Your quick guide to the technology that could save the planet
On its own, the earth has remarkably efficient mechanisms for regulating CO2. Trees come to mind, which naturally convert carbon dioxide into oxygen while storing the carbon in their trunks and roots. Peat bogs and coastal ecosystems also play a critical role in the earth’s natural carbon cycle, but one of the most important players is the oceans themselves: Covering 71% of the earth’s surface, the oceans hold more than 50 times the CO2 that the atmosphere does, and are estimated to consume more than 2 billion tons (or 2 gigatons) of CO2 per year. Global warming, the most critical issue facing humanity today, is a result of the Earth’s natural carbon cycle being thrown out of whack by human emissions. We currently release around 40 gigatons of carbon into the atmosphere every year – a number so high that decarbonization (eliminating our current carbon emissions) is critical but no longer enough: We need new ways to remove CO2 in order to stave off the worst effects of climate change.
That’s where carbon dioxide removal comes in. Ultimately, it refers to human-led approaches (sometimes collaborating with natural climate solutions) to remove more CO2 than they emit and sequester that CO2 permanently (or at least for a significant length of time).
Below we explore four examples of pathways that are used to remove carbon, as well as some of the challenges that scientists, geologists, biologists, engineers, and innovators are working on to unlock each pathway’s maximum potential.
Direct Air Capture
Direct air capture (DAC) refers to any means of taking CO2 directly out of the air. Most DAC technologies that exist today use giant fans to suck in air and strip the CO2 from it using a chemical solution (called “sorbents”).
DAC solutions could, in principle, be situated nearly anywhere on Earth, powered entirely by renewable energy, and generate pure CO2 streams from the air. They offer radical potential for carbon removal, however they are currently limited by their cost. While the price tag of direct air capture has dropped dramatically over the past eight years, it remains very expensive. While DAC systems carry high capital costs, making it difficult to get them off the ground in the first place, the cost of direct air capture is primarily driven by the energy required to heat and cool the sorbent chemicals in the DAQ process. Today this energy also tends to come from fossil fuel power plants.
For DAC to reach its full potential, we need 1) for current market leaders to continue pushing innovation and deployment, and 2) to usher in the next wave of DAC innovators. These innovations will need to reduce their energy requirements and find a better fit with intermittent, renewable energy sources. These breakthroughs would turbo-charge the scalability of direct air capture and give us a powerful tool to draw down the buildup of CO2 in the atmosphere.
More than just a superfood, kelps and seaweeds are also especially efficient at capturing CO2 (hence their incredible growth rate). And unlike traditional land-based crops, they require zero fresh water and don’t compete with terrestrial food crops, bypassing the “food vs. fuel” debate that arises in conversations about burning or fermenting crops for energy extraction (see: bio-energy with carbon capture and storage, or BECCS).
For all these advantages, there are several hurdles that are keeping kelp and macroalgae solutions from becoming a serious vehicle to large-scale removal of carbon dioxide. The aquaculture industry is labor intensive and generally confined to shallow coastal waters. It also requires a large amount of energy to bring the crop ashore and dry it (kelp and macroalgae are roughly 80-90% water, so they’re heavy). Work is being done to drive down the cost of macroalgae production, but we’re not there yet. We not only need to drive down the cost of macroalgae cultivation, but we also need more research, funding, and testing for aquaculture carbon dioxide removal to take off.
At the far edges of ocean-based carbon removal solutions live ideas that harness various marine systems to stimulate CO2 uptake. For instance, some have proposed drawing nutrient-rich waters up from the deep ocean to stimulate the propagation of phytoplankton, which consume CO2 through photosynthesis. Others have argued that adjusting the alkalinity of the ocean would stimulate CO2 uptake. Ideas have even been floated (no pun intended) to take kelp and macroalgae crops and sink them to the deep ocean where the CO2 can be sequestered permanently.
The truth of the matter is we simply don’t know enough about the oceans and the potential ramifications of toying with these crucial systems. In theory, these ideas could prove to be valuable CO2 removal pathways. However, these ideas mostly live in research papers and are only just beginning to be seriously explored by experts. While these concepts may appear far out at first glance, the sheer capacity of the oceans make them especially compelling in the quest for gigaton CO2 removal.
Geologic sequestration consists of injecting CO2 into underground porous rock formations. The carbon dioxide reacts with the rock and mineralizes, keeping the CO2 locked underground for centuries, if not longer. Underground CO2 injection has been practiced for the last 50-60 years as a way to boost output from aging oil wells. Sequestration projects have been developed to store CO2 within saline aquifers.
