key: cord-0542822-ars6espx
authors: Marx, Werner; Haunschild, Robin; Bornmann, Lutz
title: Heat Waves -- a hot topic in climate change research
date: 2021-06-25
journal: nan
DOI: nan
sha: 439f8c80cb83d5847f1e486b4c2d474f206d27ca
doc_id: 542822
cord_uid: ars6espx

Research on heat waves (periods of excessively hot weather, which may be accompanied by high humidity) is a newly emerging research topic within the field of climate change research with high relevance for the whole of society. In this study, we analyzed the rapidly growing scientific literature dealing with heat waves. No summarizing overview has been published on this literature hitherto. We developed a suitable search query to retrieve the relevant literature covered by the Web of Science (WoS) as complete as possible and to exclude irrelevant literature (n = 8,011 papers). The time-evolution of the publications shows that research dealing with heat waves is a highly dynamic research topic, doubling within about 5 years. An analysis of the thematic content reveals the most severe heat wave events within the recent decades (1995 and 2003), the cities and countries/regions affected (United States, Europe, and Australia), and the ecological and medical impacts (drought, urban heat islands, excess hospital admissions, and mortality). Risk estimation and future strategies for adaptation to hot weather are major political issues. We identified 104 citation classics which include fundamental early works of research on heat waves and more recent works (which are characterized by a relatively strong connection to climate change).

As a consequence of the well-documented phenomenon of global warming, climate change has become a major research field in the natural and medical sciences, and more recently also in the social and political sciences. The scientific community has contributed extensively to a comprehensive understanding of the earth's climate system, providing various data and projections on the future climate as well as on the effects and risks of anticipated global warming (IPCC 2014; CSSR 2017; NCA4 2018; and the multitude of references cited therein).

During recent decades, climate change has also become a major political, economic, and environmental issue and a central theme in political and public debates.

One consequence of global warming is the increase of extreme weather events such as heat waves, droughts, floods, cyclones, and wildfires. Some severe heat waves occurring within the last few decades made heat waves a hot topic in climate change research, with "hot" having a dual meaning: high temperature and high scientific activity. "More intense, more frequent, and longer lasting heat waves in the 21 st century" is the title of a highly cited paper published 2004 in Science (Meehl and Tebaldi 2004) . This title summarizes in short what most climate researchers anticipate for the future. But what are heat waves (formerly also referred to as "heatwaves")? In general, a heat wave is a period of excessively hot weather, which may be accompanied by high humidity. Since heat waves vary according to region, there is no universal definition, but only definitions relative to the usual weather in the area and relative to normal temperatures for the season. The World Meteorological Organization (WMO) defines a heat wave as five or more consecutive days of prolonged heat in which the daily maximum temperature is higher than the average maximum temperature by 5 °C (9 °F) or more (https://www.britannica.com/science/heat-wave-meteorology).

Europe, for example, has suffered from a series of intense heat waves since the beginning of Until the end of the 20 th century, heat waves were predominantly seen as a recurrent meteorological fact with major attention to drought, being almost independent from human activities and unpredictable like earthquakes. However, since about 1950, distinct changes in extreme climate and weather events have been increasingly observed. Meanwhile, climate change research has revealed that these changes are clearly linked to the human influence on the content of greenhouse gases in the earth's atmosphere. Climate-related extremes, such as heat waves, droughts, floods, cyclones, and wildfires, reveal significant vulnerability to climate change as a result of global warming.

In recent years, research on heat waves has been established as an emerging research topic within the large field of current climate change research. Bibliometric analyses are very suitable in order to have a systematic and quantitative overview of the literature that can be assigned to an emerging topic such as research dealing with heat waves (e.g., Haunschild et al. 2016) . No summarizing overview on the entire body of heat wave literature has been published until now. However, a bibliometric analysis of research on urban heat islands as a more specific topic in connection with heat waves has been performed (Huang and Lu 2018) .

