Impact of Solar Radiation Modification on Allowable CO₂ Emissions: What Can We Learn From Multimodel Simulations?

Abstract

Solar radiation modification (SRM) is known to strengthen both land and ocean carbon uptake because of its impacts on surface temperature, solar radiation, and other potential drivers of the global carbon cycle. However, the magnitude and timing of the response of both land and ocean carbon uptake to SRM and its consequence on allowable CO2 emissions remain poorly understood. Here we use the results of six Earth system models simulating a continuous stratospheric injection of 5 Tg of sulfur dioxide per year between 2020 and 2069 under the representative concentration pathways 4.5 to investigate the impact of SRM on land and ocean carbon uptake. We find that 50 years of SRM under this protocol increases the allowable CO2 emissions by 40 ± 19 GtC; 85% of this additional uptake of carbon is stored in the land biosphere and 15% in the ocean. This increase in allowable CO2 emissions is however not sustainable after the stoppage of SRM. Earth system models predict a mean release of 8 ± 11 GtC of the carbon back to the atmosphere 20 years after the stoppage which is dominated by large uncertainties in the response of the simulated land carbon cycle to rising temperature and solar radiation. We demonstrate that the time scales of carbon dioxide removal (CDR) potential of SRM are smaller than the time scales of the geological storage assumed in well‐established CDR options. This shows that the CDR potential of SRM should be compared to well‐established CDR options with caution.

Identifying the sources of uncertainty in climate model simulations of solar radiation modification with the G6sulfur and G6solar Geoengineering Model Intercomparison Project (GeoMIP) simulations

“…Reasons for a different aerosol distribution with similar injection locations and height of SO 2 can be the different dynamical features of the simulated stratosphere and/or differences in the aerosol microphysics schemes (Pitari et al, 2014;Niemeier et al, 2020;Franke et al, 2021) resulting in different aerosol growth, transport and sedimentation, as already shown for simulations of explosive volcanic eruptions (Marshall et al, 2018;Clyne et al, 2021). The response to the presence of the aerosols themselves can in turn produce differences in stratospheric dynamics, for instance, interacting with the quasi-biennial oscillation (Aquila et al, 2014;Richter et al, 2017), strengthening the tropical confinement of the aerosols (Niemeier and Schmidt, 2017;Visioni et al, 2018b).…”

“…As mentioned before, the presence of aerosols in the stratosphere also produces a perturbation of stratospheric dynamics (Richter et al, 2017;Visioni et al, 2020a) that, in turn, might affect precipitation (Simpson et al, 2019) and temperature (Jiang et al, 2019) at the surface. The response is driven by the absorption of infrared radiation by the aerosols resulting in the heating of the stratospheric air and is thus dependent on the overall burden and the size of the particles (Pitari et al, 2016) but also on interactions with the chemical cycles in the stratosphere (Visioni et al, 2017b;Richter et al, 2017) and the incursion of water vapor from the troposphere due to the warming of the tropopause layer (Visioni et al, 2017b;Tilmes et al, 2018b;Boucher et al, 2017). In Fig.…”

Tracking Improvement in Simulated Marine Biogeochemistry Between CMIP5 and CMIP6

“…Consequently, they have helped determine the change in carbon budgets that is compatible with a given level of warming since pre-industrial times. Ocean biogeochemical models have also been used to investigate potential geoengineering solutions to climate change such as solar radiation management [37][38][39], ocean fertilization [40][41][42][43][44][45][46][47], alkalinity addition [48][49][50][51][52] and reversibility experiments (e.g. [53,54]).…”

Uncertainty and the basis for confidence in solar geoengineering research

“…We also provide our personal assessment of where each uncertainty falls on the risk register, recognizing that there is not yet an adequate basis for objectivity and that the process will constantly evolve with new information (TaBle 1). TaBle 1 is not exhaustive and other potential uncertainties could include impacts on extreme events 15 , methane chemistry 71 , the carbon cycle 91,92 , water cycling 93 , vegetation 94 , agriculture 29,30 , human health 95 and the cryosphere 13,14 . Nevertheless, TaBle 1 is a useful starting point to identify, prioritize and reduce uncertainties in SAG, as well as motivate discussion on how to reduce or manage these uncertainties.…”