Negative emissions technologies: necessary but not sufficient

Image credit: George Hodan, CC0 Public Domain
Image credit: George Hodan, CC0 Public Domain

To protect the climate, we need to rapidly cut our emissions of greenhouse gases – but that alone might not be enough. Here’s why we have to take carbon out of the atmosphere using negative emissions technologies.

The goal: a safe climate

To avoid dangerous climate change, the international Paris Agreement aims to limit the increase in global average temperature to no more than 1.5°C to 2°C above the temperatures of the pre-industrial era1. Global average temperatures have already increased by between 0.8°C and 1.2°C2.

Cutting emissions is vital for limiting climate change

To have a 66% chance of limiting climate change to 1.5°C, we can emit no more than a ‘carbon budget’ of 420 GtCO2 (that’s 420 billion tonnes of carbon dioxide), according to the latest estimates of the IPCC (Intergovernmental Panel on Climate Change)3. That’s about a decade’s worth of current emissions.

The graph below shows that, if we want to stay under 1.5°C of warming, then starting this year (2020) we need to cut emissions at “monumental” rates, reaching over 20% per year by the 2030s (you might have seen a similar graph in One flight can burn through your annual fair share of carbon).

Image credit: Robbie Andrew, Creative Commons Attribution 4.0 International.

Limiting climate change to 2°C rather than 1.5°C would still require steep emissions cuts of 6% per year if we start now, as shown below. But here are just some of the reasons why allowing more warming could be dangerous. At 2°C, compared to 1.5°C:

  • Several hundred million more people could be at risk of both poverty and climate-related harm
  • 10 million more people could be exposed to the risks of rising sea levels
  • The decrease in fish catches could be twice as large
  • There would be a higher risk of setting off an irreversible feedback loop leading to multi-metre sea level rise4.
Image credit: Robbie Andrew, Creative Commons Attribution 4.0 International.

We need negative emissions as well

Reducing emissions by double-digit percentages each year is highly unlikely to be practical. Far from falling, global CO2 emissions from fossil fuels and industry are at a record high: they rose by 1.6% in 2017 and 2.1% in 20185. Even following the recession of 2008, global emissions only fell by 1.1%, swiftly followed by a rebound of 4.9%6.

The longer we wait, the steeper the cuts will have to be. As Robbie Andrew, creator of the graphs above, comments7, “Since such steep mitigation is impossible, the only way to achieve this budget is with very large ‘negative’ emissions: pulling CO2 out of the atmosphere.”

The IPCC agrees. In most of its modelled scenarios, keeping climate change below 1.5°C means removing roughly 100 to 1000 GtCO2 from the atmosphere this century using negative emissions technologies8. For an idea of scale, if you could convert all the carbon in 1000 GtCO2 into one giant diamond, it would be bigger than Mount Everest9.

What are negative emissions technologies?

Negative emissions technologies (NETs) are any way of taking greenhouse gases (including carbon dioxide, methane and others) out of the atmosphere.

Imagine CO2 in the atmosphere as being like water in a bath: cutting emissions is like turning off the tap, while using negative emissions is like taking out the plug. The level of water will only fall if more water is escaping through the plug than is coming in through the tap.

NETs, also known as carbon capture and sequestration, drawdown technologies, air capture technologies and carbon dioxide removal, come in many different forms (as shown in the box below). I’ll cover individual NETs in more detail in later blog posts. To be notified when they are published, subscribe here.

A selection of negative emissions technologies

Restoration or conservation of ecosystems

So-called natural climate solutions involve conserving or restoring ecosystems such as forests, grasslands and wetlands. Ecosystems can store carbon as wood or organic matter in the soil10.

Benefits Drawbacks
  • Regenerated ecosystems are likely to be better for wildlife than plantations of one type of tree.
  • Restoring ecosystems for carbon storage can have other benefits, including purification of air and water.
  • Carbon capture can slow down after a few decades as forests become mature and soils reach a saturation point where they can’t absorb much more carbon11.
  • Reduces area of land available for farming.
  • Forest fires, drought, etc. could release carbon back into the atmosphere.

BECCS (bioenergy with carbon capture and storage)

Wood, straw or other plant material is burned, producing energy. The CO2 from the burning process is captured and stored12.

Benefits Drawbacks
  • Easier to capture CO2 from burning than from the atmosphere, as it is more concentrated.
  • Energy is produced.
  • Cheaper than some other NETs.
  • If crops are grown specifically to be burned, they can compete with producing food13.
  • Crop monocultures would not support much wildlife.

Direct air capture

Special filters or solvents can capture CO2 directly from the atmosphere14.

Benefits Drawbacks
  • Doesn’t use much land, so has low impacts on biodiversity.
  • Can be located anywhere, reducing need for pipes to transport CO2 to storage facilities.
  • Energy-intensive, because a large volume of air has to be processed to extract the dilute CO2.
  • The special materials used could release harmful chemicals.
  • Currently expensive.