In this study, we analyzed the publications dealing with heat waves using appropriate bibliometric methods and tools. First, we determined the amount and time-evolution of the scientific literature dealing with heat waves. The countries contributing the most papers are presented. Second, we analyzed the thematic content of the publications via keywords assigned by the WoS. Third, we identified the most important (influential) publications (and also the historical roots). We identified 104 citation classics, which include fundamental early works and more recent works with a stronger connection to climate change.

The status of the current knowledge on climate change is summarized in the Synthesis Report of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) (IPCC 2014, https://www.ipcc.ch/report/ar5/syr/). This panel is the United Nations body for assessing the science related to climate change. The Synthesis Report is based on the reports of the three IPCC Working Groups, including relevant Special Reports.

In its Summary for Policymakers it provides an integrated view of climate change as the final part of the Fifth Assessment Report (IPCC 2014, https://www.ipcc.ch/site/assets/uploads/2018/02/AR5_SYR_FINAL_SPM.pdf).

In the chapter Extreme Events, it is stated that "changes in many extreme weather and climate events have been observed since about 1950. Some of these changes have been linked to human influences, including a decrease in cold temperature extremes, an increase in warm temperature extremes, an increase in extreme high sea levels and an increase in the number of heavy precipitation events in a number of regions … It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia and Australia. It is very likely that human influence has contributed to the observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20 th century. It is likely that human influence has more than doubled the probability of occurrence of heat waves in some locations" (p. 7-8). Under Projected Changes, the document summarizes as follows: "Surface temperature is projected to rise over the 21 st century under all assessed emission scenarios. It is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions" (p. 10). In summary, climate change research expects more frequent and more severe heat wave events as a consequence of global warming. It is likely that the more frequent and longer lasting heat waves will significantly increase excess mortality, particularly in urban regions with high air pollution. Therefore, research around heat waves will become increasingly important and is much more than a temporary research fashion. We applied the search query given in the appendix to cover the relevant literature as completely as possible and to exclude irrelevant literature. We practiced an iterative query optimization by identifying and excluding the WoS subject categories with most of the nonrelevant papers. For example, heat waves are also mentioned in the field of materials science but have nothing to do with climate and weather phenomena. Unfortunately, WoS obviously assigned some heat wave papers related to climate to materials science related subject categories. Therefore, these subject categories were not excluded. By excluding the other non-relevant subject categories, 597 out of 8,568 papers have been removed, resulting in a preliminary publication set of 7,971 papers (#2 of the search query). But this is no safe method, since the excluded categories may well include some relevant papers.

Therefore, we have combined these 597 papers with search terms related to climate or weather and retrieved 62 relevant papers in addition, which we added to our preliminary paper subset, eventually receiving 8,033 publications (#3 to #5 of the search query).

Commonly, publication sets for bibliometric analyses are limited to articles, reviews, and conference proceedings as the most relevant document types and are restricted to complete publication years. In this study, however, we have included all relevant WoS document types The final publication set of 8,011 papers dealing with heat waves still contains some nonrelevant papers primarily published during the first half of the 20 th century, such as some Nature papers within the WoS category Multidisciplinary Sciences. Since these papers are assigned only to this broad subject category and have no abstracts and no keywords included, they cannot be excluded using the WoS search and refinement functions. We do not expect any bias through these papers, because their keywords do not appear in our maps. Also, they normally contain very few (if any) cited references, which could bias/impact our reference analysis.

We used the VOSviewer software package (Van Eck and Waltman 2014) to map coauthorship with regard to the countries of authors (88 countries considered) of the papers dealing with heat waves (www.vosviewer.com). The map of the cooperating countries presented is based on the number of joint publications. The distance between two nodes is proportionate to the amount of co-authorship. Hence, largely cooperating countries are positioned closer to each other. The size of the nodes is proportionate to the number of papers published by authors of the specific countries.

The method that we used for revealing the thematic content of the publication set retrieved from the WoS is based on the analysis of keywords. For better standardization, we chose the keywords allocated by the database producer (keywords plus) rather than the author keywords. We also used the VOSviewer for mapping the thematic content of the 104 key papers selected by reference analysis. This map is also based on keywords plus.

