Section 2: Carbon removal

Some further resources on CDR:

This book by Prof. Jennifer Wilcox at Stanford

The references in this 2015 review paper on biophysical and economic limits of negative emissions

The full report from the National Academies

The full report from EFI.
This is the second of a series of three blog posts intended as a primer on how technology can help to address climate change. In the last post, I covered some basics about climate and renewable energy. This post is a summary of what I learned about carbon dioxide removal technologies.

Why even talk about carbon dioxide removal?

I’ll quote the company Stripe’s recent announcement on this:

“Urgent global action is needed to halt greenhouse gas emissions, and it looks increasingly likely that in addition to emissions reduction, humanity will need to remove large amounts of carbon dioxide from the atmosphere. In its most recent summary report, the IPCC notes that most scenarios that stay below 2°C of temperature increase involve “substantial net negative emissions by 2100, on average around 2 gigatons of CO2 per year.””

Here it is directly from the IPCC, with regard to a 1.5C target:

C3. All pathways that limit global warming to 1.5°C with limited or no overshoot project the use of carbon dioxide removal (CDR) on the order of 100-1000 GtC02 over the 21st century. CDR would be used to compensate for residual emissions and, in most cases, achieve net negative emissions to return global warming to 1.5°C following a peak (high confidence). CDR deployment of several hundreds of GtCO2 is subject to multiple feasibility and sustainability constraints (high confidence). Significant near-term emissions reductions and measures to lower energy and land demand can limit CDR deployment to a few hundred GtC02 without reliance on bioenergy with carbon capture and storage (BECCS) (high confidence).

So to break that down:

  1. All pathways to 1.5℃ in the IPCC models use CDR
  2. It is used to compensate for existing emissions
  3. It is used to achieve net negative emissions
  4. There are lots of constraints on our ability to scale it currently

Here is a GIF from Glen Peters on how much negative emissions we need to stay under 1.5C warming, for different amounts of positive emissions, based on this work:

Solving the climate problem likely needs both aggressive decarbonization and negative emissions and potentially other mitigation measures.

Here is a relevant figure with a schematic 2C pathway from the NAS report, showing a key role for large-scale negative emissions:

2 degrees pathways

Even if we managed to stop releasing any new CO2 very soon, we might still have big problems, due to the carbon already up there in the atmosphere by that time. If and when we get to a certain average temperature, even the steepest emissions reductions as such don’t lead to cooling much below that temperature, except on long timescales, because the natural carbon sinks are slow. The basic idea is described eloquently in the book “Radical Abundance”:

What makes climate change an unusually difficult problem (aside from the scale of its effects and the momentum of fossil fuel consumption) is the long-term persistence of CO₂, in the atmosphere. Levels of atmospheric pollutants like soot or SO₂, closely track the rate of emissions. Turn off the source and the pollutant soon washes out of the atmosphere. Even chlorofluorocarbons and methane have lifetimes measured in a small span of years. CO₂, is different. It exits the atmosphere on a timescale best measured in decades or centuries. Rain doesn't scrub CO₂, from the atmosphere; instead, CO₂, is slowly absorbed, primarily by the seas.* At any moment, CO₂, levels are only slightly elevated by last year's emissions and are in- stead the cumulative result of emissions that span the industrial era. Deep cuts in emissions wouldn't lower CO₂, levels in proportion, but would instead merely slow down the rate of their rise. The system behaves more like a bathtub with a drain that's almost closed: If the tap is left open, the tub will eventually overflow.

Here is a relevant figure from MacKay:

keeling curve
The introduction to the National Academies report on negative emissions has a deeper discussion of natural versus artificial sinks on atmospheric CO2, which adds some color to this picture: they don’t claim that negative emissions are the only way to bring down atmospheric CO2 over time, but they do say that “NETs provide the only means to achieve deep (i.e., >100 ppm)… reductions, beyond the capacity of the natural sinks”.


As we’ll discuss below, sucking CO2 out of the atmosphere requires a large scale of energy or other resources, and doing so efficiently may require deployment of new kinds of technology that could induce some unique environmental issues of their own. Depending on the scheme, economic incentives may well be needed to bring negative emissions to a truly relevant scale.

Moral hazard

Some do not view the reliance of IPCC scenarios on negative emissions as a good thing. As Anderson and Peters argued in a perspective piece in Science:

“Negative-emission technologies are not an insurance policy, but rather an unjust and high-stakes gamble… the mitigation agenda should proceed on the premise that they will not work at scale. The implications of failing to do otherwise are a moral hazard par excellence.”

There is a risk that some would misperceive, or falsely label, negative emissions technologies as “quick techno-fixes” that remove the responsibility for global coordination around complex tradeoffs and priorities for emissions reduction. Some with vested interests in the fossil fuel industry could use this false narrative to push against the implementation of crucial policies that incentivize broader decarbonization throughout the economy. There has been some attempt to empirically study the extent to which people’s preferences are actually influenced by such moral hazard, albeit in a limited setting. It is a real concern. To be clear: investing in negative emissions is not an excuse not to reduce positive emissions quickly, but some might try to use it that way.

The moral hazard risk has to be weighed carefully against the importance of developing the technologies. I don’t claim to know how best to manage that tradeoff in society, except to say: we should limit the ability of fossil fuel interests to act upon moral hazard, by constraining their negative externalities through carbon taxes, and by out-competing them economically with cheap, clean technologies and appropriate incentives to make them even cheaper, and we should do so regardless of whether negative emissions technology ever looks poised to become truly viable at scale. But frankly, it looks like we need negative emissions technology, in addition to these other things.

In any case, this is now entering the mainstream public discourse.


Different carbon removal approaches have vastly different characteristics in terms of things like land or energy use. While there is a lot of information in this summary figure it gives a good overview of the types of tradeoffs we will be looking at below. E.g DAC uses very few natural resources (land, water etc) but currently has a massive cost and energy footprint, whereas something like forests use vast amounts of natural resources but have a net energy gain from their use, as well as biodiversity benefits.

keeling curve

Additional resources

Background: I recommend John Baez’s blog posts and the accompanying thread for an intro to this area. He also links to MacKay’s discussion of the topic. Y Combinator has a great introductory website focused on carbon removal technologies. The Drawdown project is also interesting. The National Academies has a great report on negative emissions that we will look at a bit below — I highly recommend reading that. Here is also a nice report from the Energy Futures Initiative — also highly recommended: it was released just as I was finishing this post, and it covers some closely related ground. There is also a review in Nature on CO2 utilization pathways.