A document by Mira Adhikari. Click on the document to view its contents.

Walker Raymond Leea, Michael Diamondb, Pete Irvinec, Jesse Reynoldsd, Daniele VisionieaClimate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO, USAbDepartment of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL, USAcEarth Sciences, University College London, London, UKdThe DEGREES Initiative, London, UKeDepartment of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USACorr. author: Walker Raymond Lee, [email protected] 5/31/24 to Nature Communications Earth & Environment as Matters Arising regarding the study "Radiative forcing geoengineering causes higher risk of wildfires and permafrost thawing over the Arctic regions" (R. C. Müller, et al, 2024, https://doi.org/10.1038/s43247-024-01329-3)The study “Radiative forcing geoengineering causes higher risk of wildfires and permafrost thawing over the Arctic regions”1 (henceforth “Müller, et al.”) examines three scenarios of radiative forcing geoengineering (RFG) - stratospheric aerosol injection (SAI), marine cloud brightening (MCB), and cirrus cloud thinning (CCT) - as simulated by the Norwegian Earth System Model (NorESM2), comparing high-latitude (>50°N) boreal summer maximum temperatures (TXx) and winter minimum temperatures (TNn) for the RFG scenarios to the high-warming RCP8.5 and moderate-warming RCP4.5 scenarios. They conclude that all three RFG interventions worsen the risk of wildfire and permafrost thaw relative to RCP4.5. We have significant concerns about how this paper’s results and conclusions are framed. First and foremost, the title of the study claims that RFG increases risk of wildfires and permafrost thaw; instead, what the authors show is that RFG reduces these risks, but not as much as an equivalent scenario of emissions cuts. Secondly, the authors overgeneralize from a limited set of simulations even though it is now well known that regional impacts are highly dependent on the specific RFG strategy employed2.Our first concern relates to how Müller, et al. characterize “risk”. All three RFG interventions were simulated in a context of RCP8.5 emissions and designed to achieve the same global radiative balance as RCP4.5. It is clear from Figure 1 of Müller, et al. that the interventions substantially reduce global and Arctic mean temperatures relative to RCP8.5 by 2100. While it may be the case that, relative to RCP8.5, the greenhouse gas mitigation represented by RCP4.5 more efficiently reduces risk than any of the RFG interventions (assuming they were used as a substitute for that mitigation), the title misattributes the impacts of increased GHGs plus RFG to RFG alone; their Figures 2-6 present results with respect to RCP4.5, which is not, on its own, a suitable frame of reference to determine the impacts of RFG. International assessments of RFG underscore that such methods should not be considered as a substitute to emissions reduction3, not least because the environmental consequences of GHGs and RFG can be very different4. Thus, to have a clear and accurate sense of their potential consequences, an assessment of RFG’s potential climatic risks must consider them in relation to, not isolated from, the counterfactual risks of a world where warming is unabated by RFG. In Figure 1, we plot July maximum (TXx) and January minimum (TNn) temperature differences for each RFG realization to both RCP8.5 and RCP4.5 using the authors’ data. The authors’ data show a reduction of risk of wildfires and permafrost thaw in the RFG intervention scenarios compared to a world with the same CO2 concentrations but without RFG (in line with other studies5,6. Müller, et al. mischaracterize the response to RFG as an increase in these risks because they compare against the wrong baseline, ignoring the appropriate counterfactual.

Projected rates of emissions reductions are unlikely to keep global temperatures from crossing the Paris Agreement temperature targets. Large-scale carbon dioxide removal (CDR) could help recover a target temperature after it has been exceeded, producing an overshoot scenario. Solar radiation management (SRM) is the proposal to cool the planet by increasing the reflection of incoming solar radiation. It could be used in an overshoot scenario for peak shaving, where SRM is deployed to maintain a temperature target during the overshoot. Here, we quantify the effect of peak shaving on the duration of the overshoot using an adapted extension of the SSP2-4.5 scenario and an ensemble of variants of the FaIR simple climate model. We find a substantial reduction in overshoot duration, which ranges from ∼5% for decadal overshoots up to ∼20% for multi-century overshoots. The shortening is predominantly driven by the ocean response to peak shaving. Peak shaving results in lower ocean temperatures relative to the overshoot scenario, inducing a stronger surface temperature response to decreasing and negative emissions, driving overshoot shortening. Our results also indicate that peak shaving with SRM would reduce the cumulative net negative emissions needed to end temperature overshoot by ∼27%. Thus, SRM, when deployed as a complement to emissions reductions and CDR, could end overshoot decades earlier than otherwise and at a substantially lower cost.