The term maps (keywords plus) are based on co-occurrence for positioning the nodes on the maps. The distance between two nodes is proportionate to the co-occurrence of the terms.

The size of the nodes is proportionate to the number of papers with a specific keyword. The nodes on the map are assigned by VOSviewer to clusters based on a specific cluster algorithm (the clusters are highlighted in different colors). These clusters identify closely related (frequently co-occurring) nodes, where each node is assigned to only one cluster. 

In this study, we have considered all WoS document types for a preferably comprehensive coverage of the literature of the research topic analyzed. The vast majority of the papers of our publication set, however, has been assigned to the document types "article" (n=6.738, 84.1 %), "proceedings paper" (n=485, 6.1 %), and "review" (n=395 papers, 4.9 %). Note that some papers belong to more than one document type.

In Fig. 1 

In Table 1 The country-specific percentages from Table 1 Table 1 . Countries with large deviations between heat wave output and overall WoS output are labeled. Countries with an absolute percentage of more than two percentage points higher (lower) in heat wave research than in overall WoS output are marked blue (red). Countries with a relative percentage at least twice as high (at most half as much) in heat wave research than in overall WoS output are marked green (yellow). Table 1 , the United States are most productive in heat wave research. This is not unexpected, because the US publication output is at the top for most research fields. However, this aside, the United States have been heavily affected by heat wave events and are leading with regard to the emergence of the topic. Australia appears as another major player and is strongly connected with the US publications within the co-authorship network and thus appears as a large node near the US node in the map.

Next, the leading European countries England, France, Germany, Italy, and Spain appear. In order to compare the thematic content of the complete publication set with the earlier literature on heat waves, we have analyzed the pre-2000 publications (n=297) separately. Major nodes in Fig. 5 are: heat waves (searched), temperature, United States, mortality, with climate change appearing only as a smaller node here. Obviously, the connection between heat waves and climate change was not yet pronounced, which can also be seen from Fig. 1 .

Compared with Fig. 4 , the thematic content of the clusters is less clear and the clusters presented in Fig. 5 can hardly be assigned to specific research areas. For a better overview of the thematic content of the early publications dealing with heat waves, we have assigned the connected keywords of Fig. 5 for better resolving the historical roots. Fig. 8 shows the spectrogram for the period 2000-2020, comprising the cited references from the bulk of the publication set analyzed.

The gray bars (Fig. 6 ) and red lines (Figs. 7-8) in the graphs visualize the number of cited references per reference publication year. In order to identify those publication years with significantly more cited references than other years, the (absolute) deviation of the number of cited references in each year from the median of the number of cited references in the two previous, the current, and the two following years (t-2; t-1; t; t+1; t+2) is also visualized (blue lines). This deviation from the five-year median provides a curve smoother than the one in terms of absolute numbers. We inspected both curves for the identification of the peak papers.

We use the number of cited references (N_CR) as a measure of the citation impact within the topic specific literature of our publication set. Which papers are most important for the scientific community performing research on heat waves? N_CR should not be confused with the overall number of citations of the papers as given by the WoS citation counts (times cited). These citation counts are based on all citing papers covered by the complete database (rather than a topic-specific publication set) and are usually much higher. papers were exported and their keywords (keywords plus) were displayed in Fig. 9 for revealing the thematic content of the key papers from the RPYS analysis at a glance. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the clusters via the following URL: https://tinyurl.com/4vwpc4t2

Overall, the keywords mapped in Fig. 9 are rather similar to the keywords presented in Fig.   4 . Besides climate change, temperature, weather, and air-pollution, the keywords deaths, health, mortality, and United-States appear as the most pronounced terms.

The key papers presented in Table B1 can be categorized as follows: (1) papers dealing with specific heat wave events, (2) the impact of heat waves on human health, (3) heat wave related excess mortality and implications for prevention, (4) the interaction between air pollution and high temperature, (5) circulation pattern and the meteorological basis, (6) future perspectives and risks, (7) climate models, indicators, and statistics.

Today, the hypothesis of a human-induced climate change is no longer abstract but has be an increasingly important issue in the future, when more and larger urban areas are affected by heat waves beyond the limit of human survival.

