State of knowledge and implications for adaptation and mitigation Editor: Ralf Döscher Authors: Danijel Belusic (SMHI), Peter Berg (SMHI), Denica Bozhinova (SMHI), Lars Bärring (SMHI), Ralf Döscher (SMHI), Anna Eronn (SMHI), Erik Kjellström (SMHI), Katharina Klehmet (SMHI), Helena Martins (SMHI), Carin Nilsson (Climate and Culture), Jonas Olsson (SMHI), Christiana Photiadou (SMHI), David Segersson (SMHI), Gustav Strandberg (SMHI) Acknowledgement: This knowledge synthesis was possible due to the Formas-supported project "Klimatextremer för Sverige". We would like to thank the engaged discussions, valuable suggestions and input coming from the members of our advisory board: Lotta Andersson (SMHI), Cecilia Alfredsson (MSB), Karl-Erik Grevendahl (Karl-Erik Grevendahl Development AB Consultants), Kaj Hellner (Länsstyrelsen i Södermanland), Barbro Näslund Landenmark (MSB), Katarina Skoglycka (Oskarshamn municipality), Roger Street (UKCIP - UK Climate Impact Programme), Rob Swart (Wageningen University, The Netherlands) We are also greatful to Erik Engström from SMHI reviewing the report. First published September, 2019 DOI: https://doi.org/10.17200/Climate_Extremes_Sweden CONTENT 1. INTRODUCTION: WHY DO EXTREMES MATTER? 04 2. METHODS AND PROCEDURES FOR THIS SYNTHESIS REVIEW 06 3. UPDATED STATE OF KNOWLEDGE ABOUT CLIMATE EXTREMES 09 AND THEIR TRENDS 4. SOCIETAL IMPACTS OF CLIMATE EXTREMES 26 5. APPLICATIONS OF CLIMATE EXTREMES INFORMATION 30 IN SELECTED USER STORIES 6. GAPS IN KNOWLEDGE AND UNDERSTANDING 42 7. SUMMARY AND CONCLUSIONS FOR STAKEHOLDERS 50 AND CLIMATE SCIENTISTS APPENDIX 52 GLOSSARY 54 REFERENCES 56 CLIMATE EXTREMES FOR SWEDEN CONTENT 3 »Extreme: reaching a high or the highest degree, very great; not usual; exceptional; very severe or serious« (Oxford Dictionary, 2018). 1. INTRODUCTION: WHY DO EXTREMES MATTER? Extremes of weather and climate are part of the human verse, and needs to be set for individual cases. It includes experience with nature. Societies have always been vulne- even temporal sequences of events or compound events rable to e.g. flooding, extreme precipitation, windstorms involving combinations of several meteorological and and heat waves, and sought to adapt in order to avoid hydrological phenomena and variables, each not necessa- extreme impacts on living conditions. rily extreme by itself, which together constitute an extre- The term “climate” describes the long-term state and me impact (Leonard et al., 2014). Accordingly, exposure behaviour of the climate system and the atmosphere, typi- and vulnerability in Swedish municipalities, enterprises cally the 30-year average of weather, as well as anomalous and other societal sectors vary dependent on the specific and rare events on top of the long-term average. “Climate nature of climate extremes and the sectoral risk. extremes” are generally defined as the occurrence of a Based on existing scientific literature, there is clear climate variable above (or below) a certain threshold evidence that the climate and its extremes are changing value near the upper (or lower) range of observed values. (e.g. Westra et al., 2014; Lehmann et al., 2015; Schleussner Both extreme weather and extreme climate events are et al., 2017) in a way that goes beyond our present expe- collectively referred to as climate extremes (Field et al., rience and that exceeds our preparedness, i.e. our adapta- 2012). IPCC further distinguishes between “Weather and tion level (e.g. Im et al., 2017). Changes of extremes such Climate Extremes” (such as precipitation, temperature as hot/cold days, warm-spell duration, heavy rainfall, incl. hot/cold spells, wind) and “Impacts on the Natural aggregated over global observational networks exhibit an Physical Environment” (such as floods, droughts, sea level increase that is substantially larger than what would be and more). expected by chance (Schleussner et al., 2017) in a stable While the mean climate is changing and the transfor- climate. mation can be largely attributed to anthropogenic emis- There is high confidence that certain observed chang- sions (Field et al., 2012), climate extremes are changing as es in temperature extremes can be attributed to anthropo- well. The character and severity of impacts from climate genic forcing (Masson-Delmotte et al., 2018; Bindoff et al., extremes depend not only on the extremes themselves but 2013). Enhanced extreme peaks in a number of additional also on exposure and vulnerability of societal structures. climate variables on a global scale can be attributed to the Therefore, for this report, the overarching definition of an changing climate. Attribution of extreme events is a extreme is extended by the perspective of users and their growing field of science and there has been a rapid increa- respective vulnerabilities. The complete resulting defini- se with time in the number of peer-reviewed papers (such tion of a climate extreme becomes case sensitive and di- as collected by Herring et al., 2019). 