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1. An increase in marine heatwaves without significant changes in surface ocean temperature variability

   https://doi.org/10.1038/s41467-022-34934-x  

   Xu, Tongtong; Newman, Matthew; Capotondi, Antonietta; Stevenson, Samantha; Di Lorenzo, Emanuele; Alexander, Michael A. (December 2022, Nature Communications)

   Abstract Marine heatwaves (MHWs)—extremely warm, persistent sea surface temperature (SST) anomalies causing substantial ecological and economic consequences—have increased worldwide in recent decades. Concurrent increases in global temperatures suggest that climate change impacted MHW occurrences, beyond random changes arising from natural internal variability. Moreover, the long-term SST warming trend was not constant but instead had more rapid warming in recent decades. Here we show that this nonlinear trend can—on its own—appear to increase SST variance and hence MHW frequency. Using a Linear Inverse Model to separate climate change contributions to SST means and internal variability, both in observations and CMIP6 historical simulations, we find that most MHW increases resulted from regional mean climate trends that alone increased the probability of SSTs exceeding a MHW threshold. Our results suggest the need to carefully attribute global warming-induced changes in climate extremes, which may not always reflect underlying changes in variability. 

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2. Divergent global-scale temperature effects from identical aerosols emitted in different regions

   https://doi.org/10.1038/s41467-018-05838-6  

   Persad, Geeta G.; Caldeira, Ken (August 2018, Nature Communications)

   Abstract The distribution of anthropogenic aerosols’ climate effects depends on the geographic distribution of the aerosols themselves. Yet many scientific and policy discussions ignore the role of emission location when evaluating aerosols’ climate impacts. Here, we present new climate model results demonstrating divergent climate responses to a fixed amount and composition of aerosol—emulating China’s present-day emissions—emitted from 8 key geopolitical regions. The aerosols’ global-mean cooling effect is fourteen times greater when emitted from the highest impact emitting region (Western Europe) than from the lowest (India). Further, radiative forcing, a widely used climate response proxy, fails as an effective predictor of global-mean cooling for national-scale aerosol emissions in our simulations; global-mean forcing-to-cooling efficacy differs fivefold depending on emitting region. This suggests that climate accounting should differentiate between aerosols emitted from different countries and that aerosol emissions’ evolving geographic distribution will impact the global-scale magnitude and spatial distribution of climate change. 

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3. Intensified Likelihood of Concurrent Warm and Dry Months Attributed to Anthropogenic Climate Change

   https://doi.org/10.1029/2021WR030411  

   Chiang, Felicia; Greve, Peter; Mazdiyasni, Omid; Wada, Yoshihide; AghaKouchak, Amir (June 2022, Water Resources Research)

   Abstract Detection and attribution studies generally examine individual climate variables such as temperature and precipitation. Thus, we lack a strong understanding of climate change impacts on correlated climate extremes and compound events, which have become more common in recent years. Here we present a monthly‐scale compound warm and dry attribution study, examining CMIP6 climate models with and without the influence of anthropogenic forcing. We show that most regions have experienced large increases in concurrent warm and dry months in historical simulations with human emissions, while no coherent change has occurred in historical natural‐only simulations without human emissions. At the global scale, the likelihood of compound warm‐dry months has increased 2.7 times due to anthropogenic emissions. With this multivariate perspective, we highlight that anthropogenic emissions have not only impacted individual extremes but also compound extremes. Due to amplified risks from multivariate extremes, our results can provide important insights on the risks of associated climate impacts. 

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4. Paris Agreement's Ambiguity About Aerosols Drives Uncertain Health and Climate Outcomes

   https://doi.org/10.1029/2020EF001787  

   Polonik, Pascal; Ricke, Katharine; Burney, Jennifer (May 2021, Earth's Future)

   Abstract Anthropogenic aerosols are hazardous to human health but have helped offset warming from greenhouse gases (GHGs), creating a potential regulatory tradeoff. As countries implement their GHG reduction targets under the Paris climate agreement, the co‐emissions of aerosols and their precursors will also change. Since these co‐emissions vary by country and by economic sector, each country will face different tradeoffs between aerosol‐driven health or temperature co‐benefits. We combine simple parameterizations of physical processes and health outcomes to examine three idealized climate policy approaches that are consistent with the Paris Agreement targets, which (i) optimize for local air quality, (ii) reduce global temperature change, or (iii) reduce emissions equally from all domestic economic sectors. We evaluate aerosol impacts on premature mortality and global mean temperature change under these three policy approaches and find that by 2030 the three policies yield differences of over 1 million annual premature deaths and global temperature differences of the same magnitude as those from GHG reductions. We also show that implementing equal reductions between all economic sectors can actually result in less beneficial health and temperature outcomes than either of the other options, especially in less industrialized regions. We therefore conclude that aerosol‐related co‐benefits and aerosol accounting guidelines should be explicitly considered in setting international climate policy. 

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5. The Response of Precipitation Extremes to the Twentieth‐ and Twenty‐First‐Century Global Temperature Change in a Comprehensive Suite of CESM1 Large Ensemble Simulation: Revisiting the Role of Forcing Agents Vs. the Role of Forcing Magnitudes

   https://doi.org/10.1029/2021EA002010  

   Xu, Yangyang; Lin, Lei; Diao, Chenrui; Wang, Zhili; Bates, Susan; Arblaster, Julie (January 2022, Earth and Space Science)

   Abstract The response of precipitation extremes (PEs) to global warming is found to be nonlinear in Community Earth System Model version 1 (CESM1) and other global climate models (Pendergrass et al., 2019), which led to the concern that it is not accurate to approximate the response of PE to a single forcing as the difference between simulations with all forcing agents and those that exclude one specific forcing. This calls into question previous model‐based results that the sensitivity of PE with warming due to aerosol forcing is significantly larger than that due to greenhouse gases (GHGs). We reevaluate the PE sensitivity to GHGs and aerosols using available CESM1 ensemble simulations. We show that although the PE response to warming is nonlinear in CESM1, especially for the high warming projected in the twenty‐first‐century, PE sensitivity to aerosols is still significantly stronger than that due to GHGs when the comparison is made within similar warming regimes to avoid the bias induced by the nonlinear behavior. But the difference is smaller than previously estimated. We also conclude that the additivity assumption is largely valid to isolate the PE response due to aerosol forcing from the paired simulations including the “all forcing” experiment when the warming regime is small (e.g., 1°C–2°C in the next few decades when aerosol forcing is projected to decline and becomes a major source of uncertainty for model projection). 

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