The importance of heat waves for the medical area is underlined by the large portion of papers discussing excess hospital admissions and excess mortality during intense heat wave events, particularly in urban areas with a high population density. As was the case during the boom phase of the Covid-19 pandemic, local medical health care systems may become overstressed by long-lasting heat wave events and thus adaptation strategies are presented and discussed. Finally, the analysis of the keywords in this study reveals the connection of heat wave events with air pollution in urban regions. There seems to be evidence of an interaction between air pollution and high temperatures in the causation of excess mortality (e.g., Katsouyanni et al. 1993) . Two more recent papers discuss the global risk of deadly heat The results of this study should be interpreted in terms of its limitations:

(1) We tried to include every relevant heat wave paper in our bibliometric analyses. Our long-standing experience in professional information retrieval has shown, however, that it is Two other limitations of this study also refer to the RPYS of the heat wave paper set:

(2) There are numerous rather highly cited references retrieved by RPYS via CRExplorer but not considered in the listing of ( 1984 OR 1967 OR 1983 OR 1966 OR 1982 OR 1965 OR 1999 OR 1981 OR 1964 OR 1998 OR 1963 OR 1979 OR 1962 OR 1978 OR 1961 OR 1977 OR 1959 OR 1994 OR 1976 OR 1954 OR 1993 OR 1975 OR 1949 OR 1992 OR 1974 OR 1940 OR 1991 OR 1973 OR 1938 OR 1990 OR 1972 OR 1930 OR 1989 OR 1971 OR 1926 OR 1988 OR 1970 OR 1914 OR 1987 OR 1969 OR 1912 OR 1986 OR 1968 OR 1906 OR 1985 Indexes=SCI ("global temperature*" OR "global warm*" OR "greenhouse effect" OR "greenhouse gas*" OR "greenhouse warm*") Indexes=SCI-EXPANDED, SSCI, A&HCI, CPCI-S, CPCI-SSH, BKCI-S, BKCI-SSH, ESCI Timespan=All years # 7 331,8 78 TOPIC: ("climate chang*" OR "climatic chang*" OR "climate varia*" OR "climatic varia*" OR "climate warm*" OR "climatic warm*") 

Weather-related mortality how heat, cold, and heat waves affect mortality in the United States

Heat waves in the United States: Mortality risk during heat waves and effect modification by heat wave characteristics in 43

The wisdom of citing scientists

Exceptional atmospheric circulation during the "Dust Bowl

Dramatically increasing chance of extremely hot summers since the 2003 European heatwave

Detecting seminal research contributions to the development and use of the global positioning system by Reference Publication Year Spectroscopy

The weakening summer circulation in the Northern Hemisphere mid-latitudes

Excess mortality related to the August 2003 heat wave in France

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Deadly heat waves projected in the densely populated agricultural regions of South Asia

Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team

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North China Plain threatened by deadly heatwaves due to climate change and irrigation

Evidence for interaction between air-pollution and high-temperature in the causation of excess mortality

The 2006 California heat wave: Impacts on hospitalizations and emergency department visits

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More intense, more frequent, and longer lasting heat waves in the 21 st century

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Longer and more frequent marine heatwaves over the past century

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Excess hospital admissions during the July 1995 heat wave in Chicago

Marine heatwaves threaten global biodiversity and the provision of ecosystem services

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City size and urban heat island 10

The assessment of sultriness. Part I: A temperature-humidity index based on human physiology and clothing science

Photosynthetic response and adaptation to temperature in higher

The energetic basis of the urban heat island

Morbidity and mortality associated with the

Risk-factors for heat-stroke -A case-control study

A universal scale of apparent temperature 10

Extreme high-temperature events -Changes in their probabilities with changes in mean temperature

An evaluation of summer discomfort in the United-States using a relative climatological index

Increased platelet and red-cell counts, blood-viscosity, and plasma-cholesterol levels during heat stress

Thermal comfort of man in different urban environments 10.1007/BF00866252 1989 87* Kalkstein, L.S. Weather and human mortality -An evaluation of demographic and interregional responses in the United-States

