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Has the climate become more variable or extreme? Progress 1992-2006 Neville Nicholls and Lisa Alexander Progress in Physical Geography 2007 31: 77 DOI: 10.1177/0309133307073885 The online version of this article can be found at: http://ppg.sagepub.com/content/31/1/77

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Has the climate become more variable or extreme? Progress 1992–2006 Neville Nicholls* and Lisa Alexander School of Geography and Environmental Science, Building 11, Monash University, Victoria 3800, Australia Abstract: In 1990 and 1992 the Intergovernmental Panel on Climate Change (IPCC), in its first assessment of climate change and its supplement, did not consider whether extreme weather events had increased in frequency and/or intensity globally, because data were too sparse to make this a worthwhile exercise. In 1995 the IPCC, in its second assessment, did examine this question, but concluded that data and analyses of changes in extreme events were ‘not comprehensive’ and thus the question could not be answered with any confidence. Since then, concerted multinational efforts have been undertaken to collate, quality control, and analyse data on weather and climate extremes. A comprehensive examination of the question of whether extreme events have changed in frequency or intensity is now more feasible than it was 15 years ago. The processes that have led to this position are described, along with current understanding of possible changes in some extreme weather and climate events. Key words: climate change, frost, greenhouse effect, heat waves, weather extremes.

I Introduction – why are extremes important? Extreme weather and climate events can produce severe impacts on our society and environment. For instance, heat waves can be devastating for societies that are not used to coping with such extremes. More than 500 people died from heat-related illnesses in the 1995 Chicago heatwave (Karl and Knight, 1997). Over 30,000 deaths were attributable to the 2003 heatwave in Europe (about

15,000 deaths in France alone; IFRCRC, 2004; Poumadere et al., 2005) which also led to the destruction of large areas of forests by fire, and effects on water ecosystems and glaciers (Gruber et al., 2004; Koppe et al., 2004; Kovats et al., 2004; Schär and Jendritzky, 2004). European drought conditions during the summer of 2003 caused crop losses of around US$13 billion, while forest fires in Portugal were responsible for an additional US$1.6 billion in damage (Schär and

*Author for correspondence. Email: [email protected] © 2007 SAGE Publications

DOI: 10.1177/0309133307073885


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Progress in Physical Geography 31(1)

Jendritzky, 2004). A Brisbane, Australia, heatwave in January 2000 is believed to have contributed to the deaths of 77 elderly people, over 100 hospitalizations, and 280 heatstroke cases attended by ambulance officers (Bureau of Meteorology, 2006). A heatwave in February 2004 in the same area led to newspaper headlines such as ‘Hot spell hits with collapses’ (Sunday Mail, Adelaide, 8 February), ‘Taking the heat in state of distress’ (Daily Telegraph, Sydney, 12 February), and ‘Sweltering temperatures make school children sick’ (Queensland Times, 19 February). Brisbane recorded 41.7°C on the weekend of 21–22 February, exceeding the previous February record by nearly one degree. That weekend the Queensland ambulance service recorded ‘a 53% increase in ambulance callouts’, and the ambulance service Commissioner described it as ‘the most significant medical emergency in the south-east corner on record’ (Canberra Times, 24 February). Other extremes such as storms and flooding can also have large impacts. Shortly before Hurricane Katrina battered the southeast coast of the United States in August 2005 killing an estimated 1300 people, heavier than usual monsoon rains caused severe floods in Gujarat and subsequent flooding in the Maharashtra state in India caused similar loss of life. In the late 1980s and early 1990s, a series of stormy winters cost the UK insurance industry over US$6 billion (Alexander et al., 2005) and in the early 2000s economic damage through flooding in central Europe exceeded €15 billion (RMS, 2003). However, these disasters may pale into insignificance compared with expected insurance losses from the impact of Hurricane Katrina. This event was undoubtedly a factor when Insurers Lloyds of London reported a loss of US$180 million in 2005 citing ‘the worst year on record for natural disasters’ (BBC website, ‘Storms push Lloyds into the red’, 6 April 2006). Even a relatively small change in the mean of a variable may lead to substantial changes in the frequency of extremes. Extremes are

the infrequent events at the high and low end of the range of values of a particular variable. The probability of occurrence of values in this range is called a probability distribution function (pdf) that is, for many variables, shaped similarly to a ‘Normal’ or ‘Gaussian’ curve (the familiar ‘bell’ curve). Figure 1 illustrates the effect a small shift (corresponding to a small change in the average or location of the distribution) can have on the frequency of extremes at either end of the distribution. An increase in the frequency of one extreme (eg, the number of hot days) will often be accompanied by a decline in the opposite extreme (in this case the number of cold days such as frosts). Of course, changes in the variability or shape of the distribution can complicate this simple picture, but the figure shows that the number of very cold nights was reduced by more than 50% as the mean temperature increased by less than a degree. The large impacts climate extremes can have, and their tendency to change substantially in frequency with even small changes in average climate, mean that changes in extremes can be the first indication that climate is changing in a way that can affect humans and the environment substantially. On the other hand, problems with data and analyses of extremes can make it very difficult to determine whether or not they are changing. II How has our assessment of changes in extremes developed over the past 15 years? The various assessments by the Intergovernmental Panel on Climate Change (IPCC) provide an indication of progress over the past 15 years in the assessment of climate extremes and their changes. The 1992 Supplement Report to the Scientific Assessment of climate change from the (IPCC) concluded that global mean surface air temperature had increased by about 0.3 to 0.6°C over the past 100 years, but did not address the question of whether extremes in temperature, precipitation or severe


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Figure 1 Illustration of the effect of an increase in mean temperature on the risk of extremes. Broken (1957–1980) and full (1981–2005) lines show for Melbourne, Australia, the probability distribution function of daily minimum temperatures in winter. Vertical thin lines show mean minimum temperatures for the two periods. Vertical short thin line shows probability of extreme cold (⬍1°C) temperature. The increase in mean minimum temperature has led to a substantial decrease in the probability of temperatures ⬍1°C (mean number of days ⬍1°C was 1.7 per year in first period, declining to 0.7 per year in second period) circulation features such as tropical cyclones had changed (Folland et al., 1992). By 1995, the Second Assessment Report (SAR) of the IPCC was specifically addressing the question ‘Has the climate become more variable or extreme?’ (Nicholls et al., 1995). They concluded that: ‘Overall, there is no evidence that extreme weather events, or climate variability, has increased, in a global sense, through the 20th century, although data and analyses are poor and not comprehensive.’ The SAR noted that the data on climate extremes and variability available at that time were inadequate to say anything about recent

global changes, although in some regions where data are available there had been changes in extreme events. The SAR also concluded that we should expect ‘an increase in the occurrence of extremely hot days and a decrease in the occurrence of extremely cold days’, in the future (Houghton et al., 1995: 7). Nicholls (1996) observed that a major problem undermining our ability to determine whether extreme weather and climate events were changing was that it is more difficult to maintain the long-term homogeneity of observations required to observe changes in extremes, compared to monitoring changes in


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means of variables. