Contribution of Global Warming and Atmospheric Circulation to the Hottest Spring in Eastern China in 2018

The spring of 2018 was the hottest on record since 1951 over eastern China based on station observations,being 2.5°C higher than the 1961−90 mean and with more than 900 stations reaching the record spring mean temperature.This event exerted serious impacts in the region on agriculture,plant phenology,electricity transmission systems,and human health.In this paper,the contributions of human-induced climate change and anomalous anticyclonic circulation to this event are investigated using the newly homogenized observations and updated Met Office Hadley Centre system for attribution of extreme events,as well as CanESM2(Second Generation Canadian Earth System Model)simulations.Results indicate that both anthropogenic influences and anomalous anticyclonic circulation played significant roles in increasing the probability of the 2018 hottest spring.Quantitative estimates of the probability ratio show that anthropogenic forcing may have increased the chance of this event by ten-fold,while the anomalous circulation increased it by approximately two-fold.The persistent anomalous anticyclonic circulation located on the north side of China blocked the air with lower temperature from high latitudes into eastern China.Without anthropogenic forcing or without the anomalous circulation in northern China,the occurrence probability of the extreme warm spring is significantly reduced.

Bivariate attribution of the compound hot and dry summer of 2022 on the Tibetan Plateau

The extraordinarily high temperatures experienced during the summer of 2022 on the Tibetan Plateau(TP)demand attention when compared with its typical climatic conditions.The absence of precipitation alongside the elevated temperatures resulted in 2022 being the hottest and driest summer on record on the TP since at least 1961.Recognizing the susceptibility of the TP to climate change,this study employed large-ensemble simulations from the HadGEM3-A-N216 attribution system,together with a copula-based joint probability distribution,to investigate the influence of anthropogenic forcing,primarily global greenhouse gas emissions,on this unprecedented compound hot and dry event(CHDE).Findings revealed that the return period for the 2022 CHDE on the TP exceeds 4000 years,as determined from the fitted joint distributions derived using observational data spanning 1961-2022.This CHDE was directly linked to large-scale circulation anomalies,including the control of equivalent-barotropic high-pressure anomalies and the northward displacement of the subtropical westerly jet stream.Moreover,anthropogenic forcing has,to some extent,promoted the surface warming and increased variability in precipitation on the TP in summer,establishing conditions conducive for the 2022 CHDE from a long-term climate change perspective.The return period for a 2022-like CHDE on the TP was estimated to be approximately 283 years(142-613 years)by the large ensemble forced by both anthropogenic activities and natural factors.Contrastingly,ensemble simulations driven solely by natural forcing indicated that the likelihood of occurrence of a 2022-like CHDE was almost negligible.These outcomes underscore that the contribution of anthropogenic forcing to the probability of a 2022-like CHDE was 100%,implying that without anthropogenically induced global warming,a comparable CHDE akin to that observed in 2022 on the TP would not be possible.

Understanding Differences in Event Attribution Results Arising from Modeling Strategy

While there is high confidence that human activities have increased the likelihood and severity of hot extreme events over many parts of the world,there is notable spread in quantitative estimates of anthropogenic influence even on a single event.To better understand the uncertainty of attribution results,here we compare different event attribution methods using the 2015 July-August record-breaking heat event in northwestern China as a case study.To address the anthropogenic influence on the likelihood of the extreme event,we employ attribution runs with two modeling strategies—atmosphere-only and coupled simulations—with different conditioning.In atmosphere-only attribution runs,given the observed sea surface boundary conditions and external forcings in 2015,it is estimated that anthropogenic forcing has increased the likelihood of hot extremes such as that observed in 2015 in the target region,by approximately 27 and 12 times in MIROC5 and HadGEM3-A-N216,respectively.In Coupled Model Intercomparison Project phase 5(CMIP5)fully coupled attribution runs,given the external forcing at the 1961-2015 level and regardless of sea surface boundary conditions,there is a 21-fold increase in the likelihood of similar heat events due to anthropogenic forcing.The differences in quantitative attribution results can arise from modeling strategies,which are tightly linked to different conditioning in attribution.Specifically,different ocean boundary conditions,external forcings,and air-sea coupling processes contribute to different attribution results between the two modeling strategies.Within each modeling strategy,model uncertainty affects quantitative attribution conclusions.The comparison of different attribution methods provides a better understanding of the uncertainty of attribution results,which is useful in synthesizing and interpreting attribution results.

Effects of anthropogenic forcing and atmospheric circulation on the record-breaking welt bulb heat event over southern China in September 2021

In September 2021,southern China witnessed an extreme high-temperature and high-humidity event.The average regional wet bulb globe temperature(WBGT)anomaly(relative to 1961-1990 mean)in 110.0°-120.0°E,27.5°-32.5°N region was the highest on record at 3.28°C and exceeded three times the observed standard deviation.To investigate the underlying causes,we examine the effects of anthropogenic forcings and anomalous circulation patterns on this event using the multi-model ensembles from the Coupled Model Intercomparison Project Phase 6.Results indicate that 2021-like events would happen extremely rarely without anthropogenic warming(would not occur in counterfactual world simulations)and have become a 1-in-16-year event in the factual world.For the threshold of the second most extreme year,the occurrence probability of extreme WBGT events increases approximately 50 times due to the impact of anthropogenic forcings.The effect of anthropogenic warming under similar atmosphere circulation increases the probability of extreme WBGT events by 13-60 times,and that of corresponding circulation patterns under the same anthropogenic warming increases the probability by 1.3-1.8 times.

