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

   https://doi.org/10.5194/acp-21-10039-2021  

   Visioni, Daniele; MacMartin, Douglas G.; Kravitz, Ben; Boucher, Olivier; Jones, Andy; Lurton, Thibaut; Martine, Michou; Mills, Michael J.; Nabat, Pierre; Niemeier, Ulrike; et al (January 2021, Atmospheric Chemistry and Physics)

   Abstract. We present here results from the Geoengineering Model Intercomparison Project (GeoMIP) simulations for the experiments G6sulfur and G6solar for six Earth system models participating in the Climate Model Intercomparison Project (CMIP) Phase 6. The aim of the experiments is to reduce the warming that results from a high-tier emission scenario (Shared Socioeconomic Pathways SSP5-8.5) to that resulting from a medium-tier emission scenario (SSP2-4.5). These simulations aim to analyze the response of climate models to a reduction in incoming surface radiation as a means to reduce global surface temperatures, and they do so either by simulating a stratospheric sulfate aerosol layer or, in a more idealized way, through a uniform reduction in the solar constant in the model. We find that over the final two decades of this century there are considerable inter-model spreads in the needed injection amounts of sulfate (29 ± 9 Tg-SO2/yr between 2081 and 2100), in the latitudinal distribution of the aerosol cloud and in the stratospheric temperature changes resulting from the added aerosol layer. Even in the simpler G6solar experiment, there is a spread in the needed solar dimming to achieve the same global temperature target (1.91 ± 0.44 %). The analyzed models already show significant differences in the response to the increasing CO2 concentrations for global mean temperatures and global mean precipitation (2.05 K ± 0.42 K and 2.28 ± 0.80 %, respectively, for SSP5-8.5 minus SSP2-4.5 averaged over 2081–2100). With aerosol injection, the differences in how the aerosols spread further change some of the underlying uncertainties, such as the global mean precipitation response (−3.79 ± 0.76 % for G6sulfur compared to −2.07 ± 0.40 % for G6solar against SSP2-4.5 between 2081 and 2100). These differences in the behavior of the aerosols also result in a larger uncertainty in the regional surface temperature response among models in the case of the G6sulfur simulations, suggesting the need to devise various, more specific experiments to single out and resolve particular sources of uncertainty. The spread in the modeled response suggests that a degree of caution is necessary when using these results for assessing specific impacts of geoengineering in various aspects of the Earth system. However, all models agree that compared to a scenario with unmitigated warming, stratospheric aerosol geoengineering has the potential to both globally and locally reduce the increase in surface temperatures. 

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2. Fires of Unusual Size: Future of Extreme and Emerging Wildfire in a Warming United States (2020–2060)

   https://doi.org/10.3390/fire9050208  

   Stephens, Jilmarie; Joseph, Maxwell; Bitters, Matthew E; Iglesias, Virginia; Tuff, Ty; Mahood, Adam; Rangwala, Imtiaz; Wolken, Jane; O’Connor, Christopher D; Balch, Jennifer K (May 2026, Fire)

   Observed increases in wildfire activity across the contiguous United States (U.S.), together with continued warming and expanding development in fire-prone landscapes, highlight the need to anticipate near-term changes in fire regimes. We apply a Bayesian statistical model that integrates projected population density (SSP2) and downscaled climate simulations under a moderate emissions scenario (RCP 4.5) to estimate future wildfire occurrence, maximum fire size (using the 90th percentile of fire size distribution), and total area burned for large fires (>1000 acres) across all EPA Level III ecoregions for 2020–2060. Relative to 1984–2019, we project nationwide increases of 56% in fire occurrence and 59% in area burned, with larger increases in maximum fire size (63%) in 2020–2060. Spatial patterns vary substantially: fire occurrence increases most strongly in the eastern U.S., including regions where large fires have historically been rare, while western ecoregions experience the largest absolute increases in burned area and extreme fire size. The disproportionate growth in maximum fire size suggests that changes in fire weather will amplify extreme events beyond increases in ignition frequency alone. These projections indicate expanding wildfire risk across diverse U.S. landscapes and underscore the need for regionally tailored fire management and preparedness strategies. 

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3. Projected Impact of Mid-21st Century Climate Change on Wildfire Hazard in a Major Urban Watershed outside Portland, Oregon USA

   https://doi.org/10.3390/fire3040070  

   McEvoy, Andy; Nielsen-Pincus, Max; Holz, Andrés; Catalano, Arielle J.; Gleason, Kelly E. (December 2020, Fire)

   null (Ed.)

