmed by increases from rapidly industrializing Third World nations.

CFCs are another environmental success story. They were developed in the early 1930s as non-toxic, non-flammable gases for use in refrigeration and air conditioning; later they were also widely employed as aerosol propellants and in making foams, among other uses. In the mid-1970s CFCs’ harmful effects on the ozone layer became clear, and it wasn’t long before the leaders of most industrial nations understood the severity of the problem (Margaret Thatcher, trained as a chemist, was one of the most ardent proponents of vigorous action).

The turning point was the Montreal Protocol of 1987, an international agreement under which the production of CFCs has been almost completely phased out worldwide. With this reprieve, the ozone layer is coming back and is expected to recover completely by the middle of this century. Here again, a major pollutant was brought under control in a couple of decades through government action. Could something similar work for carbon dioxide?

Probably not. The most important point about CFCs is that substitutes for them are readily available. For nearly every application, chemists have found other molecules that work as well or nearly as well, cost about the same, and harm the ozone layer much less. (One of these substitutes, ironically, is carbon dioxide.) But when you’re burning fossil fuels, there’s no substitute for carbon, since that’s where the energy comes from.

Moreover, just as with sulfur dioxide, the scale of the CFC problem is vastly different from that of greenhouse gases. For one thing, they were used in relatively small amounts. If you add up all the CFCs that have ever been produced in the entire world, the total is about 25 million metric tons. That’s one-thousandth of today’s annual global production of carbon dioxide from fossil fuels. Also, CFC use was concentrated in a few specific industries, whereas power and transportation, the main sources of carbon emissions, are intimately involved in just about every corner of the economy. At the same time, CFCs are much more dangerous: Unlike carbon dioxide, they are synthetic chemicals with no natural mechanisms to remove them from the air; they can linger in the upper atmosphere, causing mischief, for 50 years or more. Together, these factors made their near-complete eradication much easier and cheaper, as well as much more imperative, than even modest reductions in greenhouse gases will be.

To be sure, there are lessons to be learned from the CFC and acid-rain success stories. For example, technology can do amazing things to help keep nature pristine, and markets are a vital tool in cleaning up the environment. But the most important lesson is that the whole world can cooperate to defuse an environmental threat — if the nature of the threat is sufficiently clear, and the cost is judged to be worth it. However great and however certain the threats from climate change may be, the costs involved in preventing it will be vastly bigger that they were with CFCs and acid rain, and environmental advocates do the Earth no favors by pretending any different.