An Updated Review of Event Attribution Approaches

There have been considerable high-impact extreme events occurring around the world in the context of climate change.Event attribution studies,which seek to quantitatively answer whether and to what extent anthropogenic climate change has altered the characteristics-predominantly the probability and magnitude-of particular events,have been gaining increasing interest within the research community.This paper reviews the latest approaches used in event attribution studies through a new classification into three major categories according to how the event attribution question is framed-namely,the risk-based approach,the storyline approach,and the combined approach.Four approaches in the risk-based framing category and three in the storyline framing category are also reviewed in detail.The advantages and disadvantages of each approach are discussed.Particular attention is paid to the ability,suitability,and applicability of these approaches in attributing extreme events in China,a typical monsoonal region where climate models may not perform well.Most of these approaches are applicable in China,and some are more suitable for analyzing temperature events.There is no right or wrong among these approaches,but different approaches have different framings.The uncertainties in attribution results come from several aspects,including different categories of framing,different conditions in climate model approaches,different models,different definitions of the event,and different observational data used.Clarification of these aspects can help to understand the differences in attribution results from different studies.

Using a Game to Engage Stakeholders in Extreme Event Attribution Science

The impacts of weather and climate-related disasters are increasing,and climate change can exacerbate many disasters.Effectively communicating climate risk and integrating science into policy requires scientists and stakeholders to work together.But dialogue between scientists and policymakers can be challenging given the inherently multidimensional nature of the issues at stake when managing climate risks.Building on the growing use of serious games to create dialogue between stakeholders,we present a new game for policymakers called Climate Attribution Under Loss and Damage:Risking,Observing,Negotiating(CAULDRON).CAULDRON aims to communicate understanding of the science attributing extreme events to climate change in a memorable and compelling way,and create space for dialogue around policy decisions addressing changing risks and loss and damage from climate change.We describe the process of developing CAULDRON,and draw on observations of players and their feedback to demonstrate its potential to facilitate the interpretation of probabilistic climate information and the understanding of its relevance to informing policy.Scientists looking to engage with stakeholders can learn valuable lessons in adopting similar innovative approaches.The suitability of games depends on the policy context but,if used appropriately,experiential learning can drive coproduced understanding and meaningful dialogue.

A Review of Climate Change Attribution Studies

This paper reviews recent progress in climate change attribution studies. The focus is on the attribution of ob-served long-term changes in surface temperature, precipitation, circulation, and extremes, as well as that of specific extreme weather and climate events. Based on new methods and better models and observations, the latest studies ftLrther verify the conclusions on climate change attribution in the IPCC AR5, and enrich the evidence for anthropo-genie influences on weather and climate variables and extremes. The uncertainty of global temperature change attrib- utable to anthropogenic forcings lies in the considerable uncertainty of estimated total radiative forcing due to aero- sols, while the uncertainty of precipitation change attribution arises from the limitations of observation and model simulations along with influences from large internal variability. In terms of extreme weather and climate events, it is clear that attribution studies have provided important new insights into the changes in the intensity or frequency of some of these events caused by anthropogenic climate change. The framing of the research question, the methods se- lected, and the model and statistical methods used all have influences on the results and conclusions drawn in an event attribution study. Overall, attribution studies in China remain inadequate because of limited research focus and the complexity of the monsoon climate in East Asia. Attribution research in China has focused mainly on changes or events related to temperature, such as the attribution of changes in mean and extreme temperature and individual heat wave events. Some progress has also been made regarding the pattern of changes in precipitation and individual ex-treme rainfall events in China. Nonetheless, gaps remain with respect to the attribution of changes in extreme precip-itation, circulation, and drought, as well as to the event attribution such as those related to drought and tropical cyc-lones. It can be expected that, with the continual development of climate models, ongoing improvements to data, and the introduction of new methods in the future, climate change attribution research will develop accordingly. Addi-tionally, further improvement in climate change attribution will facilitate the development of operational attribution systems for extreme events, as well as attribution studies of climate change impacts.

Anthropogenic influence on extreme Meiyu rainfall in 2020 and its future risk

Eastern China experienced excessive Meiyu rainfall in the summer of 2020,with a long rainy season and frequent extreme rainfall events.Extreme rainfall occurred on daily to monthly time scales.In particular,persistent heavy rainfall events occurred;e.g.,the maximum accumulated rainfall over four consecutive weeks(Rx28day)in the lower reaches of the Yangtze River was 94%greater than climatology,breaking the observational record since 1961.With ongoing anthropogenic climate change,it is vital to understand the anthropogenic influence on this extreme rainfall event and its driving mechanisms.In this study,based on multi-model simulations under different external forcings that participate in the Detection and Attribution Model Intercomparison Project(DAMIP)in the Coupled Model Intercomparison Project-phase 6(CMIP6),we show that anthropogenic forcing has reduced the probability of the Rx28day extreme rainfall as that in observations in the lower reaches of the Yangtze River in 2020,by 46%(22–62%).Specifically,greenhouse gas(GHG)emissions have increased the probability by 44%as a result of atmospheric warming and moistening.However,this effect was offset by anthropogenic aerosols,which reduced the probability by 73%by reducing atmospheric moisture and weakening the East Asian summer monsoon circulation.With the continuous emissions of GHGs and reductions in aerosols in the future,similar persistent heavy rainfall events are projected to occur more frequently.A higher occurrence probability is expected under higher emission scenarios,which is estimated to be 4.6,13.6 and 27.7 times that in the present day under the SSP1-2.6,SSP2-4.5,and SSP5-8.5 emission scenarios,respectively,by the end of the 21st century.Thus,efficient mitigation measures will help to reduce the impacts related to extreme rainfall.