Crudely put, America and the world have until 2050 to eliminate all carbon dioxide emissions if we want to minimize the worst risks of global warming. That's not necessarily as daunting as it seems: For about 70 percent of human energy use — areas like transportation, electricity generation, heating and cooling residential and commercial buildings — zero-carbon technology is already here. We just have to put it in place.

Other sectors are tougher nuts to crack. In particular, heavy industry accounts for about a fifth of U.S. carbon emissions, and a quarter of the globe's. Think of steel-making, cement making, smelting aluminum for electronics, or manufacturing chemical products like ammonia — a critical ingredient in making everything from fertilizer to pharmaceuticals to plastics.

"Industry is what we categorize as a 'hard to abate sector,'" Mark Paul, a fellow at the Roosevelt Institute who recently co-authored a paper on comprehensively decarbonizing the American economy, told The Week. "We don't necessarily have all of the technology at our fingertips right now to fully and rapidly decarbonize industry."

Nor is heavy industry important simply for its carbon footprint. It also produces a lot of other forms of pollution that lead to disease and death; eliminating fossil fuels from heavy industry will eliminate those pollutants, too. At the same time, it's not as if the world's need for steel and chemical products is shrinking. And heavy industry can be a prominent source of both good-paying blue collar jobs as well as the sorts of tradable goods that can keep smaller rural economies afloat. Transforming heavy industry, rather than merely reducing it, will be key.

Obviously, these sectors draw on a lot of electrical power, and that can be decarbonized. What makes them tricky is that individual plants and factories also burn plenty of fossil fuels on site too: Steel production relies on furnaces; the cement and glass industries require kilns; chemical products involve boilers; paper and pulp industries require both steam production and heat to dry their products.

This fossil fuel burning is often dispersed between lots of individual places and businesses. Cement, for example, is heavy and costly to transport, so the industry is characterized by lots of small manufacturers scattered hither and yon — all burning their own fossil fuels — rather than a few big centralized sites. About 35 percent of heavy industries' emissions come from the need for extremely high temperatures: above 500 degrees Cesius, and above 1,200 and 1,400 degrees in the cases of steel and cement specifically. Another 20 percent of emissions come from lower temperature needs. A lot of that is coal for furnaces and kilns, and natural gas for boilers, for example.

But there are also chemical processes in various industries that release carbon dioxide as part of the reaction, and these make up the remaining 45 percent. Cement production involves the calcination of limestone, a chemical process that gives off excess carbon dioxide, along with the emissions from the heating required to set off the reaction. Steel-making has its furnaces, but it also requires the chemical reduction of iron. Creating aluminum starts with aluminum oxide, which is then run through an electrical-driven chemical reaction. Both of these processes expel copious carbon emissions. Ammonia production requires hydrogen, derived mostly from natural gas, in a conversion process that also gives off carbon dioxide.

Eliminating all those carbon emissions will require a few different changes at once.

We'll need to generate heat with zero-carbon technologies, for example. Done properly, that can be achieved with biofuels or biomass. For lower heating needs, there are also ways to gather and use solar energy directly. But a lot of it will rely on electrical technologies: electric arc furnaces for steel-making, electric kilns for cement and glass, creating hydrogen via electrolysis of water rather than using natural gas for ammonia.

The advantage of electrifying everything is that, as mentioned, we already know how to make electrical generation itself completely renewable: once the industries' processes are switched over to electricity rather than direct fossil fuels, we just have to green the grid. The downside is that, for many of these new technologies, the electrical demand is enormous; we won't have to generate the same amount of electricity we currently do but from green sources, we'll have to generate a lot more.

Another challenge will be coming up with new chemical processes that can achieve the same result without greenhouse gas emissions: Finding new raw materials from which to generate cement, for example, or using biodiesel as the starting point for ethylene production. And in a few instances where we can't get the carbon emissions out of production entirely, we may just have to use that product less. (In building construction, for example, it might behoove us to get serious about using materials other than cement and concrete.)

"I think there's no question we could cut emissions in the industrial sector by half without a problem with given technologies," Paul explained. "But fully offsetting those emissions will definitely require additional technological development."

An experiment is underway in Sweden to setup a zero-carbon steel plant: It will rely on electric arc furnaces, and use hydrogen from electrolysis to reduce the iron. It will be powered by Sweden's electrical grid, which is already 50 percent renewable, and on its way to more. Meanwhile, a company named Elysis, with backing from Apple, is working on a new zero-carbon process for creating aluminum that gives off oxygen instead — and that can be retrofitted onto existing aluminum plants. Those are two examples of the kinds of innovations we'll need. And to give you an idea of the challenge, they're not expected to be up-and-running until 2035 and 2024, respectively.

But as I said up top, we can get going on decarbonizing 70 percent of the economy right now. That gives us time to cut emissions elsewhere, then ramp up reductions in heavy industry as we get closer to the middle of the century. Which raises the question: How do we close the gap between where we are and where we need to be?

A big part will be lots more research and development, to innovate the new technologies and industrial models we'll need. To their credit, some of the Democratic presidential candidates pushing big climate proposals are including massive increases in government spending to fund green technology research. Paul pointed out that private corporations are also currently sitting on mountains of unused cash; but they're probably waiting on the U.S. government to make it clear it's going to decarbonize the economy come hell or high water.

"Once the government sets a clear target for that sector, and sets them on a path to decarbonization, I think that's when you'll see companies unleashing investments," Paul said. This would probably require a raft of new regulations which have yet to be figured out; but you could imagine something like the government's fuel economy standards, except for heavy industrial processes rather than automobiles.

Next, we need to actually deploy these technologies. That's no small matter: Entire plants and factories, which are already expected to give their investors returns for decades as it is, will have to be retrofitted or remade. That will mean big one-time costs. And while companies could certainly shoulder those costs on their own, that could endanger employment, and put them at a competitive disadvantage to companies that are more recalcitrant. Again, the government can step in here, by providing funding to industrial players that are seeking to green themselves. That can be done with direct public grants, subsidized loans, or cheap public credit provided by something like a green infrastructure bank.

As a backstop, some kind of market mechanism would also be needed, to make sure the long-term prices of going green are preferable to business as usual. A carbon tax is en vogue among many mainstream economists, but Paul and his co-authors argue a properly designed carbon cap is a better way to put the economy on a reliable long-term trajectory to zero emissions.

Finally, what of the rest of the world? A Green New Deal for America would only change American industry. And as much as the U.S. remains a major world player, China now accounts for the bulk of both production and consumption of materials like steel, cement and ammonia.

There are several ways we could push the rest of the world along: two possibilities are using carbon tariffs, or making carbon reductions a centerpiece requirement of our trade deals. But as Paul put it, the rest of the world is already well aware of the challenge. What they need, more than anything, is for countries with the size and resources to bring these new industrial technologies to maturity as fast as possible, and then spread them as widely as possible to lower their costs.

"The best thing the U.S. can do, in terms of aiding other countries and pushing for deep decarbonization, is really just deploy, deploy, deploy."

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