Enhanced weathering of rocks

CO2 reacts spontaneously (but slowly) with some rocks to form solid carbonate minerals. The process can be sped up by exposing these rocks (either above or below ground) or waste matter from mining to concentrated CO2 streams. Alternatively, rocks can be ground up and spread on soils15.

Benefits Drawbacks
  • Carbon is stored in solid form, so is less vulnerable to leaking than when stored as a liquid.
  • Speeding up the weathering process can make waste from mining or other industry less toxic.
  • Weathering of mining waste could be cheap.
  • Mining and grinding rocks specifically for carbon capture could be as expensive as direct air capture.
  • Enhanced weathering that involves pumping CO2 underground could trigger earthquakes.

The dangers of relying on negative emissions technologies

Will negative emissions technologies work?

Most NETs are not yet ready to be used at a large scale, with many still at the research or demonstration stage (including direct air capture, enhanced weathering and bioenergy with carbon capture and storage). More development will be needed before they are cheap and reliable enough to contribute significantly to stopping climate change.

The European Academies’ Science Advisory Council finds that NETs “offer only limited realistic potential to remove carbon from the atmosphere and not at the scale envisaged in some climate scenarios”16, with methods based on forests and soils being the “most technologically credible” at the moment.

It isn’t always clear how long carbon could be stored for: forest fires could rapidly burn through several decades of accumulated carbon, while underground stores of liquid CO2 could leak.

Negative impacts could be severe and hard to predict

As with any form of geoengineering, there is a risk that interfering with the Earth’s biological and geological systems could have unwanted side effects – both those that are expected and those that are hard to predict because of the Earth’s complexity.

Some NETs would use large areas of land. For example, just one wood-fired power station – Drax in Yorkshire, which is trialling bioenergy with carbon capture and storage (BECCS) – uses more wood than the UK produces17. If NETs are scaled up, there could be three-way competition between planting crops for BECCS, producing food for the world’s growing population, and protecting or regenerating natural ecosystems and the biodiversity they host.

There’s even a risk that removing CO2 wouldn’t slow climate change as quickly as we hope. The ocean currently absorbs about a quarter of our CO2 emissions18. Some scientists think that removing CO2 from the atmosphere would cause carbon to spill back into the atmosphere from the ocean, meaning that NETs would have to mop up even more CO2. Long Cao and Ken Caldeira argue19 “To maintain atmospheric CO2 and temperature at low levels, not only does anthropogenic CO2 in the atmosphere need to be removed, but anthropogenic CO2 stored in the ocean and land needs to be removed as well when it outgasses to the atmosphere.”


Perhaps the greatest danger in suggesting that NETs could solve climate change is that people, governments and businesses could become complacent and feel less motivated to cut emissions immediately. If, in the future, NETs don’t work as well as we had hoped, our failure to cut emissions as rapidly as we could have done will make dangerous climate change more likely20.

Take ExxonMobil as an example. The fossil fuel company has teamed up with startup Global Thermostat to develop direct air capture technology. At the same time, it intends to burn most of its existing oil and gas reserves21 and to hunt for more22, suggesting that “targeted investments could mitigate production-related emissions”.

Is it a good thing that large companies such as ExxonMobil and Microsoft (which recently pledged to invest $1 billion in carbon reduction and removal technologies23) are paying attention to NETs? Climate researchers Kevin Anderson and Glen Peters argue24 that NETs “…could very reasonably be the subject of research, development, and potentially deployment, but the mitigation agenda should proceed on the premise that they will not work at scale.”

Climate activist Greta Thunberg makes a similar point in an open letter25 calling for governments to restore ecosystems to capture carbon, saying “This approach should not be used as a substitute for the rapid and comprehensive decarbonisation of industrial economies.”

Further resources

It’s highly likely that negative emissions will be required to avoid dangerous climate change, but they are often missing from climate discussions because many policymakers do not understand how heavily the IPCC’s climate scenarios rely on NETs26. Despite this necessity, it’s unclear whether NETs will be able to remove as much CO2 as the IPCC assumes without causing harm to ecosystems or competing with food production. Therefore, while research into NETs is crucial, we still need to cut emissions quickly.

Personally, I wish I could end with a more cheerful message. It seems that we need both rapid emissions cuts and the development of negative emissions technologies. That most likely means minimising the area of land used to produce food, biofuel and other goods, particularly since the NETs that are currently most technologically feasible (forests and soils) require land. In that respect, the current inefficiency of the global food system presents an opportunity: reducing food waste, improving farming yields and shifting towards predominantly plant-based diets could free up large areas of farmland for other uses27.

