So, we have too much CO2 in the atmosphere and since we can’t wait for it to just disappear (because it won’t) we need to actively remove some of that excess. Removing carbon from the atmosphere is called Carbon Dioxide Removal (no surprise here) or CDR for short.
There are many different ways to remove CO2. Some we have known for centuries, like planting trees, others speed up natural processes like rock weathering or apply novel technology like direct Air Capture. This is a high-level overview of some of these approaches.
We created a huge problem by releasing gigatons of sequestered carbon in the form of CO2. Now we need many different approaches to tackle–and hopefully eliminate–the mess we made. Over the last few years an amazing variety of CDR approaches have been developed and are at various stages of development and deployment. Not all will be feasible and economical, but those who turn out to be both are our best chance to get the excess CO2 out of the atmosphere.
The climate crisis requires an ‘‘all solutions’’ mentality, with researchers and policy-makers understanding that small gains from myriad approaches is how we will conquer the crisis. We need continued rapid advances in technologies that produce avoided emissions, like renewable-energy solutions that displace fossil-energy production, together with approaches that achieve negative emissions. None of the NETs—mineralization, direct air capture (DAC), bioenergy with carbon capture and storage (BECCS) or land/ocean management—alone will provide a singular solution, but together they are the answer.
Christopher W. Jones Georgia Institute of Technology>
This is by no means a complete list–if you follow the space you’ll read about new ways of removing CO2 regularly–but this list gives a first impression of what is out there:
Planting trees - plants use CO2, water and the energy from sunlight to create carbohydrates and oxygen in a process called photosynthesis. Planting trees, or more broadly restoring lost natural ecosystems like forests, peat- and wetlands, is an effective way of reducing the CO2 concentration in the atmosphere.
Unfortunately, planting trees is no magic cure as we discuss in the next chapter but it can contribute significantly to our removal efforts. The first and most important step is to stop deforestation and focus on reforestation in the areas where it makes the most sense. Planting saplings makes the most sense from a CDR perspective in warm and humid areas. Carbon180.org has a downloadable fact sheet about forest carbon removal that provides a great overview.
Growing kelp and other marine plants (Blue Carbon sinks) - carbon dioxide capture via photosynthesis is not limited to plants on land. Kelp and marine macroalgae can bind large amounts of CO2 and once those plants die they sink to the ocean floor where the carbon is buried in sediment and permanently removed. Here is an example. Ths is actually a very efficient way of removing CO2, because deep down n the ocean, the plants don’t rot and therefore do not release the CO2 again. Plants on land, however, release it if they decompose (or burn).
Soil - soil can bind an enormous amount of CO2 and various methods can be used to increase the amount of CO2 soils bind. Soil carbon sequestration is also known as “carbon farming”. Ways to increase CO2 absorbance of soil are:
reducing soil disturbance by switching to low/no-till practices or planting perennial crops
changing planting schedules or rotations, e.g. planting cover crops or double crops instead of leaving fields fallow
managed grazing of livestock; and
applying compost or crop residues to fields
The challenge is that these approaches are not widely used by the agricultural industry and will require deep changes in how agricultural businesses operate. And as we all know, change is never easy or fast and often costly. There also is a Carbon 180 fact sheet for that, or two.
BECCS - the acronym stands for Bioenergy with Carbon Capture and Storage. BECCS is a hybrid approach, it relies on plants to capture the CO2 via photosynthesis and then uses an engineered approach for sequestration. Here is how it works: Biomass (e.g. wood, compost or other organic material) is burned to create bioenergy and the resulting CO2 is captured and then stored in geological formations or embedded in long-lasting products. As with all methods that remove CO2 from the air care must be taken that the storage of captured carbon is reliable over long timescales and the cost of capturing and storing the CO2 is not too high.
Biochar - biochar is a solid, carbon-rich product that results from pyrolysis of biomass (burning without oxygen). Biochar has many environmental benefits, e.g. it binds contaminants and makes it less likely that plants take them up. One such example is that biochar prevents the accumulation of the heavy metal cadmium in rice plants. In addition, biochar can help increase soil fertility by adding nutrients or by improving soil structure or pH; and - and here is why biochar is relevant to CDR - biochar production converts labile carbon into a recalcitrant form that can store carbon for hundreds to thousands of years (source).
Enhanced weathering - is another CDR approach that uses natural processes, namely the slow binding of CO2 to certain minerals, e.g. olivines or limestone. The process can be accelerated by grinding these CO2 binding rocks up finely to increase the surface area. The more of the mineral’s surface is exposed to the CO2 in the air, the more CO2 can be bound per time. The rock dust can be spread on land or applied to coastal areas to supercharge CO2 capture. Enhanced weathering has a number of co-benefits–for one, it makes water more alkaline, and therefore helps counteract acidification of the oceans. Adding minerals to the soil can boost nutrient levels and potentially improve crop yields and help restore degraded agricultural soils.
“We wanted to figure out the cheapest way to take carbon dioxide out of the air and we came up with something very simple: Take limestone, cook it. Now you have CO2, to store or use, and calcium oxide. Put the CaO out in the weather. It will draw down CO2 from air, to make limestone again. Repeat. This is so simple, it is almost stupid. But we are finding that we can convert 75 percent of CaO to limestone in less than two weeks, just reacting with air in the lab. And, because the process is so simple, it currently has the lowest peer-reviewed cost estimate, of any proposed method for direct air capture.”
Peter B. Kelemen, research scientist at Lamont-Doherty Earth Observatory and the Arthur D. Storke Memorial Professor in the Department of Earth and Environmental Sciences
Another advantage of enhanced weathering is that the minerals needed to bind CO2 are very abundant.
Global reserves of peridotite (olivine and pyroxenes) and serpentine are estimated to be sufficient to sequester all carbon that may be emitted from the presently recognized reserves of coal, oil and natural gas.
Haque, et al in energies “Alkaline Mineral Soil Amendment: A Climate Change ‘Stabilization Wedge’?” June 2019
- Direct Air Capture (DAC or DACC for direct air carbon capture) - this is the most high-tech (and most controversial) of the CDR approaches. It uses fans to blow air by materials (solid or liquid) that bind CO2. Once these materials have bound CO2 to capacity, that is, when they are “full”, the CO2 gets released - typically by heating the material up - captured, and permanently stored. The capture material gets reused.
Requires little land (e.g. compared to reforestation),
Can be located virtually anywhere, and
Allows for very exact quantification of how much carbon was captured (as opposed to say a tree, where it is very difficult to measure that).
Requires a lot energy currently (for the fans as well as heating the capture materials)
Very expensive at present.
Find much more information about DAC on the DAC Coalition website.
This is just a high-level overview. New companies and organizations dedicated to removing CO2 are emerging all the time, and a variety of approaches are under development.
Here is the overview table from the CDR Primer to give you a bit more detail on different approaches (click to enlarge):