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Is Net Zero Emissions an Impossible Goal?

What it would take to suck more carbon dioxide out of the air than we put in.

Water rushes into Venice’s city council chamber just minutes after the local government rejects measures to combat climate change.…By John Baez

Water rushes into Venice’s city council chamber just minutes after the local government rejects measures to combat climate change. Wildfires consume eastern Australia as fire danger soars past “severe” and “extreme” to “catastrophic” in parts of New South Wales. Ice levels in the Chukchi Sea, north of Alaska, hit record lows. England sees floods all across the country. And that’s just this week, as I write this.

Human-caused climate change, and the disasters it brings, are here. In fact, they’re just getting started. What will things be like in another decade, or century?

It depends on what we do. If our goal is to stop global warming, the best way is to cut carbon emissions now—to zero. The United Kingdom, Denmark, and Norway have passed laws requiring net zero emissions by 2050. Sweden is aiming at 2045. But the biggest emitters—China, the United States, and India—are dragging their heels. So to keep global warming below 2 degrees Celsius over pre-industrial levels by 2100, it’s becoming more and more likely that we’ll need negative carbon emissions. That is, we’ll need to fix the air. We’ll need to suck more carbon dioxide out of the atmosphere than we put in.

In the second half of the century we should be doing things that we can’t even dream of yet.

This may seem like a laughably ambitious goal. Can we actually do it? Or is it just a fantasy? I want to give you a sense of what it would take. But first, here’s one reason this matters. Most people don’t realize that large negative carbon emissions are assumed in many of the more optimistic climate scenarios. Even some policymakers tasked with dealing with climate change don’t know this.

In 2016, climate scientists Kevin Anderson and Glen Peters published a paper on this topic, called “The trouble with negative emissions.” The title is a bit misleading, since they are not against negative emissions. They are against lulling ourselves into complacency by making plans that rely on negative emissions—because we don’t really know how to achieve them at the necessary scale. We could be caught in a serious bind, with the poorest among us taking the biggest hit.

So, how much negative carbon emissions do we need to stay below 2 degrees Celsius of warming, and how people are hoping to achieve them? Let’s dive in!

In 2018, humans put about 37 billion tonnes of carbon dioxide into the air. A “tonne” is a metric ton, a bit larger than a U.S. ton. Since the oxygen is not the problem—carbon dioxide consisting of one atom of carbon and two of oxygen—it might make more sense to count tonnes of carbon. But it’s customary to keep track of carbon by its carbon dioxide equivalent, so I’ll do that here. The National Academy of Sciences says that to keep global warming below 2 degrees Celsius by the century’s end, we will probably need to be removing about 10 billion tonnes of carbon dioxide from the air each year by 2050, and double that by 2100. How could we do this?

Whenever I talk about this, I get suggestions. Many ignore the sheer scale of the problem. For example, a company called Climeworks is building machines that suck carbon dioxide out of the air using a chemical process. They’re hoping to use these gadgets to make carbonated water for soft drinks—or create greenhouses that have lots of carbon dioxide in the air, for tastier vegetables. This sounds very exciting … until you learn that currently their method of getting carbon dioxide costs about $500 per ton. It’s much cheaper to make the stuff in other ways; beverage-grade carbon dioxide costs about a fifth as much. But even if they bring down the price and become competitive in their chosen markets, greenhouses and carbonation use only 6 million tonnes of carbon dioxide annually. This is puny compared to the amount we need to remove.

crop circle: In 2004 the world created roughly 5 billion tonnes of “crop residue”: stems, leaves, and such left over from growing food. Managing these leftovers better could make a sizable dent in our carbon emissions. Fotokostic / Shutterstock

Thus, the right way to think of Climeworks is as a tentative first step toward a technology that might someday be useful for fighting global warming—but only if it can be dramatically scaled up and made much cheaper. The idea of finding commercial uses for carbon dioxide as a stepping-stone, a way to start developing technologies and bringing prices down, is attractive. But it’s different from finding commercial uses that could make a serious dent in our carbon emissions problem.