In future posts (subscribe below to make sure you don’t miss them), I’ll review different types of negative emissions technologies and their likely side-effects in more detail. In the meantime, see the following resources for more information on negative emissions technologies:

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  1. ‘Pre-industrial’ generally refers to the time period 1850 to 1900. However, since this time period includes some large volcanic eruptions and since greenhouse gas concentrations had already started to rise, Hawkins et al. (2017) suggest that 1720 to 1800 would be a better baseline. Astonishingly, the Paris Agreement does not formally define the time period against which temperature rises are to be measured, despite setting a goal of “Holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels”.
  2. IPCC (2018), Global warming of 1.5°C, Summary for policymakers, Section A.1.
  3. IPCC (2018), Global warming of 1.5°C, Summary for policymakers. Note that this estimate has changed over time. For a discussion of the uncertainties involved in calculating carbon budgets, see Hausfather (2018), Carbon Brief, Analysis: Why the IPCC 1.5C report expanded the carbon budget.
  4. IPCC (2018), Global warming of 1.5°C, Summary for policymakers, sections B.2.1, B 2.2, B.4.4 and B.5.1.
  5. Friedlingstein et al. (2019), Global Carbon Budget 2019, section 3.3.1. “Industry” includes the production of cement, chemicals and fertilisers.
  6. Decline of 1.1% in fossil fuel and industry emissions seen between 2008 and 2009; increase of 4.9% seen between 2009 and 2010, based on Global carbon budget data. For further discussion, including the breakdown of emissions changes between “developed” and “developing” regions, see Connor (2011), The Independent, Recession did not lower CO2 emissions.
  7. Robbie Andrew, 2019 Mitigation curves 1.5°C.
  8. IPCC (2018), Mitigation Pathways Compatible with 1.5°C in the Context of Sustainable Development, p121.
  9. CO2 is 27% carbon by weight, so 1000 GtCO2 converted to diamond would weigh 273 Gt. The density of diamond is 3.51 t/m3 (source), so the giant diamond would have a volume of 7.8 x 1010 m3, i.e. a cube of 4.3 km on each side. The volume of Mount Everest has been estimated as roughly 6 x 1010 m3 (source).
  10. Monbiot (2019), Averting Climate Breakdown by Restoring Ecosystems; National Academy of Sciences (2019), Negative Emissions Technologies and Reliable Sequestration, Chapter 3, Terrestrial Carbon Removal and Sequestration.
  11. Garnett et al. (2017), Food Climate Research Network, University of Oxford, Grazed and Confused?
  12. National Academy of Sciences (2019), Negative Emissions Technologies and Reliable Sequestration, Chapter 4, Bioenergy with Carbon Capture and Sequestration.
  13. Breewood and Garnett (2020), Food Climate Research Network, University of Oxford, What is feed-food competition?
  14. National Academy of Sciences (2019), Negative Emissions Technologies and Reliable Sequestration, Chapter 5, Direct Air Capture.
  15. National Academy of Sciences (2019), Negative Emissions Technologies and Reliable Sequestration, Chapter 6, Carbon Mineralization of CO2; Taylor et al. (2016), Enhanced weathering strategies for stabilizing climate and averting ocean acidification.
  16. European Academies’ Science Advisory Council (2018), Negative emission technologies: What role in meeting Paris Agreement targets?
  17. Ocone (2019), The Conversation, Carbon capture on power stations burning woodchips is not the green gamechanger many think it is.
  18. Scripps Institution of Oceanography (2013), How Much CO2 Can The Oceans Take Up?
  19. Cao and Caldeira (2010), Atmospheric carbon dioxide removal: long-term consequences and commitment.
  20. Anderson and Peters (2016), The trouble with negative emissions; Lewis (2019), The Guardian, Sucking carbon out of the air is no magic fix for the climate emergency.
  21. ExxonMobil (2019), 2019 Energy & Carbon Summary, page 13. The report refers to “year-end 2017 proved reserves”, of which 57% are oil and 43% are gas, and says “Based on currently anticipated production schedules, we estimate that by 2040 a substantial majority of our year-end 2017 proved reserves will have been produced. Since the 2°C scenarios average implies significant use of oil and natural gas through the middle of the century, we believe these reserves face little risk from declining demand.”
  22. ExxonMobil (2019), 2019 Energy & Carbon Summary, p14: “We also continue to enhance the quality of our resources through successful exploration drilling, acquisitions, divestments, and ongoing development planning and appraisal activities… We are confident, however, that the size, diversity and continued upgrading of our undeveloped resources, along with technology developments, will enable the ongoing replenishment of our proved reserves for decades to come under a range of potential future demand scenarios.”
  23. Smith (2020), Microsoft will be carbon negative by 2030.
  24. Anderson and Peters (2016), The trouble with negative emissions.
  25. Thunberg et al. (2019), The letter – Natural Climate Solutions.
  26. Anderson and Peters (2016), The trouble with negative emissions.
  27. Alexander et al. (2019), Transforming agricultural land use through marginal gains in the food system.

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