Reforestation in southern Oregon, U.S.A. Source: https://commons.wikimedia.org/wiki/User:Downtowngal

Negative emission technologies (NETs), also known as carbon dioxide removal, refer to processes by which carbon emissions are sequestered to lower emissions in an effort to a) prevent additional accumulation of greenhouse gases in the atmosphere, and/or b) remove greenhouse gases that have already accumulated in the atmosphere. In practice, carbon emissions would be sequestered at higher rates than they’re produced, lowering the overall greenhouse gas concentration in the atmosphere (i.e. negative emissions). 

CO2 makes up less than 1% of the atmosphere by volume. Avoiding putting more molecules of CO2 into the air from the global energy system is simpler than the effort to capture that molecule back out and keep it out permanently, or, sequester it.  Therefore, the Intergovernmental Panel on Climate Change (IPCC), the National Academies of Sciences, and many peer-reviewed publications stress the importance of peaking global emissions as soon as possible followed by rapid deployment of clean energy technologies to displace fossil fuels, as opposed to relying on NETs to lower atmospheric emissions. Delays increase the need for NET approaches in order to stay below desired global temperature thresholds. 

Negative emissions can be achieved biologically by increasing natural carbon sinks or through chemical engineering processes. All NET approaches have their own set of impacts, costs and limitations (such as the permanence of the carbon stored) (Fuss 2018). Negative emissions approaches include (EASAC, 2018):

A taxonomy of negative emissions technologies (NETs). NETs are distinguished by carbon capture approach, earth system and storage medium. Major implementation options are distinguished for each NET. Source: Minx et al. 2018.

The scale of NETs needed is dictated by estimates of the remaining “carbon budget.” In this context the remaining carbon budget refers to the cumulative amount of allowable CO2 emissions associated with a reasonable chance (66%) of staying below a certain temperature limit. Analyses suggest that to meet the 2°C goal of the Paris agreement, there is a remaining carbon budget of 810 GtCO2. (Realmonte, 2019). To achieve the 1.5ºC aspiration, by the same analysis, the budget shrinks to only 220 GtCO2. For comparison, annual greenhouse gas emissions have now reached approximately 50 GtCO2e per year.

While some argue that the need for NETs can be reduced significantly by aggressive deployment of efficiency, clean energy, lifestyle changes, and adequate incentives such as a price on carbon (e.g. van Vuuren 2018), the need for negative emissions is not entirely eliminated in most scenarios.

The IPCC 1.5°C report provided four scenarios (P1-P4) with increasing roles for negative emissions. All include carbon removal in the form of Agriculture, Forestry, and Other Land Use (AFOLU) scenario P1, and P2-4 also consider varying levels of Bioenergy with Carbon Capture and Sequestration (BECCS). The amount of CO2  removal to achieve the 1.5°C goal is greatly affected by how late fossil emissions peak and how steep they decline. The amount of carbon capture in P4 is about 20 GtCO2 per year in 2100 – a massive and costly undertaking.  (IPCC 2018).