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.

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