This study isolates the influence of sea ice mean state on pre-industrial climate and transient 1850-2100 climate change within a fully coupled global model: The Community Earth System Model version 2 (CESM2). The CESM2 sea ice model physics is modified to increase surface albedo, reduce surface sea ice melt, and increase Arctic sea ice thickness and late summer cover. Importantly, increased Arctic sea ice in the modified model reduces a present-day late-summer ice cover bias. Of interest to coupled model development, this bias reduction is realized without degrading the global simulation including top-of-atmosphere energy imbalance, surface temperature, surface precipitation, and major modes of climate variability. The influence of these sea ice physics changes on transient 1850-2100 climate change is compared within a large initial condition ensemble framework. Despite similar global warming, the modified model with thicker Arctic sea ice than CESM2 has a delayed and more realistic transition to a seasonally ice free Arctic Ocean. Differences in transient climate change between the modified model and CESM2 are challenging to detect due to large internally generated climate variability. In particular, two common sea ice benchmarks - sea ice sensitivity and sea ice trends - are of limited value for comparing models with similar global warming. More broadly, these results show the importance of a reasonable Arctic sea ice mean state when simulating the transition to an ice-free Arctic Ocean in a warming world. Additionally, this work highlights the importance of large initial condition ensembles for credible model-to-model and observation-model comparisons.

Multiple 50-member ensemble simulations with the Community Earth System Model version 2 are performed to estimate the coupled climate responses to the 2019-2020 Australian wildfires and COVID-19 pandemic policies. The climate response to the pandemic is found to be weak generally, with net top-of-atmosphere radiative anomalies of +0.23+-0.14 W m/2driving a gradual global warming of 0.05+-0.04 K by the end of 2022. While regional anomalies are detectible in aerosol burdens and clear-sky radiation, few significant anomalies exist in other fields due to internal variability. In contrast, the simulated response to Australian wildfires is a strong and rapid cooling, peaking at -0.95+-0.15 W m/2 in late 2019 with an anomalous global cooling of 0.06+-0.04 K by mid-2020. Transport of fire aerosols throughout the Southern Hemisphere increases albedo and drives a strong interhemispheric radiative contrast, with simulated responses that are consistent generally with those to a Southern Hemisphere volcanic eruption.

Three new equilibrium Mid-Pliocene (MP) simulations are implemented with the Community Climate System Model version 4 (CCSM4), Community Earth System Model version 1.2 (CESM1.2), and 2 (CESM2). All simulations are carried out with the same boundary and forcing conditions following the protocol of Pliocene Model Intercomparison Project Phase 2. These simulations reveal amplified MP climate change relative to preindustrial going from CCSM4 to CESM2, seen in global mean and polar amplification of surface warming, sea ice reduction in both Arctic and Antarctic, and weakened Hadley circulation. The enhanced global mean warming arises from both enhanced Earth System Sensitivity (ESS) and Equilibrium Climate Sensitivity (ECS) to CO forcing. ESS is amplified by up to 70% in CCSM4, and up to 100% in CESM1.2 and CESM2 relative to ECSs of respective models. Simulations also agree on the strengthened Atlantic Meridional Overturning Circulation, but disagree on several other climate metrics. Compared to preindustrial, CCSM4 features small increase in both low and high cloud cover and no change in the mean climate state of the equatorial Pacific. Whereas, both CESM1.2 and 2 show reduction of cloud cover at all heights, and an anomalous El Niño-like state of the equatorial Pacific. The performances of MP simulations are assessed with a new compilation of paleo-observations of sea surface temperature (SST). CESM1.2 and 2 show better skills than CCSM4 in simulating MP global mean warming and amplified SST warming in the northern middle and high latitudes, supporting the amplified ESS compared to the CCSM4.

Future changes in tropical cyclone properties are an important component of climate change impacts and risk for many tropical and mid-latitude countries. In this study we assess the performance of a multi-model ensemble of climate models, at resolutions ranging from 250km to 25km. We use a common experimental design including both atmosphere-only and coupled simulations run over the period 1950-2050, with two tracking algorithms applied uniformly across the models. There are overall improvements in tropical cyclone frequency, spatial distribution and intensity in models at 25 km resolution, with several of them able to represent very intense storms. Projected tropical cyclone activity by 2050 generally declines in the South Indian Ocean, while changes in other ocean basins are more uncertain and sensitive to both tracking algorithm and imposed forcings. Coupled models with smaller biases suggest a slight increase in average TC 10m wind speeds by 2050.