Ever heard about negative emissions technologies (NETs) before? What about carbon removal? If not, you probably will. The more carbon dioxide (CO2) we emit into the atmosphere, the more carbon we’ll also need to remove in order to meet global climate goals, such as limiting global warming to 1.5°C (2.7°F) or 2°C (3.6°F).
We’re increasingly turning to NETs as an essential way of mitigating climate change—since it’s only by removing carbon from the atmosphere that we can reach net-zero and negative emissions.
Ready to remove some carbon? We might begin asking ourselves: why reduce emissions if we can just remove them from the atmosphere? Unfortunately, it’s not that simple.
And there’s also danger in becoming overly reliant on technology. And business-as-usual is precisely what we’re trying to avoid here. So, let’s take a deep dive into the potentials, and dangers, of negative emissions technologies.
We can divide NETs into three different categories:
- Technology-based solutions,
- Nature-based solutions
- And enhanced natural processes.
We’ll bring up some of the most common NETs within these different categories, starting with the technology solutions.
1) Technology-based Negative Emissions Technologies Solutions
a) Carbon Capture and Storage (CCS) – Many technologies, same steps
Carbon capture and storage (CCS) technologies provide a way of removing CO2 from the atmosphere and storing it. Notice how it said technologies there? There’s a lot of CCS technologies out there, going about capturing and storing carbon in different ways. But there are essentially three steps that are true for all of them:
First, we capture CO2. Again, this differs with different technologies. Generally, we capture CO2 from power plants and industrial facilities’ flue gas (the industrial emissions released into the atmosphere).
CO2 is transported to the storage site. To transport the CO2, it must convert into liquid form and move through pipelines or by ship.
The carbon is stored in the bedrock. Not just any bedrock, though, and not just at any depth. There needs to be enough pressure to liquefy the carbon, which requires a depth of at least 800 meters.
The bedrock also needs to be porous enough, meaning that there needs to be a lot of tiny spaces/holes within the rock to make room for the liquid CO2.
An impermeable bedrock must then cover the porous bedrock, ensuring that the CO2 doesn’t leak out into the atmosphere. So quite a few conditions there.
b) Carbon Capture, Utilisation and Storage (CCUS)
There can be a third element to CCS, throw a U in there, and suddenly you have a technology that not only stores CO2 but also utilises it.
The utilisation part of the CCUS is not unproblematic. CCUS typically captures CO2 at power plants or industrial facilities, where it’s separated from the other emissions and transported for usage and/or storage.
As you might have figured, using the CO2 rather than storing it means that it can’t deliver on “negative emissions” to the extent that CCS can.
In fact, CO2 use within the energy sector delivers less than 13% of the emissions reductions that CO2 storage does. And its lack of emissions reductions isn’t its only problem. This becomes clear when looking at the two primary users of CO2 from CCUS: the fertilizer industry and the oil sector.
The Fertilizer Industry
The fertilizer industry uses CO2 and ammonia to manufacture urea, a type of nitrogen fertilizer. And fertilizers can cause adverse impacts on the environment. Ammonia is toxic to marine animals, and when fertilizers leak into nearby water bodies, eutrophication occurs, where algal blooms cause oxygen depletion that eventually suffocates the marine creatures.
The Oil Sector
The oil sector also consumes a lot of CO2 for enhanced oil recovery (EOR), which is all about getting more oil. Through a gas injection, the CO2 expands in an oil reservoir and pushes previously inaccessible oil to the wellbore (which extracts the oil).
It’s possible to produce 30-60% more oil from a reservoir using EOR techniques. Using CO2 to get more oil, which releases more CO2, isn’t exactly ideal from a climate perspective.
Having said that, there’s potential for CO2 used to benefit the climate. But the technology still has a long way to go before CO2 use can contribute to climate goals. And there are a few things to consider, mainly if using CO2 from CCUS would replace a product or service with higher CO2 emissions from a life cycle perspective.
c) Bioenergy with Carbon Capture and Storage (BECCS)
Unlike other CCS technologies we have today, bio-energy with carbon capture and storage (BECCS) is relatively cost-effective and considered a crucial part in limiting global warming to 2°C (3.6°F). And by adding bioenergy into the CCS equation, BECCS contributes both to negative carbon emissions and decarbonisation.
A classic case of CCS—It works like this:
Fast-growing energy crops (low-cost, low-maintenance crops and trees) are grown for biomass fuel, and as the plants grow, they absorb CO2. When harvesting and converting biomass into energy or fuel, the BECCS facility captures the CO2 before it reaches the atmosphere and stores it in the bedrock.
