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What is Carbon Dioxide Removal?

The path to Net Zero

In order to avoid the worst outcomes of climate change, emissions of greenhouse gases, in particular carbon dioxide (CO2), need to be reduced as much as possible. Limiting global warming to 1,5 °C requires us to reach Net Zero emissions by 2050 at the very latest. However, even with great emission avoidance and reduction efforts, there still are emissions, for instance from agriculture or industrial processes, which are extremely hard and expensive to mitigate. To compensate for these, we need to actively reduce atmospheric levels of CO2 to maintain Net Zero or potentially even realize Net Negative emissions.

Enter Carbon Removal

The process to do this is called Carbon Dioxide Removal (CDR) and encompasses a range of solutions through which CO2 is removed from the atmosphere and sequestered safely for long periods of time. This is where CDR differs from emission avoidance: While avoiding emissions prevents CO2 from enterering into the atmosphere in the first place, CDR removes existing atmospheric CO2 and stores it permantenly.

Climate scenarios clearly show that we will require significant amounts of CDR if we want to stay within 1,5°C global warming: By 2050, the IPCC projects a global minumum need of 5 Gigatons of CDR annually. For perspective: This roughly equals total United States CO2 emissions in 2019. Today, only a tiny fraction of these amounts of CDR is able to be realized.

CDR is not created equal

While the CDR space is still in its infancy, it has become apparent that differences exist between CDR solution with regards to quality and price. With a shared, official framework yet to be established, a number of assessment critera have developed in practice:

PermanenceHow long will the CO2 be safely removed from the atmosphore?
AdditionalityDoes the CDR activity cause new climate benefits or would the carbon removal have happened anyway?
Carbon leakageAre emissions shifted elsewhere because of the CDR activity?
NegativityHow emission-intensive is the CDR process relative to its carbon removal potential?
VerifiabilityHow is the CO2 removal monitored and verified?
Side benefits/risksWhat are the consequences to ecosystems, biodiversity, food security, etc.?

The CDR solution continuum

There are broad range of ways to capture atmospheric CO2 and store it long-term. These CDR solutions can be categorized along a continuum, ranging from purely nature-based solutions, over hybrid solutions that combine nature-based and technical processes to purely technical solutions.

Nature-based CDR

Nature boasts one of the most established and efficient carbon removal processes: Photosynthesis. During plant growth, light energy is being converted into chemical energy and used to capture CO2 from the atmosphere. As part of this process, the captured CO2 is sequestered in the plant’s biomass. As photosynthesis is obviously not only performed by trees, nature-based CDR solutions go beyond tree-planting as well, to include, for instance, open water seaweed cultivation. Nature-based solutions do come with a number of problems, in particular with regards to the verification of sequestered CO2 as well as its permanence and the additionality of the activities.

Hybrid CDR

In an attempt to address some of the problematic issues of purely nature-based solutions whilst still making use of photosynthesis, a number of CDR pathways have emerged that combine the use of biomass with some form of technical process. In Bioenergy with Carbon Capture and Sequestration (BECCS), for instance, biomass is burned to produce power – a process that in itself would be considered carbon-neutral. Integrating Carbon Capture, however, enables the capture and long-term sequestration of the CO2 embedded in the biomass, resulting in negative CO2 emissions. Other hybrid CDR processes include the production of biochar using pyrolysis or enhanced weathering.

Technological CDR

On the other end of the spectrum, CDR solutions such as Direct Air Capture (DAC) solely rely on technology to remove CO2 from the atmosphere. Using fan-like devices, DAC sucks in ambient air, captures the embedded CO2 and releases the air essentially CO2-free. Similar to CCS applications, the CO2 is then sequestered permanently, typically in geological formations deep underground.