Here’s another example: using carbon dioxide from the air to make plastics. There’s a company called RenewCO2 that wants to do this. But even ignoring the cost, it’s clear that such a scheme could remove 10 billion tonnes of carbon dioxide from the air each year only if we drastically ramped up our production of plastics. In 2018, we made about 360 million tonnes of plastic. So, we’d have to boost plastic production almost tenfold.  Furthermore, we’d have to make all this plastic without massively increasing our use of fossil fuels. And that’s a general issue with schemes to fix the air. If we could generate a huge abundance of power in a carbon-free way—say from nuclear, solar, or wind—we could use some of that power to remove carbon dioxide from the atmosphere. But for the short term, a better use of that power is to retire carbon-burning power plants. Thus, while we can dream about energy-intensive methods of fixing the air, they will only come into their own—if ever—later in the century.

If plastics aren’t big enough to eat up 10 billion tonnes of carbon dioxide per year, what comes closer? Agriculture. I’m having trouble finding the latest data, but in 2004 the world created roughly 5 billion tonnes of “crop residue”: stems, leaves, and such left over from growing food. If we could dispose of most of this residue in a way that would sequester the carbon, that would count as serious progress. Indeed, environmental engineer Stuart Strand and physicist Gregory Benford—also a noted science-fiction writer—have teamed up to study what would happen if we dumped bales of crop residue on the ocean floor. Even though this stuff would rot, it seems that the gases produced will take hundreds of years to resurface. And there’s plenty of room on the ocean floor.

Human-caused climate change, and the disasters it brings, are here.

Short of a massive operation to sink crop residues to the bottom of the sea, there are still many other ways to improve agriculture so that the soil accumulates more carbon. For example, tilling the land less reduces the rate at which organic matter decays and carbon goes back into the air. You can actually fertilize the land with half-burnt plant material full of carbon, called “biochar.” Planting crops with bigger roots, or switching from annual crops to perennials, also helps. These are just a few of the good ideas people have had. While agriculture and soil science are complex, and you probably don’t want to get into the weeds on this, the National Academy of Sciences estimates that we could draw down 3 billion tonnes of carbon dioxide per year from improved agriculture. That’s huge.

Having mentioned agriculture, it’s time to talk about forests. Everyone loves trees. However, it’s worth noting that a mature forest doesn’t keep on pulling down carbon at a substantial rate forever. Yes, carbon from the air goes to form wood and organic material in the soil. But decaying wood and organic material releases carbon back into the air. A climax forest is close to a steady state: The rate at which it removes carbon from the air is roughly equal to the rate at which it releases this carbon. So, the time when a forest pulls down the most carbon is when it’s first growing.

In July, a paper in Science argued that the Earth has room for almost 4 million square miles of new forests. The authors claimed that as these new trees grow, they could pull down about 730 billion tonnes of carbon dioxide. At first, this sounds great. But remember, we are putting out 37 billion tonnes a year. So, the claim is that if we plant new forests over an area somewhat larger than the U.S., they will absorb the equivalent of roughly 20 years of carbon emissions. In short, this heroic endeavor would buy us time, but it wouldn’t be a permanent solution. Worse, other authors have argued that the Science paper was overly optimistic. One rebuttal points out that the Science paper mistakenly assumed treeless areas have no organic carbon in the soil already. It also counted on a large increase of forests in regions that are now grassland or savanna. With such corrections made, it’s possible that new forests could only pull down at most 150 billion tonnes of carbon dioxide.

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That’s still a lot. But getting people to plant vast new forests will be hard. Working with more realistic assumptions, the National Academy of Sciences says that in the short term we could draw down 2.5 billion tonnes of carbon dioxide per year by planting new forests and better managing existing ones. In short: If we push really hard, better agriculture and forestry could pull 5.5 billion tonnes of carbon dioxide from the air each year. One great advantage of both these methods is that they harness the marvelous ability of plants to turn carbon dioxide into complex organic compounds in a solar-powered way—much better than any technology humans have devised so far. If we ever invent new technologies that do better, it’ll probably be because we’ve learned some tricks from our green friends.

