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British Prime Minister Theresa May botched the Brexit file and, for that epic miscalculation, she lost her job. As she slinks away from 10 Downing St., she wants to ensure that her legacy includes at least one charming footnote and, to that end, she chose climate change.

Ms. May didn’t do much on the climate-change file during her three tortured years in office – it was all Brexit, all the time. She tried to rectify that omission this week by committing Britain to “net-zero” carbon emissions by 2050, the first big, wealthy country to do so. It’s a monstrously ambitious and expensive target which won’t be reached, with skeptics predicting the whole effort will get nowhere near its target, resulting in national humiliation.

But – pip, pip – it’s worth the effort even if the skeptics are partly or largely right.

Britain has done a credible job in reducing its carbon emissions as compelling evidence emerges that the planet is on course for catastrophic climate change. Since the 1992 Rio de Janeiro Earth Summit, when “global warming” entered the lexicon of fear, Britain’s emissions have fallen by more than 40 per cent.

That figure is somewhat less impressive than meets the eye. Britain has switched off a lot of its coal-burning electricity plants and switched on a lot of renewable-energy systems (wind and solar), but the country exported much of its carbon footprint. As factories closed, Britain imported manufactured goods from countries – China, India – whose industries turned the skies black with coal pollution.

Ms. May’s goal, which stands to be endorsed by Boris Johnson, the cycling fanatic who will almost certainly replace her this summer, comes with a built-in loophole. Note the term “net” in the net-zero goal. That means Britain will use carbon-offset credits to effectively bring its emissions down. For instance, if Britain funds the planting of millions of trees, or builds a solar farm, overseas, it will apply those credits against its national carbon-output tally. So, in reality, Britain’s domestic emissions will never be zero.

Her plan met with both praise and scorn. One of her own ministers, Philip Hammond, Chancellor of the Exchequer, warned that the tab to reach zero emissions might be financially crippling. In a letter to Ms. May that was leaked to the press, he said “the total cost of transitioning to a zero-carbon economy is likely to be in excess of a trillion pounds.”

The number sounds terrifying, but supporters of Ms. May’s plan point out that £1-trillion represents less than half a year of British gross domestic product; in other words, the expense is pretty small when you measure it over 30 years. Of course, Mr. Hammond’s estimate could be wildly off the mark. It could be trillions more, depending on what costs are thrown in. If the transition to zero emissions tosses hundreds of thousands of workers onto the jobless rolls, the retraining bill alone could be enormous.

Mr. Hammond’s warning gains credibility when you consider that, in Britain and a few other parts of the world, the gains from renewable energy are, relatively speaking, fairly small.

According to the World Bank’s sustainable-energy index, 17 per cent of the world’s energy consumption came from renewables in 1990. By 2015, the figure had climbed by only one percentage point, to 18 per cent. While today’s figure is no doubt higher, it’s probably not much higher. That’s because overall energy demand is surging and renewables can only meet some of that new demand. As long as hydrocarbons, especially coal, the dirtiest fuel, remain cheap, they will be the default choice for poor countries motoring up the development curve. Carbon-dioxide emissions rose 2 per cent in 2018, the biggest increase in seven years. Global oil demand keeps rising, in good part because we are enamoured with plastics.

In Britain, renewables have come on strong, and coal use is plummeting. But still, some 80 per cent of the country’s overall energy supply comes from fossil fuels, government data says. That figure will have to fall to almost zero by 2050. But how? To get there, carbon emissions from transportation (where the use of oil is still rising), home heating (where gas boilers dominate) and electricity generation will have to be largely eliminated.

Of those three areas, good progress has been made only in electricity generation and, even there, the growth curve could flatten out. The problem is plain economics – too much of a good thing depresses the value of that good thing. As renewables take a bigger share of the market, electricity prices will fall, to the point the power they spew out can be essentially worthless on the days when the wind is blowing hard and the sun is strong. Create enough electricity gluts and the renewable energy companies’ return on investment will drop, acting as a disincentive for more investment in renewables.

Britain’s effort to reach net-zero emissions will have a better chance of success if carbon taxes, and lots of them, are slapped on fossil fuels to make emissions prohibitively expensive. But “taxes” are a dirty word in every country – see the gilets jaunes protests in France and the carbon-tax rebellion in some Canadian provinces – and they hurt the poor more than the rich. Even if they are designed to be revenue-neutral, they are a hard sell.

Britain’s carbon emissions will continue to fall faster than those in most countries. Under Ms. May’s plan, the fall will certainly accelerate. But getting to zero by 2050 seems impossible. Still, the price of doing nothing would be costlier still, given the scary rate of climate change. Ms. May blew the Brexit file. The climate-change file just might have a happier ending for her even if her goal is not reached.