The truth about climate neutrality and negative emissions

Climate neutrality has become one of the new buzzwords of politicians and decision makersaround the world. This simple term seems to be the ultimate answer to the climate crisis and all its disastrous impacts. Achieving climate neutrality and thus net-zero emissions around mid-century is widely understood as magic key to fulfil the Paris Agreement and to limit globalwarming to well below 2 °C. But the truth is, magic isn't real, and the simple expectation of climate neutrality around 2050 is a false friend as it implies the need for large-scale negativeemissions. To avoid an even worse climate catastrophe, deep and fast changes as well as asconsiderable emissions cuts are urgently needed.

According to the UNFCCC Beginner's Guide to Climate Neutrality, "Climate neutrality refers to the idea of achieving net zero greenhouse gas emissions by balancing those emissions so that they are equal to (or less than) the emissions that get removed through the planet's natural absorption" (UNFCCC, 2021). What sounds quite convincing at first sight implies a significantpremise, which is not mentioned in the definition: the need for negative emissions to a)enhance natural sinks such that the sum of greenhouse gases (GHGs) really becomes net-zeroand to b) compensate for a temporary overshoot of the global carbon budget.

Enhance natural sinks

The planet's natural absorptionis driven by marine and terrestrial ecosystems (Le Quéré etal., 2009). These natural sinks absorb more than a half of each year's total carbon dioxide (CO2) emissions. The rest remains in the atmosphere. Global CO2 uptake of natural sinks is estimatedto be between 12.9 and 24.8 Gt per year (Friedlingstein et al., 2020). Natural sinks and sourcesare thus almost balanced (Yue & Gao, 2018). Anthropogenic sources of CO2, however, are not balanced within this system. Depending on how effective global decarbonization will be,natural sinks need to be enhanced to compensate for additional anthropogenic sources that are hard to abate, e.g. cement industry, steel and iron, conventional agriculture (IPCC, 2018).

‍

The global carbon budget

At the beginning of 2020, the remaining global carbon budget to limit global warming to 1.5 °Cwith a 66% probability was only 400 gigatons (Gt; 1 Gt equals 1 billion tons) (IPCC, 2021). Withannual emissions of approximately 42 GtCO2, the remaining global carbon budget will be usedup around 2030. A recent study even found that the remaining carbon budget to limit globalwarming to 1.5 °C is only 230 Gt from the end of 2020 (Matthews et al., 2021). Whether theremaining budget is 230 GtCO2 or about 340 GtCO2, a significant overshoot until mid-centuryis likely unavoidable-even if global emissions were to peak and decrease within this decade.This rather pessimistic projection is also in line with the scenarios assessed by the IPCC in its2018 special report on Global warming of 1.5 °C (IPCC, 2018). To compensate for theovershoot, negative emissions are presupposed in most scenarios. Depending on the scenario,100-1000 GtCO2 need to be removed from the atmosphere until 2100. Given the currentemissions trajectory, that figure is closer to 1,000 Gt (UNEP, 2021). And that is a serious issue because the approach of negative emissions involves at least three significant problems. 1) Atemporary overshoot of the global carbon budget implies at least a temporary overshoot of1.5 °C of global warming (IPCC, 2018). 2) The technologies to realise negative emissions arenot ready to be used at scale (Nemet et al., 2018; Lawrence et al., 2018). 3) Even if thosetechnologies were available at scale, a multitude of further problems regarding costs, energy,governance, land-use, and public acceptance would arise for which there are no real solutions,and which are only scarcely addressed in the literature (Fuss et al., 2020, Wilcox et al., 2020,Fuss et al., 2018, Nemet et al., 2018).