And here’s another way plants can help: biofuels. If we burn fuels that come from plants, we’re taking carbon out of the atmosphere and putting it right back in: net zero carbon emissions, roughly speaking. That’s better than fossil fuels, where we dig carbon up from the ground and burn it. But it would be even better if we could burn plants as fuels but then capture the carbon dioxide, compress it, and pump it underground into depleted oil and gas fields, unmineable coal seams, and the like.

To do this, we probably shouldn’t cut down forests to clear space for crops that we burn. Turning corn into ethanol is also rather inefficient, though the corn lobby in the U.S. has persuaded the government to spend lots of money on this, and about 40 percent of all corn grown in the U.S. now gets used this way. Suppose we just took all available agricultural, forestry, and municipal waste, like lawn trimmings, food waste, and such, to facilities able to burn it and pump the carbon dioxide underground. All this stuff ultimately comes from plants sucking carbon from the air. So, how much carbon dioxide could we pull out of the atmosphere this way? The National Academy of Sciences says up to 5.2 billion tonnes per year.

Of course, we can’t do this and also sink all agricultural waste into the ocean—that’s just another way of dealing with the same stuff. Furthermore, this high-end figure would require immensely better organization than we’ve been able to achieve so far. And there are risks involved in pumping lots of carbon dioxide underground.

What other activities could draw down lots of carbon? It pays to look at the biggest human industries: biggest, that is, in terms of sheer mass being processed. For example, we make lots of cement. Global cement production in 2017 was about 4.5 billion tons, with China making more than the rest of the world combined, and a large uncertainty in how much they made. As far as I know, only digging up and burning carbon is bigger: For example, 7.7 billion tons of coal is being mined per year.

Right now cement is part of the problem: To make the most commonly used kind we heat limestone until it releases carbon dioxide and becomes “quicklime.” Only about 7 percent of the total carbon we emit worldwide comes from this process—but that still counts for more than the entire aviation industry. Some scientists have invented cement that absorbs carbon dioxide as it dries. It has not yet caught on commercially, but the pressure on the industry is increasing. If we could somehow replace cement with a substance made mostly of carbon pulled from the atmosphere, and do it in an economically viable way, that would be huge. But this takes us into the realm of technologies that haven’t been invented yet.

While we can dream about energy-intensive methods of fixing the air, they will only come into their own—if ever—later in the century.

New technologies may in fact hold the key to the problem. In the second half of the century we should be doing things that we can’t even dream of yet. In the next century, even more so. But it takes time to perfect and scale up new technologies. So it makes sense to barrel ahead with what we can do now, then shift gears as other methods become practical. Merely waiting and hoping is not wise.

Totaling up some of the options I’ve listed, we could draw down 1 billion tonnes of carbon dioxide by planting trees, 1.5 billion by better forest management, 3 billion by better agricultural practices, and up to 5.2 billion by biofuels with carbon capture. This adds up to over 10 billion tonnes per year. It’s not nearly enough to cancel the 37 billion tonnes we’re dumping into the air each year now. But combined with strenuous efforts to cut emissions, we might squeak by, and keep global warming below 2 degrees Celsius.

Even if we try, we are far from guaranteed to succeed. Anderson and Peters are quite right to warn about this. But will we even try? That is more a matter of politics and economics than of science and technology. The engineer Saul Griffith said that dealing with global warming is not like the Manhattan Project—it’s like the whole of World War II but with everyone on the same side. He was half right: We are not all on the same side. Not yet, anyway. Getting leaders who are inspired by these huge challenges, rather than burying their heads in the sand, would be a big step in the right direction.

John Baez is a professor of mathematics at the University of California, Riverside and a visiting researcher at the Centre for Quantum Technologies in Singapore. He blogs about math, science, and environmental issues at Azimuth. Follow him on Twitter @johncarlosbaez.

Lead image: Stefano Mazzola

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