4 INTRODUCTION: WHY DO EXTREMES MATTER? – CLIMATE EXTREMES FOR SWEDEN On a local scale it still can be difficult to determine if a snow events. Therefore, storm events are treated more specific extreme event is attributable to the overall climate detailed than most other extreme conditions in this re- change. Often, available observational records are not port. This report is also meant to support informed risk long enough to statistically attribute single events based management and development of communication strate- on observations only. Methods for local attribution, in- gies by stakeholders. volving climate models, have been further developed and Questions relevant for stakeholders are: quasi-operational services for event attribution are on the nnWhat kind of change in extreme conditions can we way to become established. As an example, recent cases expect from global warming? of extreme events have been analysed at https://www. nnWill certain types of extreme conditions become more worldweatherattribution.org/. frequent? An example relevant to Sweden is the extremely dry nnWill extreme events under recent climate conditions period in Northern Europe during spring and summer become more or less intense? 2018, with all-time-high temperatures at many locations nnHow do changing extreme conditions impact on the since the start of observational records. Vogel et al. (2019) stakeholders sector? conclude that it is virtually certain (using IPCC calibrated nnHow does impact and vulnerability translate into best uncertainty language) that the 2018 northern hemisphere measures for adaptation to and mitigation of climate concurrent heat events would not have occurred without change? human-induced climate change. In the near future it might be possible to offer rapid The synthesis report is expected to: assessments of global warming’s connection to particular nnProvide Swedish stakeholders from local to national meteorological events with full public reports several level with knowledge on climate extremes to facilitate weeks after an event (Schiermeier et al., 2018). estimation of the impact potentials. The knowledge about climate and weather extremes, nnFacilitate communication of climate extremes in recent its underlying mechanisms, trends and attribution is and future climate. highly relevant in order to consider the risks and vulnera- nnFacilitate translation of changing climate extremes to bility for most societal sectors to changing extremes. This sectoral risk management and resilience analyses, and knowledge synthesis compiles and communicates a con- associated options for climate adaptation and mitigation. densed and balanced exposition of scattered available nnProvide examples of helpful best practises for communi- information concerning recent and possible future clima- cation of knowledge on climate extremes. te extremes in Sweden, serving growing societal expecta- nnIdentify knowledge gaps and research needs related to tions on climate adaptation. Information sources are climate extremes to support capability development for drawn from the Nordic region and climatologically similar adapting to and mitigating climate change impacts with regions, as well as from Europe and the rest of the world maximum efficacy. where applicable. Before this report, climate extremes and their impacts Communication of scientifically founded knowledge have been described both from a natural science perspec- about climate extremes needs a strategy that depends on tive and from an impact and risk management perspective. the expected impacts themselves, and on stakeholders’ Several assessments including coverage of climate extre- perception of risks. Therefore, interaction between provi- mes exists, some of them have a global scope or provide ders and recipients of information is necessarily a partici- general information (e.g. Field et al., 2012; Pachauri et al., patory process and is an integral part of this report. 2014), others have a more continental perspective (e.g. Climate extremes are an important topic also in con- AMAP, 2011; Hov et al., 2013; BACC, 2015; EEA et al. junction with the Paris Climate Accord. Mitigation mea- 2017), while a few have a Swedish focus (Holgersson et al., sures need to be planned rather quickly, which requires 2007; Kjellström et al., 2014). However, with the rapid knowledge on climate extremes themselves and how they advancement of scientific knowledge from numerous depend on connected decision options for climate ac- climate research projects, and as a consequence of recent