On its own, geologic sequestration does not capture CO2, but it does represent an incredible capacity to permanently store carbon dioxide all over the world, making it a natural partner to technology like DAC.
Scaling up these types of projects from the demonstration phase to a larger commercial roll out presents a number of challenges, such as how to properly incentivize permanent geologic sequestration of CO2. There’s EOR, yes, but that’s problematic from the standpoint that it leads to more oil production which, when refined into fuels and combusted, introduces even more carbon dioxide into the atmosphere. Governments have largely led the way on these incentive efforts, through either carbon taxes (i.e. Norway) or crediting programs (i.e. California). Bringing down the costs of geologic sequestration would also go a long way toward the proliferation of these projects.
There are other ways to use rocks to remove and sequester CO2 from the air. Mineralization, sometimes called enhanced weathering, is a process by which naturally occurring rocks and minerals can passively remove CO2 from the air and over time chemically convert it into a more stable and long-lasting form of carbon. The famous White Cliffs of Dover are an excellent example. The white color comes from the chalk that makes up those cliffs. Chalk is the common name for calcium carbonate (CaCO3), a carbon-containing mineral that naturally forms over time as a key part of Earth’s carbon cycle. Enhancing or speeding up natural mineralization processes like that one, not just with chalk but other minerals, for instance those found in ultramafic rocks, or even some wastes from mining operations (mine tailings) can be used to bind CO2 and sequester it permanently in rock. These approaches may require large amounts of rock handling and processing, but may also require very little energy to drive the mineralization process.
Salt marshes, mangroves, seagrasses, and coastal wetlands, hold huge carbon removal potential given the ability of seawater to stall microbial activity that results in CO2 release. Mangrove soils and biomass are estimated to sequester more than twice the amount of carbon as soil. Yet, many coastal ecosystem management projects around the world are being undertaken without consideration for CDR, signaling a need for more research to fully understand the CO2 flux of coastal wetlands, and to properly value the CO2 removal and sequestration opportunities these areas offer.
Then there’s the issue of competition for available coastland. Globally, coastal wetlands are capable of removing more than 800 million tons of CO2 annually. But as the population pushes toward 10 billion by 2050, and with nearly a third of the current population living within 60 miles of the ocean, fierce competition for coastline could keep ecosystem management from reaching its full carbon removal scale.
This is where terrestrial ecosystems could come in. There are numerous ways for dry land to capture and store CO2 from the air. Yet experts point to the difficulty of quantifying the carbon content in agricultural soils. Experts are working to modernize the measurement of carbon within soil systems, but more resources are needed before the agricultural sector is able to confidently value the carbon sequestration performed by land.
There are some who propose burying burned biomass underground as a way to safely store CO2 in the soil. Biochar, produced by burning biomass in the absence of oxygen, is generally more than 50% pure carbon and has been identified as a potentially cost-effective permanent CO2 storage solution. Most biochar, however, is either burned for heat or used for livestock feed. Further, the high cost of biochar coupled with a general lack of awareness of its soil enhancing properties help explain biochar’s lack of momentum as a larger carbon sink.
Each of the pathways described above hold the potential to reduce the CO2 in the atmosphere and oceans and help us meet our Paris Climate Agreement goals. Experts warn, however, that no one pathway will be enough to meaningfully reduce the glut of carbon dioxide already in the air, especially given our current rate of emissions. The best approach is for a combination of some, or ideally all, of the above to reach the gigaton scale, while also working to drastically reduce the amount of carbon dioxide we’re emitting in the first place.
The challenge now is scaling up the existing technologies and uncovering radical new ideas that blow away our current expectations, because the technologies described here only scratch the surface of the types of projects that sequester CO2. The XPRIZE Carbon Removal will support and incentivize teams in accelerating existing and discovering new carbon removal pathways. The biggest private competition in history, XPRIZE Carbon Removal will fast forward the breakthroughs in carbon removal technology that we urgently need.
To win the XPRIZE Carbon Removal, teams will need to build and demonstrate a real, working, permanent carbon removal solution. They’ll need to prove its efficacy by measuring energy use, land efficiency, and total tons CO2 removed, while accounting for any negative environmental externalities. The teams will also need to convince a panel of expert, independent judges of its ability to get to low cost when deployed in the future at scale.
To find out more about the Prize, visit https://www.xprize.org/prizes/elonmusk