It is misleading to conflate negative emissions technologies that remove CO2 from the atmosphere with proposed geoengineering techniques, such as Solar Radiation Management (SRM), as they are fundamentally different.

This conflation downplays the very high risks of geoengineering techniques and disregards viable energy system mitigation options. Geoengineering techniques such as SRM are not a solution to the climate problem.

Negative emissions technologies such as Biomass Energy Carbon Capture and Storage systems (BECCS) could, in principle, be an integral part of the energy-system and will yield net CO2 removal if deployed, as opposed to CO2 emissions by fossil-fuel combustion. Negative CO2 emissions via BECCS can no more be described as geoengineering than the burning of fossil fuels can.

Preventing dangerous interference with the climate system requires solutions that address CO2 and that are sustainable

Apart from leading to warming, rising CO2 concentrations are also causing serious damages to the world oceans through ocean acidification. As CO2 concentration in the atmosphere rises, more CO2 is absorbed by the oceans, increasing the acidity of the oceans. The effects of acidification have already been observed and will gradually worsen as acidification increases.

Of current anthropogenic CO2 emissions, about 30% is absorbed by the oceans, in response to the higher CO2 concentration of the atmosphere. The net absorption of CO2 by the oceans – and hence ocean acidification – will stop when the equilibrium is restored at the ocean’s surface. This will only occur gradually after the atmospheric CO2 concentration ceases to rise and begins to decline. As with meeting the Paris Agreement long term temperature goal, deep reductions of global CO2 emissions, peaking in the next decade and dropping to zero by 2050 are needed to halt and reverse this.

Geoengineering is not a solution to the climate problem

The lack of appropriate deep emissions reductions to date has led to increased speculation about the use of so-called ‘geoengineering’ options. The term ‘geoengineering’ (1,2) refers to a range of proposals and, most prominently, to ‘Solar Radiation Management’ (SRM) (3). SRM techniques do not address the problems posed by increased CO2 and other greenhouse gas concentrations as a result of anthropogenic emissions and will not deal with ocean acidification. Instead, they aim to reduce only some of the effects of climate change through large-scale technological applications such as stratospheric aerosol particle injection. In this approach, sulfur particles, for example, are injected into the stratosphere, causing a shading effect and reducing the amount of solar radiation available to warm the lower atmosphere – like a major volcanic explosion, but on a sustained basis. Such an alteration would come with serious adverse consequences and risks including regionally changed precipitation patterns (4). SRM would not halt ocean warming around Antarctica and would fail to counteract the increasing contribution of Antarctic melt to sea level rise (5).

If stratospheric aerosol particle injection is stopped, the planet would abruptly warm as GHGs would still have increased. The restoration of the full amount of solar radiation to the lower atmosphere would lead to an abrupt catch up warming effect. These and other considerations led the IPCC’s Fifth Assessment Report (AR5) to find clearly that SRM techniques “entail numerous uncertainties, side effects, risks and shortcomings” and “raise questions about costs, risks, governance and ethical implications of development and deployment”. Further, “SRM would not prevent the CO2 effects on ecosystems and ocean acidification that are unrelated to warming” (6). Other proposed SRM techniques are even less viable and include putting mirrors in space, increasing surface reflectivity of the earth, or altering the amount or characteristics of clouds.

Unlike SRM, some Carbon Dioxide Removal (CDR) techniques such as negative CO2 emissions technologies address the problem of increased CO2 concentrations

The term ‘geoengineering’ is also used to encompass a class of techniques referred to as Carbon Dioxide Removal. Many scientists oppose such a confused (7), as many (but not all) CDR techniques fundamentally differ from geoengineering proposals such as SRM.

As a consequence of these ambiguities the IPCC’s fifth Assessment Report (AR5) was forced to report that “Some CDR methods fall under the category of geoengineering, though this may not be the case for others” and that the “boundary between CDR and mitigation is not clear” (6). Some CDR techniques such as afforestation or bioenergy and carbon capture and storage directly address the problem of increased CO2 concentrations (8), are relatively certain, and have relatively low risks. They are therefore in line with the IPCC’s definition of mitigation as “a human intervention to reduce the sources or enhance the sinks of greenhouse gases.”

