With the rise in temperature due to anthropogenic climate change, the occurrence of hot summers, temperature extremes and heat waves is increasing globally. Projections for the coming decades to century indicate increases in the occurrence, magnitude and duration of these events. In a recent paper, Mueller et al (2016 Environ. Res. Lett. 11 044011) showed that half of summers are expected to be 'hot' (warmer than the warmest on record) across much of the world in one or two decades. While these results are consistent with earlier work, what is new here includes (i) an earlier timing of emergence of the hot summer signal and (ii) additional confidence due to the rigorous statistical examination of the observations and the analyses of the latest improved suite of model experiments. The potential impacts of these projections on society are extremely serious

Camargo, Suzana J.

Seth, Anji

Environmental Research Letters

English

Abstract
               With the rise in temperature due to anthropogenic climate change, the occurrence of hot summers, temperature extremes and heat waves is increasing globally. Projections for the coming decades to century indicate increases in the occurrence, magnitude and duration of these events. In a recent paper, Mueller et al (2016 Environ. Res. Lett. 
                  
                     11 044011) showed that half of summers are expected to be ‘hot’ (warmer than the warmest on record) across much of the world in one or two decades. While these results are consistent with earlier work, what is new here includes (i) an earlier timing of emergence of the hot summer signal and (ii) additional confidence due to the rigorous statistical examination of the observations and the analyses of the latest improved suite of model experiments. The potential impacts of these projections on society are extremely serious.</jats:p

Suzana J Camargo

Anji Seth

Crossref

Hottest summers the new normal

With the rise in temperature due to anthropogenic climate change, the occurrence of hot summers, temperature extremes and heat waves is increasing globally. Projections for the coming decades to century indicate increases in the occurrence, magnitude and duration of these events. In a recent paper, Mueller et al (2016 Environ. Res. Lett. http://dx.doi.org/10.1088/1748-9326/11/4/044011 11 http://dx.doi.org/10.1088/1748-9326/11/4/044011 ) showed that half of summers are expected to be ‘hot’ (warmer than the warmest on record) across much of the world in one or two decades. While these results are consistent with earlier work, what is new here includes (i) an earlier timing of emergence of the hot summer signal and (ii) additional confidence due to the rigorous statistical examination of the observations and the analyses of the latest improved suite of model experiments. The potential impacts of these projections on society are extremely serious