Although it’s relatively energy-intensive, it produces negative emissions as long as the stored CO2 is more than what it emits. Of course, nothing is ever that simple.
The major obstacle for BECCS to work on a global scale is land use. BECCS uses a lot of lands that are also needed to grow forests and food. Estimates suggest we need up to 700 million hectares (a third of all land suitable for growing crops) to produce enough bioenergy crops for BECCS to make a real impact in limiting global warming.
That’s an area up to two times the size of India. Other factors can also limit BECCS, such as nutrient demand. And if we need fertilizers to keep production levels high, we risk contaminating local ecosystems.
d) Direct Air Capture (DAC)
As the name implies, direct air capture (DAC), or direct air capture and storage (DACCS), capture CO2 directly from the atmosphere. This makes it slightly different from other CCS technologies, which capture the CO2 from flue gas at power plants and industrial facilities.
A common way in which DAC operates is through fans that draw in the air. The CO2 then needs to be separated from the rest of the air, generally done in two different ways. One uses a solid filter to bind the CO2, which is then subjected to heat, releasing the CO2 for capture.
We can also use a liquid solution to bind the CO2. Regardless of which process we use, the air returns to the atmosphere with significantly lower CO2 levels.
Since the CO2 in the atmosphere is not as concentrated as in industrial emissions (thankfully), DAC technologies are more expensive and energy-intensive than other NETs.
On the other hand, DAC doesn’t have the same land-use requirements that limit other technologies, such as BECCS, and can be located close to storage sites, removing the need for transporting CO2 across long distances.
However, like most NETs, the impact isn’t anywhere near where it needs to be yet. Today, the DAC captures 0.01 million tonnes of CO2, which, considering that global CO2 emissions are at 6.44 billion tonnes—really isn’t much to celebrate.
2) Nature-Based Negative Emissions Technologies Solutions
Let’s move away from all these technical forms of carbon removal for a second and turn to nature. Nature-based solutions to carbon removal focus on increasing the CO2 reduction that occurs naturally.
a) Afforestation and Reforestation (AR)
As we know, plants and trees absorb CO2 through photosynthesis, making them a natural CO2 sink. Because of these natural CO2 removal skills, and the low costs associated with planting trees, tree planting has become an essential form of mitigation.
Afforestation refers to trees planted on land that hasn’t been forested in recent history (commonly in 50 years) and,
Reforestation refers to the planting of trees on the land that was recently deforested. We’ll refer to them collectively as AR here.
One of the main issues with large-scale AR to mitigate climate change concerns the albedo effect. Albedo measures a surface’s ability to reflect sunlight. Bright surfaces reflect more sunlight from earth (high albedo), while darker surfaces absorb more sunlight (low albedo).
Since forests are rather dark, they absorb a lot of sunlight, which increases surface temperatures. This is especially true in forests in colder climates with more snow, accelerating snow, and ice melting.
Another issue is the question: Biodiversity vs. Carbon uptake. From a biodiversity perspective, AR should focus on planting native and diverse tree species. From a carbon uptake perspective, it is preferable to plant fast-growing species that absorb more carbon faster. Though, biodiversity wins the debate because of factors such as habitat quality and resilience.
b) Soil Carbon Sequestration (SCS)
There’s a lot of talk about forests, but agricultural soils are actually one of the largest carbon reservoirs on the planet, and the soil’s capacity to store carbon is larger than that of both the atmosphere and vegetation. And frankly, carbon does a lot of good when stored in the soil.
Soil carbon is beneficial for the quality of the ground, crops, and the environment. It prevents soil erosion and desertification and increases biodiversity. Since soils contain high amounts of carbon, they can act as both a carbon sink and a carbon source, depending on inputs (e.g., roots and manure) and outputs.
The output, which, if too high, turns the soil into a carbon source, occurs through soil respiration. Soil respiration comprises CO2 produced by organisms in the soil. This CO2 is emitted into the atmosphere as the soil is disturbed and exposed to the air.
Historically, the conversion of former grasslands and forests to agricultural and grazing land has caused a huge loss of soil carbon across the world. This has mainly occurred through the large increase in soil disturbance, which increases soil respiration.
And conventional agriculture has continued this trend of soil disturbance. Apart from causing CO2 emissions, losing soil carbon also leads to land degradation, where the decreased carbon levels cause lower crop yields, erosion, and a loss of biodiversity.
Soil carbon sequestration (SCS) occurs when land management increases soil carbon levels rather than depletes them, resulting in a net removal of CO2 from the atmosphere. We can do this in several ways.
Soil Carbon Sequestration: How?
