Methods for removing carbon from the atmosphere
The Intergovernmental Panel on Climate Change (IPCC) asserts that limiting global warming to 1.5˚C could avert the most catastrophic effects of climate change. In its recent report, it laid out four means of achieving this —and all of them rely on removing carbon dioxide from the atmosphere.
This is because even if we cut most of our carbon emissions down to zero, emissions from
agriculture and air travel would be difficult to eliminate altogether. And since carbon dioxide that’s already in the atmosphere can affect climate for hundreds to thousands of years, the IPCC maintains that carbon dioxide removal (CDR) technologies will be critical to get rid of 100 to 1000 gigatonnes of CO2 this century.
How can carbon dioxide be removed? There are a variety of CDR strategies, all in different stages of development, and varying in cost, benefits and risks. CDR approaches that employ trees, plants and soil to absorb carbon have been used at large scale for decades; other strategies that rely more on technology are mostly at the demonstration or pilot stages. Each strategy has pros and cons.
Afforestation and reforestation
As plants and trees grow, they take carbon dioxide from the atmosphere and turn it into sugars through photosynthesis. In this way, U.S. forests absorb 13 percent of the nation’s carbon emissions; globally, forests store almost a third of the world’s emissions.
Soil carbon sequestration
The carbon that plants absorb from the atmosphere in photosynthesis becomes part of the soil when they die and decompose. It can remain there for millennia or it can be released quickly depending on climatic conditions and how the soil is managed. Minimal tillage, cover crops, crop rotation and leaving crop residues on the field help soils store more carbon.
This strategy exploits a natural process wherein reactive materials like peridotite or basaltic lava chemically bond with CO2, forming solid carbonate minerals such as limestone that can store CO2 for millions of years.
Direct air capture
Direct air capture sucks carbon dioxide out of the air by using fans to move air over substances that bind specifically to carbon dioxide. The technology employs compounds in a liquid solution or in a coating on a solid that capture CO2 as they come into contact with it; when later exposed to heat and chemical reactions, they release the CO2, which can then be compressed and stored underground.
Rocks and soil become weathered by reacting with CO2 in the air or in acid rain, which naturally occurs when CO2 in air dissolves in rainwater. The rocks break down, creating bicarbonate, a carbon sink, which is eventually carried into the ocean where it is stored.
Ocean fertilization would add nutrients, often iron, to the ocean to prompt algal blooms, which would absorb more CO2 through photosynthesis.
Coastal blue carbon
Salt marshes, mangroves, sea grasses and other plants in tidal wetlands are responsible for more than half of the carbon sequestered in the ocean and coastal ecosystems. This blue carbon can be stored for millennia in the plants and sediments.
Each CDR technology is feasible at some level, but has uncertainties about cost, technology, the speed of possible implementation, or environmental impacts. It’s clear that no single one provides the ultimate solution to climate change.
“Carbon dioxide removal alone cannot do it,” said Kate Gordon, a fellow at the Columbia Center on Global Energy Policy. “If there’s one thing the IPCC report really underscores is that we need a portfolio—we need to reduce emissions dramatically, we need to come up with more renewable energy options to replace fossil fuels, we need to electrify a lot of things that are currently run on petroleum and then we need to do an enormous amount of carbon removal.” In the near term she would like to see more deployment and ramping up of tried and true strategies, such as tree planting trees and more sustainable agricultural practices.
In fact, a new study just determined that planting trees and improving management of grasslands, agricultural lands and wetlands could sequester 21 percent of the U.S.’s annual greenhouse gas emissions at relatively low cost.
Developing the other carbon dioxide removal strategies further is going to take substantial amounts of money.
“The climate philanthropy community actually needs to recognize this as part of the climate solution—it’s really important that [CDR] becomes part of that portfolio,” said Gordon. “We also need a pretty significant federal R&D budget dedicated to these strategies so we can start improving the technology and get a better grasp on how much it does cost to do each of these things, how effective they are and how safe they are.”
Establishing a financial incentive to remove carbon such as a carbon tax or penalties for emitting carbon would help as well.
“This is the next frontier of the energy, climate and technology conversation,” said Gordon. ”We need to be ahead of this thing if we want to stay competitive—if we want to continue to have most of the world’s clean energy and advanced energy patents…Otherwise we’ll be buying it from somebody else, because someone’s going to do.
Earth Institute, Columbia University