We Need To Take CO2 Out Of The Sky

Gone are the days where optimistic emissions reductions kept us below a 2-degree warming target.To keep below two degrees, we'll need to dramatically reduce current emissions and simultaneously remove 10-15 gigatons of CO2/yr from the atmosphere by 2050 and scale that to about 20+ gigatons annually by 2100. The atmosphere is made of a mixture of gases:Greenhouse gases are a super small part of it, so instead of describing tiny absolute percentages, we use these units to describe how much:This is what we mean by methane being far more dilute than CO2 (and therefore unrealistic to capture) -- we need to measure it in parts per billion.This sense of scale helps explain why removing CO2 is an expensive proposition, thermodynamically and hence monetarily: it’s an extremely dilute (parts per million!) gas in solution — we’re talking about separating a tiny fraction of the air from the rest of the air.Finally, taking a step back: the sensitivity of this system is really incredible -- we’re talking about changing the lives of people around the world by adding less than a basis point to the absolute concentration of a trace gas. Stripe’s Negative Emissions Commitment blogpost includes a brief overview of these and other technologies as well as some important context on adoption curves.Plant based solutions leverage a plant’s capacity to capture carbon via photosynthesis and the energy of the sun. Solutions in this category include (re)forestation, soil carbon sequestration, algae/kelp farming, and bio-energy with carbon capture and storage (often referred to as BECCS, this is basically a biomass power plant that burns wood and then is fitted with a carbon capture device to handle the smoke aka “flue gas”).Mineral-based solutions include speeding the weathering of naturally occurring rocks, e.g. Olivine.Chemical solutions include Direct Air Capture (typically coupled with geologic storage, the capture aspect is often referred to as DAC), where a big machine sucks CO2 out of the air; providing gaseous concentrated CO2 that can then be injected underground for permanent storage. For a detailed but still accessible overview of the field, see The Role of Direct Air Capture in Mitigation of Anthropogenic Greenhouse Gas Emissions.Companies currently working on this include Climeworks, Carbon Engineering, and Global Thermostat. These are promising but quite early!In any case, the result of DAC is pure gaseous CO2, which can be either utilized or sequestered.Sequestering it means permanently removing the CO2 from the carbon cycle, and literally reducing the concentration of CO2 in the atmosphere. For a deeper overview, I highly recommend Soil C Sequestration as a Biological Negative Emission Strategy.These “regenerative” mechanisms include no-till farming, using cover crops, and reducing usage of nitrogen fertilizers organic fertilizers can enable storage in soil of some of the plant’s carbon volume.Measuring soil carbon can be complex and has high variance: there’s the carbon in the biomass of the plant roots, and then the carbon expelled by the roots that can become stable in the soil as "Soil Organic Carbon". Also take a look the Rocky Mountain Institute’s overview report on land-based negative emissions solutions, as well as WRI’s land carbon removal overview.The idea of biomass power in general is that instead of burning coal or natural gas  to turn a turbine, you burn biomass (which, because it's biomass, is made of carbon that used to be in the atmosphere). For a more detailed overview, see the "Biochar Additions" section of Soil C Sequestration as a Biological Negative Emission Strategy.Properly produced biochar has the potential to remain durable (keep the C sequestered) in soil for an extended period; however its durability is subject to similar environmental conditions as Soil Organic Carbon, particularly precipitation and soil water content as it relates to microbial decay. For a more technical overview of this topic, I highly recommend An Overview of the Status and Challenges of CO2 Storage in Minerals and Geological Formations.Common rocks form carbonate minerals when exposed to CO2; often even at atmospheric concentrations; permanently binding the C as a mineral (eg CaCO3 calcium carbonate). It’s important to interpret all carbon removal solutions through a lens of a full lifecycle analysis, which can help answer the question: is the entire operation of a given negative emissions solution net-negative?For BECCS, for example, this analysis would include the greenhouse gas emissions to grow the biomass, the emissions of building the BECCS plant, the emissions of transporting the biomass to the plant, any loss of CO2 to inefficiencies in the carbon capture system, and the emissions associated with the energy to run the big machine to pump the CO2 underground.All of this would need to add up to less than the volume of CO2 sequestered -- ideally much less.Alongside lifecycle analysis, there are a number of dimensions to evaluate carbon removal solutions.