Will negative emissions technology get us to 2 degrees?

There is an elephant  in the climate mitigation room. Actually, if we’re honest, what’s in the room is quite a bit bigger than even an elephant.

For the latest report issued last year by the Intergovernmental Panel on Climate Change (IPCC), researchers looked at over 1,000 different greenhouse gas emission scenarios over the course of the rest of this century. Of these thousand scenarios, there are ones that are terrifying in the amount of carbon that could be emitted, leading to terrifying temperature increases. There are IPCC scenarios that are in line with the internationally agreed target of limiting warming to below 2° Celsius above pre-industrial temperatures. We can call these the ‘worrying’ scenarios. And there are scenarios in between. What lies between worrying and terrifying? How about we call these scenarios the ‘frightening’ ones.

Here’s the thing though. The vast majority of the scenarios that allow us to stick to the two-degree limit—the worrying ones, that is—assume that by some point in the second half of this century, we will have achieved net negative emissions. In other words, we will be taking more greenhouse gases out of the atmosphere than we put into it.

Even many of the scenarios that will likely lead to three degrees of warming—the frightening ones—still assume a large role for negative emissions. Even if we don’t manage to achieve net negative emissions, there are a lot of scenarios that require bulk CO2 removal from the atmosphere. It’s just that the amount removed does not exceed the rest of the emissions pumped out, so this will not be enough to dip below zero.

“We are late with mitigation,” economist Sabine Fuss of the Mercator Research Institute on Global Commons and Climate Change reminds Road to Paris. “As a result, many scenarios require negative emissions.”

Negative emissions depend on BECCS

Negative emissions—net or otherwise—would require the widespread adoption of a suite of technologies collectively known as carbon capture and storage (CCS), used in conjunction with the production of bioenergy.

The plants that are used to produce bioenergy take carbon out of the atmosphere as they grow. Then when we combust the bioenergy, that carbon is put back, in principle leading to no net new carbon emissions. But if bioenergy is combined with CCS, which scrubs carbon out of the combustion process and later stores this carbon dioxide underground or deep under the seabed, then we could begin to enjoy negative emissions.

Most of the merely worrying scenarios require that the world emit a total of no more than 1,200 gigatonnes of carbon by the end of the century. That’s about 30 years’ worth of carbon emissions at current levels. But these scenarios also foresee absorption of up to 1,000 gigatons of carbon via the aforementioned blend of bioenergy and CCS—a combo known by the acronym BECCS. This combo then would allow the total positive emissions to increase from 1,200 to 2,200 gigatonnes – and make the effort that much easier.

There are other options, including afforestation (planting trees), increasing the carbon stored in soil, and direct air capture of carbon. But the first two involve a sequestration of carbon dependent on land-use change that can be changed back at any time (if someone chops down a tree, for example). Soil carbon stocks are constantly at jeopardy of being disturbed. Direct capture technologies such as artificial trees and scrubbing towers are impressively gee-whiz and show great promise, but are years away from commercialization, currently even more expensive than already very expensive CCS, and we shouldn’t forget that they have a voracious energy appetite themselves.

Other possibilities such as the geoengineering techniques of ocean fertilization or enhanced weathering of natural or artificial minerals remain unproven at scale and are already raising hackles amongst some environmentalists. And these are not prominent in any of the considered scenarios. As a result, BECCS remains the top bet in the negative emissions sweepstakes.

But there are a few unanswered questions

But a bet it is, and not by any stretch a sure one. In a 2014 commentary in Nature Climate Change that has so far drawn little attention in the media, 14 prominent climate researchers with the Global Carbon Project, including Fuss, laid out the difficulties with banking on BECCS.

“Its credibility as a climate change mitigation option is unproven,” the researchers bluntly wrote with regard to BECCS, “and its widespread deployment in climate stabilization scenarios might become a dangerous distraction.”

First we have to consider what sort of bioenergy we are talking about. We already know that first-generation biofuels that use feedstocks such as palm oil, sugarcane, soy, rapeseed or cereals produce greater carbon emissions than fossil fuels due to direct and indirect land-use change. In addition, such biofuels place pressure on food prices as farmland shifts away from food production, with worrying impacts on food security. And while second-generation biofuels from waste or crop residues were once a great hope, recent research has shown there too, according to a $500,000 study funded by the U.S. Department of Energy published in 2014 in Nature, the removal of residues from cropland can release carbon trapped in the ground – known as soil carbon – and produce an overall increase in CO₂ emissions. So we are left with third-generation biofuels from algae. This at least has the benefit of not competing for arable land. But this fuel remains considerably more expensive than conventional fuels and will remain so for the near future pending more basic research. Many of the early pioneers in the field are pulling back from their bullish claims of near-term results.

Meanwhile, IPCC two-degree scenarios imply vast demands for biomass, between 100 and 300 exajoules’ worth per year by 2050. An exajoule, or one quintillion joules, is a gargantuan unit of energy: the 2011 Tohoku earthquake for example clocked in at 1.41 exajoules, while the entire energy used in the US per year comes to 94 exajoules.

These same scenarios expect a delivery from BECCS of between two and 10 gigatons annually by mid-century, which corresponds to between five and 25 percent of global CO2 emissions in 2010. The Global Carbon Project researchers make another sobering comparison: every year, oceans remove just under 10 gigatons of CO2.