‍

Overshoot of the global carbon budget

As soon as the remaining global carbon budget is used up, global warming will exceed 1.5 °C.That is a simple truth. Not that simple, however, are the impacts of exceeding 1.5 °C or even2 °C (IPCC, 2021; 2018). Even now, at only 1.2 °C of global warming, the number and intensityof extreme weather events have already increased remarkably (Otto, 2020). Just recently, for example, parts of Germany and Belgium have been devastated by massive floods, while in Northern America a deadly heatwave has led to new and "almost unimaginable" recordtemperatures (Hook et al., 2021). Hundreds have died in these events and tremendouseconomic damage has occurred. Not to mention all the victims and damages in other, oftenpoorer regions of the world. It must be clear that extreme weather events and thus its impactswill get worse with every tenth of a degree of global warming. Beside the growing threat of extreme weather events, leading climate scientists also expectveral tipping elements to be crossed between 1-2 °C of global warming (Steffen et al., 2018,Lenton et al. 2019). Tipping elements are defined as "subsystems of the Earth system that areat least subcontinental in scale and can be switched--under certain circumstances--into aqualitatively different state by small perturbations. The [climate] tipping point is thecorresponding critical point--in forcing and a feature of the system--at which the future stateof the system is qualitatively altered" (Lenton et al, 2008: 1786). Potential tipping elementsfall into three categories: entities of the cryosphere (e.g. Greenland Ice Sheet, Arctic Sea Ice),circulation patterns (e.g. Atlantic Thermohaline Circulation, El Niño Southern Oscillation), andcomponents of the biosphere (e.g. Amazon Rainforest, Boreal Forests) (PIK, 2017). Some ofthese tipping elements, including warm-water corals, parts of the Amazon Rainforest, and theWest Antarctic and Greenland ice sheets have already been crossed or are nearing (Gatti etal., 2021, Ripple et al. 2021). The impact of crossing tipping elements is irreversible and tremendous: Global warming wouldbe accelerated by additional GHG emissions, ecosystems would change dramatically, oceancurrents would be destabilized, additional tipping elements would be crossed, eventuallyresulting in a shift to a Hothouse Earth (Lenton et al., 2019, Steffen et al., 2018). Leadingclimate scientists suggest that such a cascade of tipping elements may already be reached ataround 2 °C of global warming. As "[t]he impacts of a Hothouse Earth pathway on humansocieties would likely be massive, sometimes abrupt, and undoubtedly disruptive" (Steffen etal., 2018: 8257), global warming of more than 1.5 °C, even temporarily, must be avoided.

‍

Negative Emission Technologies (NETs) are not ready to use them at scale

"Negative-emission technologies are not an insurance policy, but rather an unjust and high-stakes gamble. There is a real risk they will be unable to deliver on the scale of their promise" (Anderson & Peters, 2016: 183). Even though this quotation is already five years old, theyessage is still the same; Or even worse, no real progress has been made in the meantime,and the remaining global carbon budget has continuously decreased.

Negative Emission Technologies (NETs) are grouped in different categories: Bioenergy withCarbon Capture and Storage (BECCS), Direct Air Carbon Capture and Storage (DACCS),Enhanced Weathering, Ocean Fertilization, Afforestation and Reforestation, as well as LandManagement to increase carbon storage in soils (EASAC, 2018). Most scenarios that are consistent with limiting global warming to 1.5 °C with a 66%probability assume annual negative emissions in the second half of the 21st century of about10-12°GtCO2 (Fajardy et al., 2019; Smith et al., 2016). Of these scenarios, a majority include the implementation of BECCS (IPCC, 2018). Currently, however, only a few small-scale BECC test sites exist, which demonstrate that the technology will become rather one more problemthan a solution to the climate crisis (Geoengineering Monitor, 2021a; Nemet et al., 2018). ForDACCSa few test sites exist as well, but the technology is still at a "nascent stage", with thetotal quantity of carbon removed from the atmosphere only at a few thousand tons per year(Geoengineering Monitor, 2021b, Nemet et al., 2018). Enhanced Weathering, either marine or terrestrial, has been addressed by a few academic research programs. However, most dataregarding potentials have been derived from computer models, while field studies are veryrare (Geoengineering Monitor, 2021c, Nemet et al., 2018). For Ocean Fertilization, severalfield trials have taken place, but a large-scale deployment is, as for the aforementionedtechnologies, currently far from being feasible (Geoengineering Monitor, 2021d, Nemet et al.,2018). Afforestation and Reforestation as well as Land Management are deployed at scalesince many decades and have proven successful. However, even these 'technologies' are notready to be used at scale as they are linked to necessary behavioural changes that are rather unlikely as will be discussed in the following section.

