The deployment length of solar radiation modification: an interplay of mitigation, net-negative emissions and climate uncertainty
Geoengineering News
A growing body of literature investigates the effects of solar radiation modification (SRM) on global and regional climates. Previous studies have focused on the potentials and the side effects of SRM, with little attention being given to possible deployment timescales and the levels of carbon dioxide removal required for a phase out. Here, we investigate the deployment timescales of SRM and how they are affected by different levels of mitigation, net-negative emissions (NNEs) and climate uncertainty. We generate a large dataset of 355 emission scenarios in which SRM is deployed to keep warming levels at 1.5 ∘C global mean temperature. Probabilistic climate projections from this ensemble result in a large range of plausible future warming and cooling rates that lead to various SRM deployment timescales. In all pathways consistent with extrapolated current ambition, SRM deployment would exceed 100 years even under the most optimistic assumptions regarding climate response. As soon as the temperature threshold is exceeded, neither mitigation nor NNEs or climate sensitivity alone can guarantee short deployment timescales. Since the evolution of mitigation under SRM, the availability of carbon removal technologies and the effects of climate reversibility will be mostly unknown at its initialisation time, it is impossible to predict how temporary SRM deployment would be. Any deployment of SRM therefore comes with the risk of multi-century legacies of deployment, implying multi-generational commitments of costs, risks and negative side effects of SRM and NNEs combined.
Figure 2Calculating required SRM. (a) Calculating warming to 2035 using an NDC-like pathway (in this case, SSP2-4.5). (b) Determining a 1.5 ∘C temperature trajectory for ensemble members that have not already overshot 1.5 ∘C by 2035. (c) Determining a 1.5 ∘C temperature trajectory for ensemble members that have already overshot 1.5 ∘C by 2035. (d, e) Calculating required solar radiation modification (SRM) for each scenario–ensemble member combination, whether it overshoots 1.5 ∘C (e) or remains below 1.5 ∘C at all times (d).
Figure 3SRM deployment length for all scenarios and all ensemble members. One bar spans a range of 50 years. Marked in black are pathways consistent with current 2100 warming projections for NDCs (2.4 ∘C; CAT, 2022).
Figure 4Interdependencies of mitigation, negative emissions and climate uncertainty with SRM deployment length. (a) Relationship between cumulative CO2 emissions from 2030 until net-zero CO2 and SRM deployment length. Colour coding is according to annual average NNEs in Gt CO2 yr−1. (b) Relationship between cumulative CO2 emissions from net-zero CO2 until the reattainment of 1.5 ∘C and SRM deployment length. Colour coding is according to annual average NNEs in Gt CO2 yr−1. (c) Relationship between eTCRE ratio and SRM deployment length. Colour coding is according to cumulative CO2 emissions from the time of peak warming until the reattainment of 1.5 ∘C. Plot shows data points that fall in the 1st–99th percentile range.
Figure 5Relationship between cumulative CO2 emissions from net-zero CO2 until the reattainment of 1.5 ∘C and SRM deployment length. (a) Colour coding is according to the maximum deployed annual NNEs. (b) Colour coding is according to the entire ratio. See the clean figure without descriptions in the Supplement (Fig. S03)
Source: Europeans Geosciences Union