Our planet has a big problem: too much carbon in the atmosphere. Carbon-rich greenhouse gases, such as carbon dioxide (CO2), slow the rate at which the earth sheds heat to space. When there’s too much CO2 in the atmosphere, the planet warms up—which is what’s happening now.
One solution to this problem, obviously, is to stop adding more carbon to the atmosphere. When the tub is overflowing, you don’t start bailing water out with a bucket–first you turn off the tap. But at this point, that may not be enough. Increasingly, scientists are arguing that we also need to remove some of the carbon that’s in the atmosphere now.
Enter carbon sequestration. It’s the process of converting the carbon in greenhouse gases into solids or solutions that can be stored. This keeps it out of the atmosphere, so it can no longer contribute to global warming.
Carbon is an element that’s part of every living thing. You, me, the noisy birds outside your window, the grass growing between sidewalk cracks—they all contain carbon. So does everything made from living things, from a wooden chair to a wool sweater. Carbon is literally the stuff of life.
You can find carbon in every part of our planet. It’s in the rocks and soil under our feet, in the oceans, and in the atmosphere. And that’s a good thing, because the greenhouse gases in our atmosphere make life on Earth possible. Without them, our planet would be too cold for anything to survive.
But in the past 200 years or so, levels of greenhouse gases have risen sharply. That’s mostly on account of human activities, particularly from burning fossil fuels. Fossil fuels contain rich deposits of carbon formed over millennia, and burning them releases it all at once. As a result, our atmosphere is now retaining too much heat, putting us and all species at risk.
Carbon sequestration means storing carbon in a stable form. This already happens constantly as part of the earth’s natural carbon cycle. The ocean absorbs CO2, for instance, and plants take it in as they grow. These natural consumers of carbon are called “carbon sinks” because, like a sink, they drain off carbon from the atmosphere.
Plants, soils, and the ocean have already absorbed around 55% of the extra carbon humans have added to the planet. Without this natural carbon sequestration, the planet would be warming much faster than it is. But these natural processes can’t keep up with the vast amount of carbon humans are putting into the atmosphere.
So, when people talk about carbon sequestration, they don’t mean letting nature take its course. They’re talking about humans stepping in to speed up the process. Artificial carbon sequestration is often paired with carbon capture—removing carbon directly from the air. This usually means “scrubbing” CO2 out of smokestack emissions, compressing it, and storing it.
There are three main ways to sequester carbon. Biological sequestration converts carbon to plant matter. Geological sequestration stores it in rock. And technological sequestration stores it in new forms that don’t exist in nature. By deploying all these methods at once, we can maximize the amount of carbon we remove.
When people think about trapping carbon in plants, they usually think of trees. But trees aren’t the only plant that can store carbon. There are also vast amounts stored in grasslands, soils, and ocean plants.
Trees take in CO2 through their leaves. Through photosynthesis, they turn it into their roots, trunks, and branches, trapping the carbon in their wood. It eventually returns to the carbon cycle when they die and decay. In the U.S., forests absorb about 12% of all carbon emissions.
But this massive carbon sink is shrinking. Earth has lost one-third of its tree cover since the last Ice Age. And half this loss was in the last century alone. We’re currently losing forest at the rate of about 10 million hectares (nearly 2.5 million acres) per year. That’s an area larger than Yellowstone National Park.
Some of this loss is due to deforestation: clearing forest land for other uses, such as agriculture. However, even more is due to forest degradation, or declining health of forests. Trees perish every year due to pests, wildfires, and illegal logging. And as forests die, they don’t just stop absorbing carbon; they release what they’ve stored.
Thus, the biggest key to storing more carbon in forests is halting deforestation and degradation. That means keeping logging sustainable. It means finding better ways to grow food to stop the clearing of forests for agriculture. And it means managing forests better to protect them from pests, disease, and fire.
But we can go further and reverse this process by adding more trees. One method is reforestation: restoring lost trees in degraded forests. Another is afforestation: growing new forests in places that have none, like degraded farmland. We can also plant trees in other places, such as cities. Urban trees provide shade that keeps neighborhoods cool, reducing the need for air conditioning. Within the same city, neighborhoods with the most trees have temperatures around 15°F cooler than those with the fewest.