   Characterizing wildfire regimes where wildfires are uncommon is challenged by a lack of empirical information. Moreover, climate change is projected to lead to increasingly frequent wildfires and additional annual area burned in forests historically characterized by long fire return intervals. Western Oregon and Washington, USA (westside) have experienced few large wildfires (fires greater than 100 hectares) the past century and are characterized to infrequent large fires with return intervals greater than 500 years. We evaluated impacts of climate change on wildfire hazard in a major urban watershed outside Portland, OR, USA. We simulated wildfire occurrence and fire regime characteristics under contemporary conditions (1992–2015) and four mid-century (2040–2069) scenarios using Representative Concentration Pathway (RCP) 8.5. Simulated mid-century fire seasons expanded in most scenarios, in some cases by nearly two months. In all scenarios, average fire size and frequency projections increased significantly. Fire regime characteristics under the hottest and driest mid-century scenarios illustrate novel disturbance regimes which could result in permanent changes to forest structure and composition and the provision of ecosystem services. Managers and planners can use the range of modeled outputs and simulation results to inform robust strategies for climate adaptation and risk mitigation. 

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4. North Atlantic Oscillation response in GeoMIP experiments G6solar and G6sulfur: why detailed modelling is needed for understanding regional implications of solar radiation management

   https://doi.org/10.5194/acp-21-1287-2021  

   Jones, Andy; Haywood, Jim M.; Jones, Anthony C.; Tilmes, Simone; Kravitz, Ben; Robock, Alan (January 2021, Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. The realization of the difficulty of limiting global-meantemperatures to within 1.5 or 2.0 ∘C abovepre-industrial levels stipulated by the 21st Conference of Parties inParis has led to increased interest in solar radiation management (SRM)techniques. Proposed SRM schemes aim to increase planetary albedo to reflectmore sunlight back to space and induce a cooling that acts to partiallyoffset global warming. Under the auspices of the Geoengineering ModelIntercomparison Project, we have performed model experiments whereby globaltemperature under the high-forcing SSP5-8.5 scenario is reduced to followthat of the medium-forcing SSP2-4.5 scenario. Two different mechanisms toachieve this are employed: the first via a reduction in the solar constant(experiment G6solar) and the second via modelling injections of sulfurdioxide (experiment G6sulfur) which forms sulfate aerosol in thestratosphere. Results from two state-of-the-art coupled Earth system models(UKESM1 and CESM2-WACCM6) both show an impact on the North AtlanticOscillation (NAO) in G6sulfur but not in G6solar. Both models show apersistent positive anomaly in the NAO during the Northern Hemisphere winterseason in G6sulfur, suggesting an increase in zonal flow and an increase inNorth Atlantic storm track activity impacting the Eurasian continent and leadingto high-latitude warming over Europe and Asia. These results are broadlyconsistent with previous findings which show similar impacts fromstratospheric volcanic aerosol on the NAO and emphasize that detailedmodelling of geoengineering processes is required if accurate impacts of SRMeffects are to be simulated. Differences remain between the two models inpredicting regional changes over the continental USA and Africa, suggestingthat more models need to perform such simulations before attempting to drawany conclusions regarding potential continental-scale climate change underSRM. 

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5. Data from: Forest carbon emission sources are not equal: putting fire, harvest, and fossil fuel emissions in context

   https://doi.org/10.7923/VVZE-7642  

   Bartowitz, Kristina; Walsh, Eric; Stenzel, Jeffrey; Kolden, Crystal; Hudiburg, Tara (January 2022, University of Idaho)

   Climate change has intensified the scale of global wildfire impacts in recent decades. To help policymakers and managers avoid these unintended carbon consequences and to present carbon emission sources in the same context, we calculate western US forest fire carbon emissions and compare them with harvest and fossil fuel emissions over the same timeframe. We find that forest fire carbon emissions are on average only 6% of anthropogenic fossil fuel emissions (FFE) over the past decade. While wildfire occurrence and area burned have increased over the last three decades, per area fire emissions for extreme fire events are relatively constant. In contrast, harvest of mature trees releases a higher density of carbon emissions (e.g., per unit area) relative to wildfire (150-800%) because harvest causes a higher rate of tree mortality than wildfire. Shown in context, our results demonstrate that reducing FFEs will do more for climate mitigation potential (and subsequent reduction of fire) than increasing extractive harvest to prevent fire emissions. 

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