One of the more uncomfortable, little-discussed aspects of global warming is that we’ve reached the point where sharp cuts in emissions alone are unlikely to be enough to avoid big temperature increases.

Climate models suggest we’ll also need to figure out how to pull some carbon dioxide back out of the atmosphere toward the end of the century if we want to stay below the agreed-upon 2°C warming limit. The UN's most recent "Emissions Gap" report gives an illustration:

So how do we go "carbon-negative"? Unfortunately, no one really knows for sure. We could plant more trees, but there's only so much land to go around. We could deploy giant carbon-sucking fans, but that would be incredibly expensive and likely have limited effect.

Perhaps the most promising idea involves a technology called "bioenergy with carbon capture and sequestration." Basically, you harvest plants that are absorbing carbon dioxide from the atmosphere, burn them for electricity, and then capture the CO2 that’s emitted from those power plants and bury it underground. The result? Negative emissions. To date, however, few people have studied this technology — or its limits — in detail.

That's what makes this recent study in Nature Climate Change so fascinating. The authors, led by Daniel Sanchez of the University of California–Berkeley tried to model in detail how far western North America could actually go in deploying carbon-negative biomass plants. There are real limits: You can’t just cut down trees willy-nilly or you can actually make climate change worse.

In an optimistic case, power emissions fall 145 percent below 1990 levels by 2050

The good news? Even a limited number of carbon-negative power plants could help reduce emissions drastically, they found. In a truly optimistic case — if combined with lots of wind and solar — western North America could have a power system in 2050 with emissions that are 145 percent below 1990 levels. That is, emissions don't just fall to zero. The electricity system would actually remove carbon dioxide from the air.

The sober news is that this technology is still in its infancy, far from commercialization, and expected to be quite expensive. And it can't really substitute for other ways to cut emissions; it's more of a complement. But this modeling effort suggests the technology is worth pursuing further.

"There are all these climate models telling us we have to go carbon negative by 2060 or so," Sanchez told me. "But they don’t say much about how to get there — what power plants do you build? Where do you build them? How do you make it sustainable? That’s what we tried to do here."

How to build a carbon-negative power plant

The basic concept of a "bioenergy with carbon capture and sequestration" (BECCS) power plant is fairly straightforward.

Start with this idea: Trees and crops pull carbon dioxide out of the atmosphere as they grow. If we take those plants and burn them for fuel, that CO2 gets released back into the atmosphere. If, however, we replace those burned plants with new plants that also suck up CO2, the whole thing is carbon-neutral, in theory. (Yes, there are caveats here, we’ll get to them shortly.)

Now let’s go a step further: Say we took those plants and burned them to generate electricity. But we then captured all the CO2 that came out of the smokestack of our power plant and buried it underground, permanently. And we completely replaced those burned plants with new plants. On net, we’re actually removing CO2 from the atmosphere.

Notice that this is very similar to the concept of using carbon capture and storage for, say, coal or natural gas. Except CCS for coal or gas can only be carbon-neutral at best. CCS for plant matter can actually be carbon-negative.

There are lots of different ways to design a BECCS plant. We could use trees, crops, or leftover crop residue. There are also a variety of methods for capturing CO2 emissions from a power plant. We could scrub the CO2 out of the flue gas that comes up the smokestacks. Or we could gasify our fuel before it's burned and remove the CO2 chemically.

Finally, we have to find an underground saline aquifer or depleted oil field that can hold the CO2 permanently.

Designing a sustainable carbon-negative plant is tricky

One of the big problems with trying to use biomass as a clean energy source, however, is that it can be hard to do sustainably. That’s one of the big questions Sanchez’s study tried to figure out.

Say, for instance, we chop down a forest full of older trees and burn them for energy. We then plant a bunch of seedlings to replace them. That’s not perfectly carbon-neutral: the new trees are smaller than the trees we cut down, so they’re absorbing less CO2, at least for their first few decades. We’ve just worsened global warming. Bad news.