Grasslands are also major carbon sinks. This may seem surprising, since grasses are much smaller than trees. But most of the stored carbon isn’t in the visible part of the grass, but in the roots. Grasslands currently store about 12% of all terrestrial carbon.
A 2018 study at UC Davis found that in California, grasslands make more reliable carbon sinks than forests. That’s because they’re less vulnerable to damage from drought and fire. Forests capture carbon mostly in the trees themselves, so a forest fire can release it all. But because grasslands store it mainly underground, it remains in the soil even if the grass burns.
Grasslands absorb and store carbon in the soil naturally. But it’s possible to turn the soil on farms and ranches into a carbon sink as well. Ways to do this include:
Currently, about 45% of all habitable land is devoted to growing crops or grazing animals. All this land has the potential to store quite a lot of carbon. According to most estimates, soil could sequester the equivalent of 2 to 5 gigatons of CO2 each year.
The ocean stores carbon in multiple ways. For one, ocean plants like plankton absorb and photosynthesize it. These plants can then feed other sea creatures, like fish, transferring the carbon to them. When the animals die, their remains fall to seafloor, containing the carbon. The ocean also absorbs carbon directly, as carbon dioxide in the air dissolves into seawater at the ocean’s surface.
The ocean absorbs quite a lot of CO2 already. Currently, about 30% of human-caused CO2 emissions go into its upper layers. Unfortunately, this has resulted in harmful ocean acidification. When the ocean is more acidic, it’s harder for creatures such as corals to form shells. Other sea creatures, such as sea snails, have a harder time escaping from predators. And aside from these problems, there’s a limit to how much carbon the ocean can absorb naturally.
There are several ways to trap more carbon in the ocean. One is deep ocean storage. If you inject CO2 at least 1,000 meters below surface, it will sink to the seafloor. However, doing this can make ocean acidity worse.
Another approach depends on ocean plants. Plants like algae and seaweed absorb carbon naturally as they grow and release it when they die. However, there are ways to stop this from happening. You can harvest the plants and convert them into products like bioplastics. Or you can sink the plant matter to the ocean floor, trapping its carbon.
An alternate solution is enhanced weathering. When rocks like limestone and basalt erode, their material runs off into the ocean, making it more alkaline. This, in turn, allows it to absorb more CO2 from the atmosphere. We could speed up this process by grinding up alkaline minerals and scattering their dust on beaches.
There are several ways to contain carbon within rocks. One method, deep geological storage, involves pumping liquid CO2 deep underground and trapping it in rock formations. Carbon can be stored in coal seams, depleted oil and gas fields, and saltwater aquifers—on land or below the seabed.
Another approach is to turn the carbon into minerals. CO2 reacts with metals to form minerals like calcite and magnesite. In these forms, it remains stable for decades.
Many methods of sequestering carbon depend on natural processes. Technological carbon sequestration is quite different. It uses entirely new processes to convert carbon into stable forms. For instance, carbon can be made into:
The Carbon XPrize is a contest to come up with the best products made from captured carbon. Products currently vying for the $20 million prize purse include vodka, hand sanitizer, sunglasses, drinking straws, jewelry, and pens.
Carbon sequestration is an important tool for controlling climate change. By reducing the amount of carbon in the atmosphere, it improves our chances of keeping global warming in check. That’s important not just for humans, but for the health of our forests, oceans, and every part of the planet.
However, carbon sequestration has its drawbacks, too. For one, some methods of carbon sequestration can damage the environment. For instance, deep ocean storage could make ocean acidification worse.
But even methods that aren’t directly harmful have another problem: They’re expensive. It typically costs more to sequester a ton of CO2 than to avoid emitting a ton somewhere else. And every dollar we spend on sequestering carbon is a dollar we can’t spend on reducing emissions directly.
Worse, carbon sequestration could distract us from the urgent task of cutting emissions. Governments might decide there’s no need to phase out fossil fuels because we can always sequester the carbon later. That would be a costly mistake.
Despite its drawbacks, carbon sequestration is an essential piece of the climate puzzle. At this point, we have almost no chance of avoiding catastrophic levels of warming without it.
You can think of our planet as a boat that’s taking on water. To keep it from sinking, we need to bail out the excess water (that is, atmospheric carbon) through sequestration. But we also need to plug the leak by keeping more carbon from entering the atmosphere.
In other words, carbon sequestration isn’t an alternative to building the clean energy economy. But it may be a necessary supplement to it.