Put another way, our use of BECCS would be like trying to add a whole extra carbon sink on the scale of the oceans to the world’s carbon cycle.

“Huge upscaling efforts will be needed to reach this level,” the researchers dryly remark.

Then with respect to the other part of the BECCS combo from bioenergy, the CCS bit, the researchers note that the International Energy Agency’s CCS roadmap indicates again that huge upscaling efforts will be needed to achieve the level of CCS implementation required by the aforementioned scenarios. As of this year, despite considerable research and development and while the carbon scrubbing processes have been successfully demonstrated, there is only one commercial scale industrial application worldwide, a rebuilt coal-fired generation unit with carbon capture technology in Saskatchewan, Canada.

Not to mention some risk

Meanwhile, all CCS development is predicated on the assumption that stored carbon won’t leak out.

“[P]rogress in deploying CCS has stalled,” declared a 2013 survey of the state of play appearing in Nature Climate Change. “Governments have to either increase commitment to CCS through much more active market support and emissions regulation, or accept its failure.”

There are just nine scenarios out of the thousand considered that manage to achieve the 2° cut-off but don’t depend on BECCS. But each of these nine have extreme mitigation rates, often involving very large deployment of nuclear, wind, solar, or bioenergy without CCS.

“So if we don’t have BECCS, then we will need a lot of something else,” says Glen Peters of the Center for International Climate and Environmental Research (CICERO) in Oslo. “If we are good at CCS and BECCS, it may crowd out nuclear and renewables. If we are good at renewables, and accept nuclear, then it may crowd out many CCS options. But, it all comes back to the same ultimate point. More mitigation now, the less effort later—including BECCS.”

The researchers are careful to stress that they are not dismissing BECCS, noting that some of the non-carbon-dioxide greenhouse gases such as methane and nitrous oxide from agriculture and some CO2 from industry such as steel and cement production will be very difficult indeed to mitigate completely. But policymakers need a much more detailed understanding of the challenges involved.

“Determining how safe it is to bet on negative emissions in the second half of this century to avoid dangerous climate change should be among our top priorities,” they argue.

Because so many of the current scenarios depend on negative emissions despite so little being known about how to achieve this, the researchers, perhaps unsurprisingly, want to see more research, done carefully and quickly, but also call for rigorous monitoring, reporting and verification in the event of any deployment.

We know we need to go negative, and quickly. But we also need to know how we’re going to do that, rather than just assuming that somehow the green pixie of BECCS will appear and magic away vast, leviathan quantities of carbon.


  1. Gerry Dunn

    This was an incredibly valuable article. With the constant barrage of plans from various organizations and governments, this is the best comparative analysis I’ve seen. Now if we could just turn it into a graphic :-)

  2. Dr. Greg H. Rau

    A sad day, indeed, when decision makers are again fed the myth that “Negative emissions depend on BECCS” – that expensively making supercritical CO2 from biomass, pumping it in the ground, and praying it stays there is viewed as the poster child of CDR and only game in town. How about taking the broader and more inclusive view that negative emissions are needed, we don’t yet know the best ways to do this, plenty of options exist, more research is needed, and by the way Nature is already removing nearly 20 GT CO2/yr from the atmosphere (for free) while we figure out what to do. To quote: “The Global Carbon Project researchers make another sobering comparison: every year, oceans remove just under 10 gigatons of CO2.” Doesn’t this provide a clue that rather than expensively (re)inventing the wheel from the ground up (CCS), that we might want to copy/improve upon/enhance existing, successful, natural CDR systems? (OK, passive diffusion of excess CO2 into the ocean is not one we want to copy, but there a plenty of other ways the ocean could participate (and will if we don’t act): http://link.springer.com/referenceworkentry/10.1007/978-94-007-5784-4_54)

    That “BECCS remains the top bet in the negative emissions sweepstakes” apparently speaks to: i) the human need, in desperate times, to raise hopes by picking a winner regardless of the realities, ii) the massive (though thus far failed) investment in CCS for point-source mitigation, and iii) the resulting large CCS lobby and PR machine who view other approaches to carbon management as a threat to their existence (and BE/CDR as an opportunity to expand their influence).

    Do we really want to bet the future of the planet on making and storing supercritical CO2 while continuing to ignore other proven CDR methods and failing to encourage new, better, cheaper and safer ideas?

  3. Craig Hill (user name Extinctiongangbang)

    Unless some gargantuan new carbon collecting system debuts within a decade, we are literally cooked. That inventor would deserve the adulation of every person on Earth. Short of the appearance of that white knight and his or her planetary ecosystem-saving gizmo, we’re toast. When you listen to climatologists like Guy McPherson, formerly of U of AZ, none of the several accelerating feedback loops of heat even figure in on the optimists’ childlike predictions deluding readers in articles like this. Those feedback loops bring us to 2C no later than 2030 and possibly sooner. No human has ever lived in the 2C range. The conclusions McPherson and Malcolm Light reach, among others, is a Northern Hemisphere uninhabitable due to the heat alone by ca 2033, and the same for the Southern Hemisphere 10 years later max. What Homo “Sapien” Hath Wrought.

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