Or let’s say we want to use plant residue — branches that have fallen on the forest floor, or the corncobs and stalks left over on farms in Iowa. Surely that’s all useless stuff that would have decayed and released its CO2 anyway, right? Well, maybe not. Some of that residue might have helped soils absorb more carbon. So we have to be careful. (Most research suggests only a fraction of the residue can be harvested sustainably.)

In their Nature Climate Change paper, Sanchez and his coauthors were careful to take into account research on sustainable practices. And that put real limits on things. They found that there was only enough sustainable biomass in the western United States to provide about 1.9 × 10^9 MMBtu of energy by 2050.

To put that in perspective, that’s only enough to satisfy about 7 to 9 percent of projected electricity demand in western North America by 2050. Not a ton. The good news, however, is that even a small number of plants could make a big difference.

Even a few carbon-negative plants can radically shrink emissions

At the moment, California has a goal of cutting emissions to 80 percent below 1990 levels by 2050. Policymakers are hoping to do that with some mix of solar, wind, hydro, nuclear, and energy storage, plus perhaps gas plants that capture and bury their emissions.

In the Nature Climate Change paper, Sanchez found that the western US could go much, much further than that by simply adding a few carbon-negative biomass plants to the mix. In the most optimistic case, power plant emissions could fall 145 percent below 1990 levels by 2050:

It all depends on what other energy sources are being used. So, for instance, if the west was still using lots of natural-gas plants in 2050, it could build a few carbon-negative biomass plants and get an 86 percent reduction in emissions, compared with 1990 levels.

On the other hand, if the region was relying heavily on carbon-free renewables like wind and solar, then throwing these carbon-negative plants in the mix could lead to a point where the power sector, on the whole, is taking more CO2 out of the atmosphere than it’s putting back in.

So what’s stopping us from building carbon-negative plants?

One downside here is that this is all expensive — the authors estimate that BECCS plants would only start to become economically competitive in the western United States if we priced carbon dioxide at around $74 per ton. More drastic emissions cuts would require even higher costs.

To put that in perspective, that’s a fair bit more expensive than the cost of replanting additional forests (between $5 and $40 per ton of CO2). But it’s significantly cheaper than estimates of what it would take to filter carbon dioxide out of the air directly (a whopping $1,000 per ton). That's because it's much easier to filter carbon dioxide out of, say, a smokestack, where it's concentrated, than it is to take it out of the air — after it's diffused into the atmosphere.

There are also technological hurdles to any carbon-capture and storage technology. Right now, there is only one power plant in North America that actually captures its own carbon dioxide and buries it underground — the Boundary Dam Carbon Capture Project in Saskatchewan, which opened in 2014. (Another coal CCS demonstration plant in Kemper, Mississippi, has faced delays and overruns, now costs $6.2 billion, has relied on federal government support, and won't open until mid-2016 at the earliest.)

This technology is still very much in its early stages, with much of the cost in figuring out how best to separate out the carbon dioxide in order to bury it. And apart from China, many governments are increasingly shying away from further R&D, scared off by the high costs.

"The next step is tackling that commercialization question," Sanchez says. "Right now, we don’t have commercial scale deployment. So how can we start building commercial scale plants? What R&D do we need for better biomass gasifiers?"

There are also questions about what happens to the carbon dioxide once we pump it into old saline aquifers or depleted oil and gas fields. In theory, it should stay there permanently. But we need to be sure. Sanchez notes that even very small leakage rates would add up very quickly and would blunt the carbon benefit.

Even so, he notes, it’s a technology well worth researching —especially since pulling carbon dioxide out of the atmosphere is looking increasingly essential to stopping drastic global warming.

Further reading

-- A press release from UC Berkeley on Sanchez's Nature Climate Change paper.

-- How to stop global warming, in 7 steps (There's more discussion here of the need for negative emissions.)

-- Is China the last hope for carbon capture technology?