Other proposed – but less certain – negative emission technologies include direct-air capture and storage of CO2, biochar or enhanced weathering that may be part of a portfolio solution of negative emission technologies, but will not provide the scale of negative emissions required. In addition, some CDR proposals clearly qualify as geoengineering proposals such as, for example, ocean fertilisation. The science is clear that ocean fertilisation will not work at the scale required, if at all, while at the same introducing numerous negative side effects related to ocean acidification9 and de-oxygenation.10, Ocean fertilisation is also illegal under international law. (11)

Conflating negative CO2 emissions technologies and techniques such as SRM under the same ‘Geoengineering’ label is misleading

It strongly downplays the very high risks of geoengineering techniques, and effectively denigrates viable mitigation options that could be provided by some negative emissions technologies.

While large-scale applications of negative emissions technologies could also have negative side effects,12the risks related to SRM and negative emissions technologies are not comparable. Sustainable deployment potentials for afforestation or bioenergy and carbon capture and storage have been identified. (12,13)

Known viable negative emissions technologies do not, and cannot, replace the need for early and stringent reductions of CO2 and other anthropogenic greenhouse gas emissions

Regrettably, negative emission technologies need to be deployed at scale from the 2050’s following the early, and rapid deployment of a full portfolio of mitigation options to bring CO2 emissions to zero globally by around 2050 to achieving the long term temperature goal in the Paris Agreement. Early and rapid reduction of CO2 emissions are needed now to minimise this need, but cannot eliminate this.

The application of CDR methods in these scenarios – principally afforestation and BECCs – is not a replacement for stringent decarbonisation, but a necessity additional to the rapid reduction to zero CO2 emissions by 2050.

At this moment, the option that is seen as most likely to achieve large-scale negative emissions combines modern biomass energy systems with carbon capture and storage (BECCS). All elements of this technology are available, and demonstration plants are already functional, and scenarios project very rapid upscaling of this technology in the 2030-2050 period in stringent mitigation pathways consistent with 1.5°C and 2°C. In any case, early and rapid action to reduce emissions in the coming decades is essential.

References

1 Schneider, S. H. Geoengineering: Could? or should? we do it? Clim. Change 33, 291–302 (1996).
2 Kiehl, J. T. Geoengineering climate change: Treating the symptom over the cause? An editorial comment. Clim. Change 77, 227–228 (2006).
3 Climate Analytics. Unacceptable risks posed by ‘Climate Neutrality’ replacing ‘GHG emission reductions’ in the Paris Agreement. (2015).
4 Kravitz, B. et al. A multi-model assessment of regional climate disparities caused by solar geoengineering. Environ. Res. Lett. 9, 74013 (2014).
5 Mccusker, K. E., Battisti, D. S. & Bitz, C. M. Inability of stratospheric sulfate aerosol injections to preserve the West Antarctic Ice sheet. Geophys. Res. Let. 1–9 (2015). doi:10.1002/2015GL064314.impacts
6 IPCC. Climate Change 2014: Synthesis Report. (Cambridge Univ. Press, 2014).
7 McNutt, M. K., Abdalati, W., Caldeira, K., Doney, S. C. & Falkowski, P. G. Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration. National Research Council of the National Academies (2015). doi:10.17226/18805
9 Caldeira, K., Bala, G. & Cao, L. The Science of Geoengineering. Annu. Rev. Earth Planet. Sci. 41, 231–256 (2013). 10 Stocker, T. F. et al. Technical Summary. Clim. Chang. 2013 Phys. Sci. Basis. Contrib. Work. Gr. I to Fifth Assess. Rep. Intergov. Panel Clim. Chang. 33–115 (2013). doi:10.1017/ CBO9781107415324.005
11 Freestone, D., Rayfuse R,. “Iron ocean fertilization and international law.” Marine Ecology Progress 9 (2008).
12 Smith, P. et al. Biophysical and economic limits to negative CO2 emissions. Nat. Clim. Chang. (2015). doi:10.1038/nclimate2870
13 Creutzig, F. et al. Bioenergy and climate change mitigation : an assessment. GCB Bioenergy 1–29 (2014). doi:10.1111/gcbb.12205