Introduction to CDR
The Earth’s climate system is incredibly complex. It involves constant interactions between the atmosphere, oceans, land, and living organisms, making it difficult to predict how the planet responds when removing carbon dioxide (CO2) from the air. To make sense of this complexity, researchers use Earth System Models (ESMs) to understand how temperature and other climate variables might change in response to various carbon dioxide removal (CDR) methods.
Through ESMs, scientists have found that when CO2 is removed from the atmosphere, only a portion of it remains permanently removed. This is because of a “rebound effect,” where the ocean and soils respond to lower CO2 in the atmosphere by releasing some CO2 back into the air.
CDR also affects the climate in ways beyond lowering CO2 concentrations. Different removal methods may produce other greenhouse gas emissions (like nitrous oxide from bioenergy with carbon capture and storage, known as BECCS) and can change physical and biological processes across the planet.
Understanding how various carbon removal methods affect the carbon cycle and the climate as a whole is essential for evaluating the real climate response to CDR and for guiding future climate policies and emissions planning.
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Carbon dioxide removal (CDR) involves extracting carbon dioxide from the atmosphere and storing it in land, ocean or geological reservoirs. This may sound straightforward, but the Earth system comprises many interacting parts, making it hard to predict exactly how the climate responds to CDR.
Evaluating climate response to CDR with the use of Earth System Models (ESMs)
Achieving net-zero emissions — where greenhouse gas removal fully balances emissions — does not automatically stabilize the climate. Different CDR methods can alter land surfaces or generate other non-CO2 greenhouse gases. To understand these effects, scientists use sophisticated computer simulations that integrate physics, chemistry, biology, and climate science, known as Earth System Models (ESMs). These models can vary in complexity depending on the focus of the research.
By running ESMs with different CDR scenarios, scientists can project how removing specific amounts of CO₂ will affect global temperatures, precipitation patterns, ocean chemistry, and carbon cycling over time.
Most climate models today represent CDR in a simplified way: for every tonne of CO2 removed from the atmosphere, atmospheric CO2 levels decrease by one tonne. However, these models don’t fully account for other effects of some CDR methods, such as changes in how much sunlight the Earth reflects (albedo) or changes in N2O emissions. That’s why researchers are working to develop better ways to predict future atmospheric CO2 levels that account for these and other responses to various CDR methods. This will help provide a more complete understanding of how different CDR methods affect global temperatures and climate change.
The rebound effect: how much carbon stays removed
One important process that ESMs help researchers understand is how the Earth’s natural carbon sinks respond to CO2 removal. When CDR reduces atmospheric CO2, the land and ocean systems respond by releasing previously stored carbon back into the atmosphere. Right now, plants and oceans are soaking up extra CO2 because there’s so much of it in the air. Removing CO2 using CDR means the pressure to absorb CO2 eases, so land and ecosystems stop pulling in as much carbon as before. Basically, the land and ocean have been storing extra carbon because CO2 levels were high. Once CO2 levels are lowered, part of the reason they were storing that extra carbon goes away, causing some of it to drift back out. That’s the “rebound.”
This “rebound effect” means only a portion of removed CO2 remains sequestered, typically 60-85% depending on initial emissions levels. The higher the initial levels, the higher the effectiveness of the CDR method. This is because when CO2 levels are already high, plants and oceans don’t react as strongly to a drop in CO2 as they would if CO2 were lower to begin with.
CDR effects beyond carbon reduction
When there are alterations to land surface, it affects how heat, water and energy are absorbed, reflected or transferred through the environment — called biogeophysical effects. For instance, spreading dark rock across land surfaces during Enhanced Mineralization decreases albedo (the amount of sunlight a surface reflects back into space). Think of it like wearing a black shirt versus a white shirt on a sunny day: the black shirt absorbs more sunlight and gets noticeably hotter, while the white shirt reflects more of it and stays cooler. Land surfaces work the same way. A lighter surface (like parent rock or soil) reflects more sunlight back into space, while a darker surface absorbs more of that sunlight as energy, which then radiates as heat. This means that when dark rock is spread across land, the surface absorbs more sunlight than before, leading to higher surface temperatures, counteracting the cooling effect of decreasing CO2 levels in the air.
Certain CDR methods may also unintentionally increase emissions of other greenhouse gases. For instance, BECCS may involve growing dedicated energy crops. These crops often require a fertilizer that releases nitrous oxide (N2O), a potent greenhouse gas. This happens because plants can’t absorb all the nitrogen in the fertilizer and the leftover nitrogen in the soil is broken down by naturally occurring soil bacteria, converting some of the nitrogen into N2O gas, which then escapes into the atmosphere. The more fertilizer used to grow energy crops at scale, the more of this excess nitrogen there is for bacteria to convert into N2O.
Last Edited - Sept. 28, 2025