Governments and companies are increasingly betting on a mix of technologies to remove huge amounts of carbon dioxide (CO2) from the air as the effects of climate change become more and more pronounced.

“Many people are looking for an easy answer for scaling [carbon] removals,” said Eve Tamme, managing director of Climate Principles, a climate policy advisory group based in the Netherlands. “But it’s not just one thing. It’s a growing ecosystem of solutions where every removal method has limitations, and incentives and policy mechanisms for developing these will vary.”

Let’s explore five prominent proposed solutions. We start with an emerging—and much discussed—novel engineered solution, followed by solutions that combine new technologies with natural processes and end with nature-based methods.

We’re not getting into the plethora of actions by corporations—ranging from the Frontier coalition to Microsoft—in this space, but we’ll tackle those in future articles.

A novel engineered solution

Direct air capture (DAC) is an up-and-coming technology that uses chemical reactions to capture CO2 from the atmosphere by pulling air through a series of fans. It’s also probably the carbon removal strategy you’ve heard about the most.

The process is different than carbon capture and storage (CCS), which captures CO2 at a particular source (like a power plant) before it can be released into the atmosphere.

Eighteen DAC plants are operational in Europe, the United States and Canada, mostly small facilities that capture CO2 for re-use—in fizzy drinks, for example—although there are also plans to use the carbon to extract oil, which can limit the tech’s climate benefit.

Ultimately, most carbon will need to be stored underground, where it’s trapped as a liquid or solid in the tiny pores of subterranean rocks. Only two DAC plants currently store the captured CO2 in geological formations, according to the International Energy Agency.

The world’s largest operating DAC plant, which is in Iceland and run by industry leader Climeworks, can remove 4,000 metric tons of CO2 a year. That’s a blip compared to what is needed, illustrating how early the industry’s efforts are in scaling up the tech.

DAC has a relatively small physical footprint, taking up far less space than planted trees to absorb the same amount of CO2 (more on that below), and potentially storing the carbon for 10,000 years or longer. But DAC is energy-intensive, due to the chemical heating processes used, and it could compete with other sectors for renewable power as efforts scale up.

Another core hurdle is DAC’s hefty price tag. Last year, Climeworks said it was removing one metric ton of CO2 for around $800 at its Iceland plant. A Climeworks spokesperson told Cipher last week the company plans to bring costs down to $100-$300 per metric ton “in the long term.” The U.S. Energy Department’s goal is to reach a cost of $100 per metric ton or less by the end of the decade.

Nature-inspired solutions with an engineered twist

Bioenergy with carbon capture and storage (BECCS – pronounced “becks”) is the only CDR technique that can also provide energy.

This technology involves burning crops and wood residues to create electricity or biofuels and then using CCS technology to capture the resulting CO2 and store it underground.

Bioenergy is the subject of a lot of debates we don’t have space to delve into here, but in short:

Electricity from biomass is considered a renewable energy source even though the plant matter produces emissions when combusted because, in theory, new plants will absorb an equivalent amount of CO2. When the CO2 is trapped instead, the method is considered a carbon removal solution.

BECCS reduces reliance on fossil fuels, but crops grown for energy production can compete for land with crops grown for food production. Some environmental groups contend the technology is unproven and could have unintended consequences.

The world has just 2% of the amount of BECCS it would need by 2030 to reach net-zero emissions by 2050, according to the IEA. Bioethanol production, which makes up most of that 2%, is mature and commercial, while other BECCS types are still at the pilot stage, the IEA found.

An adjacent method called biomass carbon removal and storage (BiCRS) converts unwanted agricultural residues like corn stalks into a carbon-rich bio-oil through a high-heat process called pyrolysis. The bio-oil is then injected deep underground.

The ocean, which also soaks up carbon, has already absorbed 42 times more carbon than the atmosphere. Various efforts are underway to supercharge this absorption capacity, including fertilization and alkalinization.

Fertilization involves adding nutrients to sunlit parts of the ocean to stimulate photosynthesis and increase natural CO2 absorption. Alkalinization involves increasing the ocean’s ability to absorb CO2 by adding alkaline materials, such as lime, into the water.

Both approaches are still in experimental stages and come with hard-to-anticipate risks to marine ecosystems.

Nature-based solutions

Planting trees—which naturally absorb CO2 from the air—is the most common nature-based carbon removal strategy. Reforestation (growing new forests in places where trees have been chopped down) and afforestation (converting degraded agricultural land into forests) are the two main approaches.

These methods are cost-effective, important and already practiced at scale. But only so much land can be devoted to forests. What’s more, the carbon is only stored for the lifetime of the trees, normally decades or centuries, but sometimes shorter if trees are killed by wildfires or logging, for example.