A study led by a team scientists found that the most effective path to limiting global warming to 1.5 degrees Celsius by the end of the century likely requires a mix of technologies that can pull carbon dioxide from Earth’s atmosphere and oceans. The research found that overreliance on any one carbon removal method may bring undue risk. All methods likely will need to be called on to remove the necessary amount of carbon dioxide—estimated at 10 gigatons annually—to limit warming to 1.5 degrees by 2100.
The work was published in early March in the journal Nature Climate Change. Its authors included researchers from the Energy Department’s Pacific Northwest National Laboratory, the ClimateWorks Foundation and the University of Virginia. The paper outlined the carbon-removing potential of six methods, which ranged from restoring deforested lands to spreading crushed rock across landscapes, a method known as enhanced weathering.
The study is one of the first attempts to incorporate all carbon dioxide removal approaches recognized in U.S. legislation into a single model that projects how their interactions could measure up on a global scale. It also considered how those methods could influence factors like water use, energy demand or available crop land.
The authors explored the potential of these carbon removal methods by modeling decarbonization scenarios: hypothetical futures that were intended to demonstrate what kind of interactions could emerge if the technologies were deployed under varying conditions.
One so-called pathway included a future where no climate policy is applied and where warming rises to 3.5 degrees as a result.
A second pathway looked at what amount of carbon would need to be removed using the technologies under a policy in which carbon emissions are targeted to fall to net-zero by mid-century and net-negative by late-century. Doing so would limit end-of-century warming to less than 1.5 degrees.
A third scenario was paired with behavioral and technological changes, such as low material consumption and rapid electrification. Here, societal changes translated to fewer overall emissions released, helping to reduce the amount of residual greenhouse gas emissions that would need to be offset with carbon removal to meet the 1.5-degree goal.
To meet that target—the original goal of the Paris Agreement—the authors found that roughly 10 gigatons of carbon dioxide must be removed per year. That amount remained the same even if countries were to strengthen efforts to reduce carbon dioxide emissions from all sources.
Some of the technologies could contribute a great deal, with the potential to remove several gigatons of carbon dioxide per year. Others offered less, yet still could play an important role. Enhanced weathering, for example, could remove up to four gigatons of carbon dioxide annually by mid-century.
Under this method, finely ground rock spread over cropland converts carbon dioxide in the atmosphere into carbonate minerals on the ground. It was among the most cost-effective methods identified in the study.
In comparison, direct ocean capture with carbon storage, where carbon dioxide is stripped from seawater and stored in Earth’s subsurface, would likely remove much less carbon. On its own, the technology is currently too expensive, according to the authors. Pairing this method with desalination plants in regions where demand for desalinated water is high, however, could drive down the cost while delivering more meaningful carbon reductions.
Other technologies under study included biochar, direct air capture with carbon storage, and bioenergy paired with carbon capture and storage.
Each of the technologies modeled brings unique advantages, costs and consequences. Many of those factors are tied to specific regions. The authors pointed to Sub-Saharan Africa as place where biochar, enhanced weathering and bioenergy with carbon capture and storage could contribute “significant reductions.”
The authors said that work is needed to address greenhouse gases other than carbon dioxide, such as methane and nitrous oxide. Many of these gases are several times more potent and more difficult to target than carbon dioxide.