‍

Problems with large-scale NETs

In most models, NETs are crucial to limit global warming to 1.5 °C or even 2 °C. Yet, NETs arenot ready to be deployed at scale. But even if they were, a multitude of further problemswould arise.

BECCS is still discussed as the major NET as it promises not only to reduce atmospheric CO2,but also to produce bioenergy. However, the removal of about 10 GtCO2 annually from theatmosphere would require vast amounts of arable land to be converted into large-scalebiomass plantations-up to 46% of all arable land (Fuss et al., 2018; Smith et al., 2016). Sucha massive conversion would not only generate additional GHG emissions as soil carbon wouldfurther be depleted and released in huge quantities, but also strongly impact food productionand prices as well as biodiversity (Fajardy et al., 2020; Smith et al., 2016). Another set of offurther problems is generated by Carbon Capture and Storage (CCS) (Fuss et al., 2018). Due to overpressure, potable water could be polluted, seismic activity could occur, and leaks could emerge, causing environmental as well as health damage. Even though the technology is mature, only a few dozen commercial scale projects exist, public acceptability is little, and thepolitical economy is rather uninterested (Bui et al., 2018). For BECCS, costs are assumed to beabout US$ 100-200 per t CO2 in 2050 (Fuss et al., 2018). Implementation costs are not included in this estimate but would be very considerable - several hundred billion US$ - for a global-scale deployment (Smith, 2016).

The ecological and land footprint of DACCS is magnitudes smaller than for BECCS (Realmonte et al., 2019). Yet, potential barriers of a large-scale deployment are costs, the large demandfor energy and chemical pollution due to the massive need for sorbents (Fuss et al., 2018;Realmonte et al., 2019). As for BECCS, the problems regarding CCS are further barriers to thedeployment of DACCS. Running costs are estimated between US$ 100-300 per t CO2 in 2050(Fuss et al., 2018). Again, implementation costs are not included in this estimate but would beat a similar level as for BECCS (Smith, 2016).

Enhanced Weathering of rock material is another proposed NET. Major problems to this 'technology' are the vast amounts of energy needed to grind suitable rock material into small-size particles (≤ 20 μm) and the quantities of rock powder needed to remove carbon from theatmosphere (to remove 1 GtCO2 would require more than 3 Gt of rock powder) (Strefler et al.,2018; Hartmann et al., 2013). In addition, various side effects regarding the environment, e.g.soil properties, water pH, changes in dissolved inorganic carbon and total alkalinity, areexpected (Fuss et al., 2018; Hartmann, 2013). Costs are estimated to be about US$ 50-200 pert CO2 in 2050 with implementation costs not included (Fuss et al., 2018).

The potential of ocean fertilisation to store carbon permanently is controversial and ratherlow (Fuss et al., 2018). Scientists also expect this NET to alter marine ecosystem properties to an increased phytoplankton production and thus impact the food cycle. Even more,toxic algae blooms may occur, and deep-water oxygen levels may decline. Costs of this 'technology' vary from US$ 2 to US$ 457 per t CO2.