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Environ. Res. Lett. 11 (2016) 081001 doi:10.1088/1748-9326/11/8/081001PERSPECTIVEHottest summers the new normalSuzana JCamargo1 andAnji Seth21 Lamont-Doherty EarthObservatory, ColumbiaUniversity, NewYork, NY,USA2 University of Connecticut, Storrs, CT,USAKeywords: summer temperatures, heat waves, climate change, detection and attribution, impactsAbstractWith the rise in temperature due to anthropogenic climate change, the occurrence of hot summers,temperature extremes and heat waves is increasing globally. Projections for the coming decades tocentury indicate increases in the occurrence,magnitude and duration of these events. In a recentpaper,Mueller et al (2016Environ. Res. Lett. 11 044011) showed that half of summers are expected tobe ‘hot’ (warmer than thewarmest on record) acrossmuch of theworld in one or two decades.Whilethese results are consistent with earlier work, what is newhere includes (i) an earlier timing ofemergence of the hot summer signal and (ii) additional confidence due to the rigorous statisticalexamination of the observations and the analyses of the latest improved suite ofmodel experiments.The potential impacts of these projections on society are extremely serious.Among all the consequences of global climate change,the rise in temperature is the most well known,understood and discussed. However, the potentialimpacts of global warming require analysis beyondglobal mean temperature increase. A small shifttowards higher mean temperature, for example, canlead to increases in the occurrence of hot and extremehot temperatures. As the mean temperature shiftduring summer months becomes larger in the comingdecades, average summer temperatures can approachor exceed the threshold for what was considered ‘hot’in the past.Heat waves and hot summers have numeroussocio-economic impacts regionally. While the defini-tion of heat waves can vary, they are often implicatedin the hottest summers and their consequences arewell known [2]. In particular, health impacts in themost vulnerable populations are seen in hot summers,as well as in agriculture, power outages and wildfires[3]. The 2003 European heat wave, which led to thou-sands of deaths in western Europe, mainly in theelderly population, is one of the many examples ofimportant impacts of heat waves [4, 5]. It is often askedwhether these anomalously hot summers and heatwaves are already more common than they were in thepast, and also how frequently they might occur in thefuture.With a focus on food security, Battisti andNay-lor [6] examined shifts in the probability distributionsof model projected mean summer temperaturesglobally and found large areas in the tropics and sub-tropics where 50% of summers would be hotter thanthe hottest on record by mid-century, and more than90% of summers would be as such by end of the 21stcentury. A similar question is posed again recentlywith updated climate projections by Mueller and co-authors, who take further steps to show recentobserved hot summers in many regions are 10 timesmore likely to occur than without greenhouse gas for-cing. They also examine the fraction of human popu-lation to be affected by the increasing frequency of thehottest summers.Recent trends in global surface temperature areseasonally dependent with the largest regional con-tributor to global temperature trends over the past twodecades being land surface temperatures in the north-ern hemisphere extra-tropics, especially during sum-mer seasons [7]. On the other hand, there has been acontinued increase of hot extremes over land and thistrend is greater for the most extreme events [8]. Theseextreme heat events tend occur during heat waves,especially in urban areas [9]. While heat waves last afew days, the increase in their frequency and durationcan be linked to the higher occurrence of the hottestsummers. By the end of the 21st century climatemodel projections suggest increases in the frequency,magnitude and duration of heat waves [10]. Thesechanges are a direct consequence of the increase in sea-sonal mean temperatures. The expected increase inOPEN ACCESSPUBLISHED1August 2016Original content from thisworkmay be used underthe terms of the CreativeCommonsAttribution 3.0licence.Any further distribution ofthis workmustmaintainattribution to theauthor(s) and the title ofthework, journal citationandDOI.© 2016 IOPPublishing Ltdtemperature variability on intraseasonal to interannualtime-scales will also contribute to the changes in theoccurrence and characteristics of heatwaves (e.g. [11]).There is a high level of confidence in the attribu-tion of heat waves to anthropogenic climate change,and a good understanding of the role anthropogenicclimate change plays in heat waves, as discussed in therecently released National Academy of Sciences reporton extreme events [12]. Already a rich literature isdeveloping to attribute individual heat wave events toanthropogenic climate change, as exemplified by a dis-cussion of the western Russian heat waves of 2010using different methodologies [13], as well as heatwaves in various regions of the globe in the past fewyears in the annual special supplement on extremeevents of the Bulletin of the American MeteorologicalSociety (e.g. [14]). Furthermore, in almost all of the2014 heat wave events analyzed, an increase in the like-lihood of their occurrence was found in regions acrossthe world, including Australia, South Korea, Europe,China andArgentina [15].Temperature extremes are amplified by soil-moisture feedbacks in the model projections, leadingto the largest projected increases in the magnitude ofwarm extremes to be expected over mid-latitude con-tinental areas [16, 17]. The occurrence of heat waves isalso commonly associated with an atmospheric pat-tern known as blocking, in which a high-pressure sys-tem is locked in place for an extended period of time,allowing tropical air to move poleward and remainintact in the mid-latitudes. In a warming climate thequestion arises if atmospheric blocking will occurmoreoften. The answer to this question is still a topic ofdebate [18, 19], with some scientists arguing for anincrease of the blocking frequency related to Articamplification [20, 21]. Others, however, do notencounter evidence for this in the current model pro-jections [22, 23], or find a reduction of blocking fre-quency [24]. Further research into the physics ofblocking, as well its response to anthropogenic forcingis clearly necessary in order to clarify the role of block-ing inheatwaves and thehottest summers in the future.In their recent paper Mueller and co-authors [1]describe a worrisome picture for the world’s popula-tion regarding summer temperatures. In their analy-sis, hot summers are defined as summers with highermean temperatures than the historically hottest sum-mer over a region. Their results indicate that ‘withinthe next two decades, half of the world’s populationwill regularly (every second summer on average)experience regional summer mean temperatures thatexceed those of the historically hottest summer,’ evenunder a moderate emissions pathway. This earlyoccurrence of regular hot summers is a new andimportant result.Mueller and co-authors provide pro-jections of likelihood of hot summers in variousregions of the world using a statistical technique,which takes into account the models’ performanceduring the historical period constraining futureprojections. This statistical analysis is another impor-tant feature of their analysis, which is posing the ques-tions about hot summers on a more rigorous way.Their results suggest that the Sahara region will be thefirst to experience hot summers, with frequency ofnine out of ten summers within fifteen years, due tothe region’s strong warming trend and low naturalvariability. Eastern Asia is another region that will besoon affected, with one out of two summers being hotby 2020. Hot summers are projected to soon be normin the Mediterranean, large parts of Asia, as well aswestern US and Canada. In their analysis, under astronger emissions pathway, half of the populationwill be affected by hot summers in only a decade.Recently, the attribution of heat waves to anthro-pogenic warming has become widespread and fre-quent. Mueller and co-authors also show that hotsummers are now about ten timesmore likely inmanyregions of the world than they would have been in theabsence of greenhouse gases. They describe a world inwhichmost people will suffer under frequent hot sum-mers and that hot summers will become the norm fora large part of the world within one or two decades(depending on the emissions scenario). In the part-icular case of heath related deaths, Li and co-authors[25] showed that in the future hot summers could leadto a positive net increase of annual temperature-rela-ted deaths. There has been large observed trends inwildfires in the western US and continuing changes inclimate are expected to contribute to larger, more fre-quent fires in the region [26].While hot summers havesubstantial impacts on society, infrastructure and eco-systems, additional factors in a warming climate couldmoderate or further aggravate the effects of temper-ature increases. Human and ecosystem health, as wellas agriculture often depend on the combined effects ofhumidity and temperature [27]. Future research willneed to examine the combined influence of heat,humidity and wind changes on various impacts.Nevertheless, the results paint a grim picture and sug-gest greater urgency to both mitigate global warmingand to prepare for the coming heat.References[1] Mueller B, ZhangX andZwiers FW2016Historically hottestsummers projected to be the norm formore than half of theworld’s populationwithin 20 years Environ. Res. Lett. 11044011[2] MeehlGA andTebaldi C 2004More intense,more frequent,and longer lasting heat waves in the 21st century Science 305994–7[3] TrenberthKE 2012 Framing theway to relate climate extremesto climate changeClim. Change 115 283–90[4] Stott PA, StoneDA andAllenMR2004Human contributionto the European heatwave of 2003Nature 432 610–4[5] García-Herrera R,Díaz J, Trigo RM, Luterbacher J andFisher EM2010A review of the European summer heat waveof 2003Crit. Rev. Environ. Sci. Technol. 40 267–306[6] Battisti D S andNaylor R L 2009Historical warnings of futurefood insecurity with unprecedent seasonal heat Science 323240–42Environ. Res. Lett. 11 (2016) 081001 S J Camargo andA Seth[7] Cohen J L, Furtado JC, BarlowM,Alexeev VA andCherry J E2012Asymmetric seasonal temperature trendsGeophys. Res.Lett. 39 L04705[8] Seneviratne S I, DonatMG,Mueller B andAlexander L 2014Nopause in the increase of hot temperature extremesNat.Clim. Change 4 161–3[9] MishraV,Ganguly AR,Nijssen B and LettenmaierDP 2015Changes in observed climate extremes Environ. Res. Lett. 10024005[10] Stocker T F,QinD, Plattner G-K, TignorM, Allen SK,Doschung J,Nauels A, Xia Y, BexV andMidgley PM (ed) 2013Summary for policymakersClimate Change 2013: The PhysicalScience Basis. Contribution ofWorkingGroup I to the FifthAssessment Report of the Intergovernmental Panel on ClimateChange (Cambridge: CambridgeUniversity Press) pp 3–29[11] Vidale P L, LüthiD,WegmannR and ShärC 2007 Europeansummer climate variability in a hetereogeneousmulti-modelensembleClim. Change 81 209–32[12] National Academies of Sciences, Engineering andMedicine2016Attribution of Extreme Events in the Context of ClimateChange (Washington, DC: TheNational Academies Press)(doi:10.17226/21852)[13] Otto F E L,MasseyN, vanOldenborghG J, Jones RG andAllenMR2012Reconciling two approaches to attribution ofthe 2010Russian heat waveGeophys. Res. Lett. 39 L04702[14] Herring SC,HoerlingMP, Peterson TC and Stott PA 2014Explaining extreme events of 2013 from a climate perspectiveBull. Am.Meteor. Soc. 95 S1–96[15] Herring SC,Hoerling J P, Kossin J P, Peterson TC andStott PA 2015 Explaining extreme events of 2014 from aclimate perspectiveBull. Am.Meteor. Soc. 96 S1–72[16] DiffenbaughNS, Pal J S, Giorgi F andGaoX J 2007Heat stressintensification in theMediterranean climate change hotspotGeophys. Res. Lett. 34 L11706[17] KharinVV, Zwiers FW, ZhangXB andHegerl GC 2007Changes in temperature and precipitation extremes in theIPCC ensemble of global coupled simulations J. Clim. 201417–44[18] Hoskins B andWoolings T 2015 Persistent extratropicalregimes and climate extremesCurr. Clim. Change Rep. 1115–24[19] Barnes EA and Screen J 2015The impact of Arctic warming onthemidlatitude jet-stream: Can it?Has it?Will it?WIREs Clim.Change 41 638–44[20] Francis J A andVavrus S J 2012 Evidence linking Arcticamplification to extremeweather inmid-latitudeGeophys. Res.Lett. 39 L06801[21] Francis J A andVavrus S J 2015 Evidence for awavier jet streamin response to rapid arctic warming Environ. Res. Lett. 10014005[22] Barnes EA,Dunn-Sigouin E,MasatoG andWoolings T 2014Exploring recent trends in northern hemisphere blockingGeophys. Res. Lett. 41 638–44[23] Barnes EA and Polvani LM2015CMIP5 projections of Arcticamplification, of theNorthAmerican/NorthAtlanticcirculation, and of their relationship J. Clim. 28 5254–71[24] Hassanzadeh P,Kuang Z and Farrell B F 2014Responses ofmidlatitude blocks andwave amplitude to changes in themeridional temperature gradient in an idealized dryGCMGeophys. Res. Lett. 41 5223–32[25] Li J et al 2013 ElNiñomodulations over the past sevencenturiesNat. Clim. Change 3 822–6[26] Dennison P E, Brewer SC, Arnold JD andMoritzMA2014Largewildfire trends in thewesternUnited States, 1984–2011Geophys. Res. Lett. 41 2928–33[27] Fischer EMandKnutti R 2013Robust projections ofcombined humidity and temperature extremesNat. Clim.Change 3 126–303Environ. Res. Lett. 11 (2016) 081001 S J Camargo andA Seth

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