Primarily, reducing soil disturbance will increase soil carbon levels. We can achieve that by lowering tillage (digging or turning the soil, usually when planting new seeds) and planting perennial crops. Perennials, unlike annual crops, do not need to be replanted each year, which reduces tillage.
Planting cover crops rather than leaving the fields fallow will also benefit soil carbon levels since it reduces soil respiration, and adding compost and/or crop residues will also add more carbon to the soil. If we apply these techniques to conventional agriculture, there’s potential for SCS to store close to 14% of today’s global CO2 emissions each year. Bingo!
There’s one drawback, though—the soil cannot contain an endless amount of carbon. It gets saturated. And when the ground can’t sequester any more carbon, it no longer functions as a NET. This occurs after 10-100 years, depending on the type of soil, climate, and SCS techniques.
c) Blue Carbon
In fact, these so-called “blue carbon ecosystems” are ten times more effective at sequestering CO2 (per area/per year) compared with forests and twice as effective at storing carbon in their soil and biomass. It’s a shame they only make up two percent of the global surface area.
And just like soil carbon, carbon sequestration and storage is only one part of the benefit they provide. These ecosystems also improve the water quality, benefit biodiversity, and provide shoreline protection for communities, protecting against sea-level rise and floods.
And just like forests, they are increasingly going from carbon sink to carbon source. Blue carbon is captured and stored in coastal and marine ecosystems, primarily mangroves, salt marshes, and seagrasses. These highly productive ecosystems are effective at sequestering (removing carbon from the atmosphere) and storing carbon.
Through deforestation, coastal development, agriculture, and aquaculture (farming of aquatic creatures), it’s estimated that we’ve lost a third of these ecosystems over the past few decades. This causes them to release their stored carbon back into the atmosphere.
3) Enhanced Natural Processes Negative Emissions Technologies Solutions
Let’s move into a bit of a grey area with these last couple of negative emissions technologies and take on hybrid approaches that combine nature-based solutions with technological removal.
a) Enhanced Weathering (EW)
Weathering is a natural process of breaking down rocks because of rain, wind, or temperatures. Since the minerals in the rock take up CO2 as they dissolve, enhanced weathering (EW) uses technology to speed up this process by distributing a rock powder on land and/or the ocean.
As the minerals in the rock powder dissolve, CO2 is sequestered. Apart from taking up CO2, EW can also provide soils with nutrients and decrease ocean acidification (which threatens marine organisms as the ocean absorbs too much CO2).
The danger with EW is that, although it mimics a natural occurrence, it’s not natural. And since it’s a technology relying on chemical reactions, environmental impacts, as well as the amount of carbon sequestered, vary depending on the type of rocks used in the powder (there’s, for instance, risk of heavy metals polluting the environment), the ecosystem it’s applied to and the local climate.
Speeding up the rate at which we add these substances to ecosystems could threaten the stability of the ecosystems.
Biochar is a type of charcoal produced by burning biomass in the absence of oxygen. Sounds more like a carbon source than sinks?
Stay with us for a minute…
When a plant decays, or burns for that matter, it releases CO2. Well, you can’t spell CO2 without O., And you know what the O stands for…? That’s right, oxygen! By removing oxygen from the equation, burning the biomass doesn’t emit any CO2, and the carbon stays in the produced biochar.
Biochar can do a lot of good in combination with SCS since adding biochar increases the carbon levels in the soil, providing all the benefits that come with it.
However, one side effect of adding biochar to the soil is the darkening of the soil surface. This would decrease the surface’s albedo (remember this from the AR section?), which would decrease its mitigation potential.
Biochar could also reduce the air quality since it could release tiny particles into the atmosphere during production, transport, and through wind distribution of the soil.
Still in its cradle with negative emissions technologies
CCS, CCUS, BECCS, DACCS, AR, SCS, Blue Carbon, EW, Biochar… It’s a veritable jungle out there with all the negative emissions technologies—NETs.
But before you get too carried away with the potentials of carbon removal, there’s something you should know. Even though NETs such as CCS have been around since 1972, the technology is still in its cradle.
Today, CCS technologies capture about 40 million tonnes of CO2 each year. This represents 0.1% of annual CO2 emissions at the global scale.
And even with nature-based solutions that are fully understood and developed, there are other constraints to carbon removal. For AR, it would be the vast amount of land required, while for SCS, it’s sink saturation.
So remember, while NETs, negative emissions technologies, could be part of the solution to limiting further global warming, it’s not the solution.
At the end of the day, there’s only one proper solution to mitigating climate change: reducing greenhouse gas emissions. Now.
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