Afforestation and Reforestation as well as Land Management are both part of so-calledNatural Climate Solutions (NCS) (Griscom et al., 2017). These 'technologies' offer a hugepotential to mitigate climate change at low costs, to reduce atmospheric CO2, to ameliorateseveral ecosystem properties and to enhance biodiversity. However, globally, deforestation rates are increasing as well as the frequency of droughts and wildfire, thereby reversing thebenefits of NCS and accelerating climate change even more. Common agricultural practicesset soil carbon free and land-use change is still a source of GHG emissions, too (Bossio et al.,2020). To use NCS as a NET would require behavioural changes, e.g. ending deforestation, morenatural based farming practices, a more plant-based diet, and the renaturation of wetlandsand forests (Griscom et al., 2017). If these changes were achieved, NCS could annually remove 23.8 Gt of carbon dioxide from the atmosphere. Yet, current practices do not point in thatdirection.

Given all these arguments, it should be obvious that Negative Emission Technologies are nota solution to the climate crisis we should bet on. We and particularly future generations wouldlose this bet with dramatic consequences for the planet and for humanity. Even more, it must be noted that this list of problems regarding NETs is not comprehensive. For a detaileddiscussion of NETs, its hypothetical potentials and various side effects see Minx et al. (2018), Fuss et al. (2018) and Nemet et al. (2018). Natural Climate Solutions, however, offer a hugepotential to mitigate climate change but require behavioural changes that do not appear tooplausible in the coming years given the current trends.

Conclusion

To limit global warming to well below 2 °C or even 1.5 °C by 2100, the Paris Agreement callsfor climate neutrality by mid-century (UNFCCC, 2015). In regard of all the risks anduncertainties, it should be imperative to limit global warming to 1.5 °C. Yet, to fulfil the Paris Agreement, large-scale negative emissions in the second half of the 21st century are almostunavoidable. As has been shown above, it does not appear too plausible that the quantitiesof negative emissions needed can be achieved. Therefore, climate neutrality by 2050 is justnot adequate to limit global warming to 1.5 °C. What it really takes to avoid an unthinkableclimate catastrophe is a fast transition to a global 100% renewable energy system by 2030, abroad and deep decarbonization of global economies by 2030, as well as lifestyle changesregarding transport and diets as soon as possible. Otherwise, it will be too late. This is our last chance to get a grip on the climate crisis. It must be noted that realizing the greentransition is not a synonym for renunciation. But real action and real changes need to happenurgently. According to a recent study from Anderson et al. (2020), GHG emissions ofindustrialized countries like Germany need to decrease by 12% annually in the coming decadeto stay within a path consistent with 1.5°C. It is still possible to safe our climate, but it needsto be done much faster. It is a tremendous challenge, but it is not impossible. We are not lost already, but we areheading for it with still accelerating high speed. We need to change, and we need to act now.

‍

Patrick Hohlwegler, Energy and Climate Policy Officer, ansvar 2030 & The Climate Task Force

‍

References

Anderson, K., Broderick, J.F., Stoddard, I. (2020). A factor of two: how the mitigation plans of 'climate progressive' nations fall short of Paris-compliant pathways. In: Climate Policy,20(10). 1290-1304. https://doi.org/10.1080/14693062.2020.1728209

Anderson, K, Peters, G. (2016). The trouble with negative emissions. In: Science, 354(6309).182-183. https://doi.org/10.1126/science.aah4567

