President Trump’s new proposal to erect a wall — this one a tariff-fueled steel and aluminum curtain — masks an inconvenient truth: The U.S. steel industry has been holding its own, thanks to advances in productivity and the spread of cheap natural gas and gas-fired electricity.

Trump has called his tariffs a way to halt the slide in U.S. jobs. Yet the slide in U.S. steelworker jobs has in large part been the result of increases in efficiency.

Labor productivity has seen a fivefold increase since the early 1980s, going from an average of 10 hours of work for each finished ton to an average of two hours in 2016, according to the American Iron and Steel Institute. Many North American plants were producing a ton of finished steel in less than one person-hour, AISI said.

Thanks to automation, workers in control rooms operate equipment that used to require hundreds of people. Open vessels once used to melt raw material have been replaced by more labor- and energy-efficient furnaces.

“The steel producers that survived did so by aggressively increasing physical productivity,” said Edward “Ned” Hill, a professor of economic development and urban planning at Ohio State University. “Labor costs are a very small part of production costs at this point. When you consider old pictures of cluttered steel mills, the level of automation has been fantastic.”

Hill said that the steel plant that Commerce Secretary Wilbur Ross, then a corporate restructuring expert, took out of its last bankruptcy in 2002 now uses one person-hour for every ton of steel it makes.

Trump’s proposed 25 percent tariff on steel imports wouldn’t stimulate “massive rehiring because there aren’t that many plants to fill,” Hill said. He said he was aware of only one idle plant in Ohio with equipment still intact. “You really can’t turn on a valve and — boom — up comes the steel plant,” he said.

Hill said it was more likely that workers at existing plants would get some overtime pay and that companies would be able to fatten up their profit margins.

Still, some economic studies say hiring gains in the steel industry could be substantial. A Washington-based consulting firm called the Trade Partnership estimates that the Trump tariffs would create 33,464 jobs in iron, steel and aluminum industries.

But the Trade Partnership said that job losses in other industries would far outweigh gains in the steel sector. For every steel job created, five would be destroyed in other sectors, resulting in an overall job loss of nearly 146,000 jobs.

Two-thirds of the job losses would hit production workers and those holding positions requiring low skills, the group said. The loss of jobs in other manufacturing industries would exceed the jobs created in steel and aluminum industries.

The consulting group said it used the same model as the Commerce Department in calculating the tariff rates needed to bring the U.S. steel industry to 80 percent of its capacity. It said it used more-recent data and took both direct and indirect job changes into account.

“The bet that the administration is trying to make is that the steel companies will increase their prices only to the point where they will undercut foreign competitors and pick up market share,” Hill said. “I say you forgot about step two in this dance: When you increase the price of your product, demand for your product will go down.”

The surge in shale gas from fracking is another reason domestic steel plants have been able to better compete with foreign manufacturers. Over the past decade, they have changed their fuel mix, saving money while reducing greenhouse gas emissions.

The natural gas boom has dovetailed with changes in the U.S. steel industry. Steel companies have switched about two-thirds of U.S. production away from blast furnaces producing molten iron to electric arc furnaces that use scrap as their main input. In an electric arc furnace, scrap is melted using electricity.

And thanks to the shale drilling boom, more of that U.S. electricity is generated by natural gas, which can also be used directly by steel factories.

According to the Energy Information Administration, the share of natural gas in the broader manufacturing fuel mix jumped more than sixfold from 2010 to 2014 and has jumped far more since. Natural gas has become cheaper than coal, and in combustion it emits about half the greenhouse gases.

This is important in helping U.S. firms compete with China, which was first described as the main target of Trump’s tariffs.

China does pose a significant threat, however. Its steel sector has flooded the world with excess capacity. A trickle of Chinese steel accounts for about 6 percent of U.S. steel imports, but China has contributed to a global glut that has affected prices and markets worldwide. Its capacity is eight times that of the next biggest producer, Japan, and a relatively slight increase in Chinese output would hurt prices and profitability globally.

Chinese steel plants have benefited and continue to benefit from cheap capital, sweetheart loans and the protection of provincial authorities who want to keep workers on steel payrolls. Often the steel firms have not paid for the land they occupy.

“The simple fact is that a great deal of investment in China is not conducted on a commercial basis,” said Thomas Rawski, an economist specializing in China at the University of Pittsburgh. “And that is certainly true in steel.”

However, while China’s steel industry was nurtured in part by cheap energy costs in the 1990s and 2000s, the Chinese government has been steadily raising those costs. Now Chinese industry pays about 50 percent more for electricity than manufacturers pay in the United States, according to Rawski.

The Chinese government has also launched campaigns to reduce capacity in steel and other industries.

That doesn’t always work out. The central government sets targets for provincial governments, but the provincial governments often favor inefficient state-owned enterprises that keep people employed.

The International Monetary Fund in its periodic review of China’s economy — known as an Article IV review — said that the government was cutting the number of steelworkers, but the overall capacity remained roughly the same.

“China’s protracted excess capacity has contributed to downward pressure on global prices, rising market share for Chinese firms, and tensions with key trading partners,” the IMF said.

But China, which sells mostly to its own domestic market, poses a smaller threat than it once did. David Fickling, a columnist for Bloomberg News, wrote: “The thing that China's steelmakers care much more about is their domestic market, which consumes about half of the world's steel and has been doing rather well of late.”

The U.S. steel industry today looks nothing like it did in the high-employment, high-wage glory days that the tariffs are designed to resurrect. The U.S. steel industry of that era — when firms made finished products from iron ore — has more in common with the Chinese steelworks of today than it does with modern U.S. domestic steel producers.

Unlike China, America’s steelmakers have pivoted toward “minimills,” more flexible, often smaller operations that process scrap and other materials without the overhead of a full steelworks.

This new, more energy-efficient and automated model — which currently exports steel to Canada and Mexico — might not need a tariff wall to make America great again.