Diversification reduces risk. That’s the gist of a key finding from a new study led by scientists at the Energy Department’s Pacific Northwest National Laboratory. An effective path to limiting global warming to 1.5 degrees Celsius by the end of this century will likely require a mix of technologies that can pull carbon dioxide out of the Earth’s atmosphere and oceans.
Over-reliance on any method of carbon removal can carry undue risk, the authors warn. And we’ll likely need all of them to remove the necessary amount of carbon dioxide — 10 gigatons per year — to ensure just 1.5 degrees of warming by 2100.
The new paper, published today in the journal Nature Climate Change, describes the carbon removal potential of six different methods. They range from restoring deforested areas to spreading crushed rock across landscapes, a method known as enhanced erosion.
This study marks the first attempt to integrate all carbon removal approaches recognized in US law into a single integrated model that projects how their interactions might be measured on a global scale. It does so while showing how these methods could affect factors such as water use, energy demand or land available for crops.
The authors explore the potential of these carbon removal methods by modeling decarbonization scenarios: hypothetical futures that demonstrate what kinds of interactions might occur if the technologies were developed under different conditions. They explore pathways, for example, where no climate policy is implemented (and warming increases to 3.5 degrees as a result).
A second pathway shows how much carbon would need to be removed using the technologies under an ambitious policy in which carbon emissions are limited to net zero by mid-century and net negative by the end of the century to limit the end of century warming below 1.5 degrees.
The third scenario follows the same emissions path but combines behavioral and technological changes such as low hardware consumption and rapid electrification. In this scenario, these societal changes translate into fewer overall emissions being released, which helps reduce the amount of residual greenhouse gas emissions that would need to be offset by carbon removal to meet the 1.5 degree target.
To reach this goal—the original goal of the Paris Agreement—the authors find that about 10 gigatons of carbon dioxide must be removed annually. This amount remains the same even if countries stepped up efforts to reduce carbon dioxide emissions from all sources.
“Getting us back to 1.5 degrees by the end of the century will require a balanced approach,” said lead author PNNL scientist Jay Fuhrman, whose work is from the Joint Global Change Research Institute. “If one of these technologies fails to materialize or scale, we don’t want too many eggs in that basket. If we use a globally diverse portfolio of carbon removal strategies, we can mitigate risk while reducing emissions.”
Some of the technologies make a significant contribution, with the potential to remove several gigatons of carbon dioxide annually. Others offer less, but still play an important role. Enhanced weather patterns, for example, could remove up to four gigatons of carbon dioxide per year by mid-century.
According to this method, finely ground rocks spread over arable land convert carbon dioxide in the atmosphere into carbonate minerals in the soil. It is one of the most efficient methods identified in the study.
By comparison, direct capture of the oceans with carbon storage, where carbon dioxide is removed from seawater and stored in the Earth’s subsurface, would likely remove far less carbon. On its own, the nascent technology is prohibitively expensive, according to the authors. However, combining this method with desalination plants in areas where demand for desalinated water is high could reduce costs while delivering more substantial carbon reductions.
In addition to the removal methods listed above, technologies under study include biochar, direct air capture with carbon storage, and bioenergy combined with carbon capture and storage.
Each of the technologies modeled brings unique advantages, costs, and consequences. Many of these factors are linked to specific regions. The authors point to sub-Saharan Africa as an example, where biochar, enhanced erosion and bioenergy with carbon capture and storage contribute to significant reductions.
However, the authors find that much work is needed to address greenhouse gases other than carbon dioxide, such as methane and nitrous oxide. Many of them are not CO2 The gases are several times more powerful and at the same time more difficult to target than carbon dioxide.
While some of the abstraction methods discussed in the new paper are well studied, their interactions with other, newer methods are less well understood. The work comes from the Joint Global Change Research Institute, a collaboration between PNNL and the University of Maryland, where researchers investigate the interactions between human, energy and environmental systems.
Their work focuses on projecting the trade-offs that may arise from a range of possible decarbonisation scenarios. The authors seek to better understand how these methods interact so that policymakers can inform their decarbonization efforts.
“This study highlights the need for continued research into carbon dioxide removal approaches and their potential impacts,” said corresponding author and PNNL scientist Haewon McJeon. “While each approach has its own unique benefits and costs, a diverse portfolio of carbon removal approaches is necessary to effectively address climate change. By better understanding the potential impacts of each approach, we can develop a more comprehensive and effective strategy to reduce greenhouse gas emissions and limit global warming”.
In addition to Fuhrman and McJeon, PNNL authors include Candelaria Bergero and Maridee Weber. Seth Monteith and Frances M. Wang of the ClimateWorks Foundation, as well as Andres F. Clarens, Scott C. Doney, and William Shobe of the University of Virginia also contributed to this work. This work was supported by the ClimateWorks Foundation, the Alfred P. Sloan Foundation, and the University of Virginia Institute for Environmental Resilience.