18.1: Introduction

Detailed studies of our options to keep the planet livable tell us that the technologies the world is currently focused on—renewables and electrification—will not be enough. We will need to actively clean up the atmosphere—essentially decarbonize it back to a safe level. That is a huge challenge. Figure 18.1.1 shows two scenarios—or trajectories—we have for emissions of greenhouse gases. In yellow is the trajectory for business as usual, which represents no change in our current emissions. This is the trajectory of carbon emissions we will follow if we continue our current activities—it will have devastating results for temperature and climate, as you have learned in previous chapters.

Figure 18.1.1 Two trajectories for future greenhouse gas emissions, showing all greenhouse gases (GHGs) as their equivalent in carbon dioxide emissions (GTCO₂e). The red curve would give a 66% chance of reaching less than 2°C in global temperature rise. The brown field represents current efforts to reduce GHG emissions. Adapted from Fuss et al. 2018.

Our ambition to keep temperature increase well below 2°C requires us to follow the red trajectory of worldwide carbon emissions. The models tell us it would give us about a 66% chance of staying below 2°C of global total temperature increase. The goal of 1.5°C would be much harder to achieve.

But the range of current tools available—renewables, electrification of transportation, efficiency, basically eliminating almost all fossil fuels— will only give us the reduction shown in brown. These are the massive changes in our energy use and economic activity that the other chapters in this book and the Bending the Curve course have discussed. But it’s not enough. Even if we stop all the greenhouse gas emissions represented by the brown field, we won’t reach the desired rate of emissions to stay below the 2°C goal. Why is that?

It’s mainly because of greenhouse gases like methane, nitrous oxide, and the other heat-trapping gases you have learned about. But it is also partially because some CO₂ is going to be really hard to remove from the economy, like that from airplane fuel. It will be very difficult to eliminate all emissions, and it will take a long time to achieve the maximum reductions. We call these residual emissions. Agriculture is a particularly difficult case. For instance, nitrous oxide is emitted from fertilizer use, and of course cattle emit methane. Are we going to stop using fertilizer and eating meat? These are choices that the world could make, but that would represent dramatic changes in our food system and agricultural economy. Other chapters have discussed how to address these problems, but we expect some emissions to remain.

And it’s worse than that. We’ve already put so much CO₂ in the air that we would need negative emissions, shown in blue in Figure 18.1.2, even if we could get our current emissions to zero, which we can’t do, because of the residual emissions. But the sum of the slow action and residual emissions is large—10 billion tons by 2050, 20 billion tons by 2100.

Figure 18.1.2 The green wedge represents the negative emissions—removal of CO₂ from the atmosphere—required to offset residual emissions from other greenhouse gases and unmitigated CO₂ and to keep worldwide greenhouse gas emissions below that required to meet a 2°C future. (GTCO₂e = GHG equivalent in CO₂.) Adapted from Fuss et al. 2018.

One approach for dealing with residual emissions is to create negative emissions, basically removing CO₂ from the atmosphere. (In this chapter we expressly limit the phrase negative emissions to CO₂ that is removed from the atmosphere—and not any of the reductions in emissions that you have learned about previously). The green wedge in Figure 18.1.2 represents the required negative emissions in order to meet the trajectory of well below 2°C emissions. (The trajectories shown in Figures 18.1.1 and 18.1.2 represent averages of many models that change the rates of electrification, efficiency, and other economic parameters to achieve the 2°C future at the lowest cost to the world economy given today’s knowledge of technology options.)

The green wedge, the required negative emissions, grows in slowly to represent realistic growth rates of the technologies required to remove CO₂ (even though we do not yet have a good understanding of those technologies, as we will discuss in this chapter). But the size of the required negative emissions is daunting—we will need to remove around 1 billion tons in 2030, 10 billion tons in 2050, and 20 billion tons in 2100. That means between now and 2050, we need to create an industry to clean up the atmosphere that moves twice the material of today’s oil industry. In 2030, the 1 gigaton we need to remove per year is about equal to the weight of all humans on Earth, or the weight of all the corn we harvest each year.

How will we create the negative emissions required to meet a 2°C future, or better yet to keep temperature rise significantly lower than 2°C? We have to find the right technologies, and then quickly get them to scale. We have to create technology that can address negative emissions and also to understand how full-scale negative emissions ecosystems—capture, transportation, and storage—can be created. We have to catch billions of tons of CO₂ at an affordable price. A really important aspect is how we encourage and enable the creation of businesses that can do this job. You don’t move billions of tons of anything without businesses that make money, even if that money is from taxes or government subsidies (like our trash removal today). Most importantly, we need to do all of this in a way that does not do unacceptable damage to the people and natural environment of the Earth that we are seeking to protect.

This chapter discusses these challenges and how we might go about addressing them. I address the major issues associated with the removal of carbon and its ultimate permanent storage. Five methods are listed here with approximate values for how much annual negative emissions they might provide:

1. Regrowing forests (1–5 billion tons per year).
2. Putting CO₂ back into soils as soil carbon (450 billion tons total, but slowly, at 2–5 billion tons per year).
3. Using biomass to remove CO₂, either while making fuel and energy, or to restore soil carbon (2–5 billion tons per year).
4. Enhancing the natural reactions of minerals with the air (2–4 billion tons per year).
5. Directly removing CO₂ via chemistry and machinery, that is, direct air capture (limited only by the availability of renewable energy to power the devices, but probably 2–5 billion tons per year).

The biomass and direct removal methods generate CO₂ that must be stored permanently out of the atmosphere. These approaches also have limitations. I describe two in detail:

1. Making carbon-based products from CO₂ instead of oil (1–5 billion tons/year).
2. Putting CO₂ underground as liquid (unlimited capacity but expensive).

Some consideration has been given to increasing the rate of biological uptake in the oceans by fertilizing plankton or enhancing the growth of large algae (like kelp), but those methods are subject to even more environmental conflicts than the seven activities listed above, and an adequate estimate of how they might affect our negative emissions activities is not yet available.