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Evidence for interaction between air-pollution and hightemperature in the causation of excess mortality

Temperature, ambient ozone levels, and mortality during summer

Heat-related deaths during the

The NCEP/NCAR 40-year reanalysis project

Impacts and responses to the 1995 heat wave: A call to action

heat wave in the midwest: A climatic perspective and critical weather factors

The Philadelphia hot weather-health watch warning system: Development and application, summer

Mortality in Chicago attributed to the

An evaluation of climate/mortality relationships in large US cities and the possible impacts of a climate change

Chicago heat wave: How likely is a recurrence?

Radiative transfer for inhomogeneous atmospheres: RRTM, a validated correlated-k model for the longwave 10

Cold exposure and winter mortality from ischaemic heart disease, cerebrovascular disease, respiratory disease, and all causes in warm and cold regions of Europe Not available

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Mortality as a function of temperature. A study in Valencia, Spain, 1991-1993 , C. Excess mortality in England and Wales, and in Greater London, during the 1995 heatwave

Near-fatal heat stroke during the 1995 heat wave in Chicago

A comparative analysis of heat waves and associated mortality in

Increased summertime heat stress in the US

Excess hospital admissions during the

The physiological equivalent temperature -A universal index for the biometeorological assessment of the thermal environment 10

Applications of a universal thermal index: physiological equivalent temperature

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On the definition of a heat wave 10

Relation between elevated ambient temperature and mortality: A review of the epidemiologic evidence 10

The role of increasing temperature variability in European summer heatwaves

Human contribution to the European heatwave of

heat wave in Europe: A shape of things to come? An analysis based on Swiss climatological data and model simulations 10

European seasonal and annual temperature variability, trends, and extremes since 1500

Contrasting patterns of mortality and hospital admissions during hot weather and heat waves in Greater London

Europe-wide reduction in primary productivity caused by the heat and drought in

Impact of regional climate change on human health

Epidemiologic study of mortality during the Summer

Excess mortality related to the

Global observed changes in daily climate extremes of temperature and precipitation 10

Land-atmosphere coupling and climate change in Europe

heat wave in France: Risk factors for death of elderly people living at home 10

Impact of high temperatures on mortality -Is there an added heat wave effect?

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Doubled length of western European summer heat waves since 1880 10

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Heat effects on mortality in 15 European cities 10

Climate change and extreme heat events 10

Has the impact of heat waves on mortality changed in France since the European heat wave of summer 2003? A study of the

Weather-related mortality: How heat, cold, and heat waves affect mortality in the United States

High ambient temperature and mortality: A review of epidemiologic studies from

California heat wave: Impacts on hospitalizations and emergency department visits

The impact of heat waves on mortality in 9 European cities: results from the EuroHEAT project 10

Investigating soil moisture-climate interactions in a changing climate: A review 10

A review of the European summer heat wave of

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The hot summer of 2010: Redrawing the temperature record map of Europe

Heat waves in the United States: Mortality risk during heat waves and effect modification by heat wave characteristics in 43 U

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The Impact of Heat Waves on Mortality

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On the measurement of heat waves 10

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Mega-heatwave temperatures due to combined soil desiccation and atmospheric heat accumulation 10

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A review on the scientific understanding of heatwaves -Their measurement, driving mechanisms, and changes at the global scale 10

Top ten European heatwaves since 1950 and their occurrence in the coming decades 10

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Dramatically increasing chance of extremely hot summers since the

A hierarchical approach to defining marine heatwaves

Climate-driven regime shift of a temperate marine ecosystem 10

Multi-year persistence of the 2014/15 North Pacific marine heatwave 10

Global risk of deadly heat 10

Global warming and recurrent mass bleaching of corals 10

Longer and more frequent marine heatwaves over the past century 10

Marine heatwaves under global warming

Marine heatwaves threaten global biodiversity and the provision of ecosystem services 10

*: N_TOP10 >9, the N_TOP10 indicator is the number of reference publication years in which a focal cited reference belongs to the 10% most referenced publications