Arias, P. A., N. Bellouin, E. Coppola, R. G. Jones, G. Krinner, J. Marotzke, V. Naik, M. D. Palmer,G-K. Plattner, J. Rogelj, M. Rojas, J. Sillmann, T. Storelvmo, P. W. Thorne, B. Trewin, K.Achuta Rao, B. Adhikary, R. P. Allan, K. Armour, G. Bala, R. Barimalala, S. Berger, J. G.Canadell, C. Cassou, A. Cherchi, W. Collins, W. D. Collins, S. L. Connors, S. Corti, F. Cruz,F. J. Dentener, C. Dereczynski, A. Di Luca, A. Diongue Niang, F. J. Doblas-Reyes, A. Dosio,H. Douville, F. Engelbrecht, V. Eyring, E. Fischer, P. Forster, B. Fox-Kemper, J. S.Fuglestvedt, J. C. Fyfe, N. P. Gillett, L. Goldfarb, I. Gorodetskaya, J. M. Gutierrez, R.Hamdi, E. Hawkins, H. T. Hewitt, P. Hope, A. S. Islam, C. Jones, D. S. Kaufman, R. E. Kopp,Y. Kosaka, J. Kossin, S. Krakovska, J-Y. Lee, J. Li, T. Mauritsen, T. K. Maycock, M.Meinshausen, S-K. Min, P. M. S. Monteiro, T. Ngo-Duc, F. Otto, I. Pinto, A. Pirani, K.Raghavan, R. Ranasinghe, A. C. Ruane, L. Ruiz, J-B. Sallée, B. H. Samset, S.Sathyendranath, S. I. Seneviratne, A. A. Sörensson, S. Szopa, I. Takayabu, A-M. Treguier,B. van den Hurk, R. Vautard, K. von Schuckmann, S. Zaehle, X. Zhang, K. Zickfeld, 2021,Technical Summary. In: Climate Change 2021: The Physical Science Basis. Contributionof Working Group I to the Sixth Assessment Report of the Intergovernmental Panel onClimate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger,N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R.Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. CambridgeUniversity Press. In Press. https://www.ipcc.ch/report/ar6/wg1/#TS

Bui, M., Adjiman, C.S., Bardow, A., Anthony, E.J., Boston, A., Brown, S., Fennell, P.S., Fuss, S.,Galindo, A., Hackett, L.A., Hallett, J.P., Herzog, H.J., Jackson, G., Kemper, J., Krevor, S.,Maitland, G.C., Matuszewski, M., Metcalfe, I.S., Petit, C., Puxty, G., Reimer, J., Reiner,D.M., Rubin, E.S., Scott, S.A., Shah, N., Smit, B., Trusler, J.P.M., Webley, P., Wilcox, J.,Mac Dowell, N. (2018) Carbon capture and storage (CCS): the way forward. In: EnergyEnviron Sci, 11(1062). https://doi.org/10.1039/C7EE02342A

EASAC (2018). Negative emission technologies: What role in meeting Paris Agreementtargets? EASAC policy report 35. ISBN: 978-3-8047-3841-6.https://unfccc.int/sites/default/files/resource/28_EASAC%20Report%20on%20Negative%20Emission%20Technologies.pdf

Fajardy, M., Morris, J., GUrgel, A., Herzog, H., Mac Dowell, N., Paltsev, S. (2020). Theeconomics of bioenergy with carbon capture and storage (BECCS) deployment in a 1.5°Cor 2°C world. In: Joint Program Report Series Report 345.http://globalchange.mit.edu/publication/17489

Fajardy, M., Köberle, A., Mac Dowell, N., Fantuzzi, A. (2019). BECCS deployment: a realitycheck. Grantham Institute Briefing paper No 28.https://www.imperial.ac.uk/media/imperial-college/grantham-institute/public/publications/briefing-papers/BECCS-deployment---a-reality-check.pdf

Friedlingstein, P., O'Sullivan, M., Jones, M.W., Andrew, R.M., Hauck, J., Olsen, A., Peters, G.P.,Peters, W., Pongratz, J., Sitch, S., Le Quéré, C., Canadell, J.G., Ciais, P., Jackson, R.B., Alin,S., Aragão, L.E.O.C., Arneth, A., Arora, V., Bates, N.R., Becker, M., Benoit-Cattin, A., Bittig,H.C., Bopp, L., Bultan, S., Chandra, N., Chevallier, F., Chini, L.P., Evans, W., Florentie, L.,

Forster, P.M., Gasser, T., Gehlen, M., Gilfillan, D., Gkritzalis, T., Gregor, L., Gruber, N.,Harris, I., Hartung, K., Haverd, V., Houghton, R.A., Ilyina, T., Jain, A.K., Joetzjer, E.,Kadono, K., Kato, E., Kitidis, V., Korsbakken, J.I., Landschützer, P., Lefèvre, N., Lenton, A.,Lienert, S., Liu, Z., Lombardozzi, D., Marland, G., Metzl, N., Munro, D.R., Nabel, J.E.M.S.,Nakaoka, S-I., Niwa, Y., O'Brien, K., Ono, T., Palmer, P.I., Pierrot, D., Poulter, B.,Resplandy, L., Robertson, E., Rödenbeck, C., Schwinger, J., Séférian, R., Skjelvan, I.,Smith, A.J.P., Sutton, A.J., Tanhua, T., Tans, P.P., Tian, H., Tilbrook, B., van der Werf, G.,Vuichard, N., Walker, A.P., Wanninkhof, R., Watson, A.J., Willis, D., Wiltshire, A.J., Yuan,W., Yue, X., Zaehle, S. (2020). Global Carbon Budget 2020. in: Earth Syst. Sci. Data. 12.3269-3340. https://doi.org/10.5194/essd-12-3269-2020

Fuss, S., Canadell, J.G., Ciais, P., Jackson, R.B., Jones, C.D., Lyngfelt, A., Peters, G.P., VanVuuren, D.P. (2020). Moving towards Net-Zero Emissions Requires New Alliances for

Carbon Dioxide Removal. In: One Earth, 3. 145-149. https://doi.org/10.1016/j.oneear.2020.08.002
Fuss, S., Lamb, W.F., Callaghan, M.W., Hilaire, J., Creutzig, F., Amann, T., Beringer, R., de

Oliveira Garcia, W., Hartmann, J., Khanna, T., Luderer, G., Nemet, G.F., Rogelj, J., Smith,P., Vicente, J.L.V., Wilcox, J., del Mar Zamora Dominguez, M., Minx, J.C. (2018). NegativeEmissions--Part 2: Costs, potentials and side effects. In: Environ. Res. Lett . 13(6).063002. https://iopscience.iop.org/article/10.1088/1748-9326/aabf9f

Gatti, L.V., Basso, L.S., Miller, J.B., Gloor, M., Gatti Domingues, L., Cassol, H.L.G., Tejada, G.,Aragão, L.E.O.C., Nobre, C., Peteres, W., Marani, L., Arai, E., Sanches, A.H., Corrêa, S.M.,Anderson, L., Von Randow, C., Correia, C.S.C., Crispim, S.P., Neves, R.A.L. (2021).Amazonia as a carbon source linked to deforestation and climate change. In: Nature,595. 388-393. https://doi.org/10.1038/s41586-021-03629-6

Geoengineering Monitor (2021a). Bioenergy with Carbon Capture & Storage (BECCS). In:Geoengineering Technologies Briefing Jan 2021.https://www.geoengineeringmonitor.org/2021/04/bio-energy-with-carbon-capture-and-storage-beccs/

Geoengineering Monitor (2021b). Direct Air Capture (DAC). In: Geoengineering TechnologiesBriefing Jan 2021. https://www.geoengineeringmonitor.org/2021/02/direct-air-capture-technology-briefing/

Geoengineering Monitor (2021c). Enhanced Weathering (marine & terrestrial). In:Geoengineering Technologies Briefing Jan 2021.https://www.geoengineeringmonitor.org/2021/04/enhanced-weathering-factsheet/

Geoengineering Monitor (2021d). Ocean fertilization. In: Geoengineering TechnologiesBriefing Jan 2021. https://www.geoengineeringmonitor.org/2021/04/ocean-fertilization/

Griscom, B. W., Adams, J., Ellis, P. W., Houghton, R. A., Lomax, G., Miteva, D. A., Schlesinger,W.H., Shoch, D., Siikamäki, J.V., Smith, P., Woodbury, P., Zganjar, C., Blackman, A.,Campari, J., Conant, R.T., Delgado, C., Elias, P., Gopalakrishna, T., Hamsik, M.R., Herrero,M., Kiesecker, J., Landis, E., Laestadius, L., Leavitt, S.M., Minnemeyer, S., Polasky, S.,Potapov, P., Putz, F.E., Sandermann, J., Silvius, M., Wollenberg, E., Fargione, J. (2017).Natural climate solutions. PNAS, 114(44). 11645–11650.https://doi.org/10.1073/pnas.1710465114

Hartmann, J., West, A.J., Renforth, P., Köhler, P., De La Rocha, C.L., Wolf-Gladrow, D.A., Dürr,H.H., Scheffran, J. (2013). Enhanced chemical weathering as a geoengineering strategyto reduce atmospheric carbon dioxide, supply nutrients, and mitigate oceanacidification. In: Rev. Geophys., 51(2), 113-149. https://doi.org/10.1002/rog.20004

Hook, L., Shepherd, C., Astrasheuskaya, N. (2021). Extreme weather takes climate change models 'off the scale'. In: Financial Times. https://www.ft.com/content/9a647a51-ede8-480e-ba78-cbf14ad878b7?shareType=nongift

IPCC, 2018: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warmingof 1.5°C above pre-industrial levels and related global greenhouse gas emissionpathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte,V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia,C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy,T. Maycock, M. Tignor, and T. Waterfield (eds.)]. In Press. https://www.ipcc.ch/sr15/

Lawrence, M.G., Schäfer, S., Muri, H., Scott, V., Oschlies, A., Vaughan, N.E., Boucher, O.,Schmidt, H., Haywood, J., Scheffran, J. (2018) Evaluating climate geoengineeringproposals in the context of the Paris Agreement temperature goals. In: NatureCommunications, 9(3734). https://doi.org/10.1038/s41467-018-05938-3

Lenton, T.M., Rockström, J., Gaffney, O., Rahmstorf, S., Richardson, K., Steffen, W.,Schellenhuber, H.J. (2019). Climate tipping points - too risky to bet against. In: Nature.575. 592-595. https://doi.org/10.1038/d41586-019-03595-0

Le Quéré, C., Raupach, M.R., Candell, J.G., Marland, G., Bopp, L., Ciais, P., Conway, T.J., Doney,S.C., Feely, R.A., Foster, P., Friedlingstein, P., Gurney, K., Houghton, R.A., House, J.I.,Huntingford, C., Levy, P.E., Lomas, M.R.Majkut, J., Metzl, N., Ometto, J.P:, Peters, G.P.,Prentice, I.C., Randerson, J.T., Running, S.W., Sarmiento, J.L:, Schuster, U., Sitch, S.,Takahashi, T., Viovy, N., van der Werf, G.R:, Woodward, F.I. (2009). Trends in the sourcesand sinks of carbon dioxide. In: Nature Geosci. 2. 831-836.https://doi.org/10.1038/ngeo689

Matthews, H.D., Tokarska, K.B., Rogelj, J., Smith, C.J., MacDougall, A.H., Haustein, K., Mengis,N., Sippel, S., Forster, P.M., Knutti, R. (2021). An integrated approach to quantifyinguncertainties in the remaining carbon budget. In: Commun Earth Environ, 2(7).https://doi.org/10.1038/s43247-020-00064-9

Nemet, G.F., Callaghan, M.W., Creutzig, F., Fuss, S., Hartmann, J., Hilaire J., Lamb, W.F., Minx,J.C., Rogers, S., Smith, P. (2018). Negative emissions--Part 3: Innovation and upscaling.Environ. Res. Lett . 13(6). 063003. https://iopscience.iop.org/article/10.1088/1748-9326/aabff4

Otto, F.E.L. (2020). Extreme Weather Events and Local Impacts of Climate Change. TheScientific Perspective. In: Brüggemann, M., Rödder, S. (Eds.): Global Warming in LocalDiscourses: How Communities around the World Make Sense of Climate Change.Cambridge, UK. Open Book Publishers. https://doi.org/10.11647/OBP.0212

PIK (2017). The tipping elements map. https://www.pik-potsdam.de/en/output/infodesk/tipping-elements#k-ppen-climate-classification

Realmonte, G., Drouet, L., Gambhir, A., Glynn, J., Hawkes, A., Köberle, A.C., Tavoni, M. (2019).An inter-model assessment if the role of direct air capture in deep mitigation pathways.In: Nature Communications, 10(3277). https://doi.org/10.1038/s41467-019-10842-5

Ripple, W.J., Wolf, C., Newsome, T.M., Gregg, J.W., Lenton, T.M., Palomo, I., Eikelboom, J.A.J.,Law, B.E., Huq, S., Duffy, P.B., Rockström, J. (2021). World Scientists' Warning of aClimate Emergency 2021. In: BioScience, biab079.https://doi.org/10.1093/biosci/biab079

Smith, P., Davis, S. J., Creutzig, F., Fuss, S., Minx, J., Gabrielle, B., Kato, E., Jackson, R.B., Cowie,A., Kriegler, E., van Vuuren, D.P., Rogelj, J., Ciais, P., Milne, J., Canadall, J.G., McCollum,D., Peters, G., Andrew, R., Krey, V., Shrestha, G., Friedlingstein, P., Gasser, T., Grübler,
A., Heidug, W.K., Jonas, M., Jones, C.D., Kraxner, F., Littleton, E., Lowe, J., Moreira, J.R.,Nakicenovic, N., Obersteiner, M., Patwardhan, A., Rogner, M., Rubin, E., Sharifi, A.,Torvanger, A., Yamagata, Y., Edmonds, J., Yongsung, C. (2016). Biophysical and economiclimits to negative CO2 emissions, in Nature Climate Change, 6(1). 42-50.https://doi.org/10.1038/nclimate2870

Steffen, W., Rockström, J., Richardson, K., Lenton, T.M., Folke, C., Liverman, D., Summerhayes,C.P., Barnosky, A.D., Cornell, S.E., Crucifix, M., Donges, J.F., Fetzer, I., Lade, S.J., Scheffer,M., Winkelmann, R., Schellnhuber, H.J. (2018). Trajectories of the Earth System in theAnthropocene. In: PNAS. 115(33). 8252-8259.https://doi.org/10.1073/pnas.1810141115

Strefler, J., Amann, T., Ba<uer, N., Kriegler, E., Hartmann, J. (2018). Potential and costs ofcarbon dioxide removal by enhanced weathering of rocks. In: Environ. Res. Lett.,13(034010). https://doi.org/10.1088/1748-9326/aaa9c4

UNEP (2020). Emissions Gap Report 2020. Nairobi. https://www.unep.org/emissions-gap-report-2020

UNFCCC (2021). A Beginners Guide to Climate Neutrality. https://unfccc.int/blog/a-beginner-s-guide-to-climate-neutrality

UNFCCC (2015): Adoption of the Paris Agreement. Report No. FCCC/CP/2015/L.9/Rev.1.http://unfccc.int/resource/docs/2015/cop21/eng/l09r01.pdf

Wilcox, J., Renforth, P., Kraxner, F., eds. (2020). The Role of Negative Emission Technologies inAddressing Our Climate Goals. Lausanne: Frontiers Media SA. doi: 10.3389/978-2-88963-672-3

Yue, X.-L., Gao, Q.-X. (2018). Contributions of natural systems and human activity togreenhouse gas emissions. In: Advances in Climate Change Research, 9(4). 243-252.https://doi.org/10.1016/j.accre.2018.12.003

‍

Image: Cristi Goi/ unsplash

‍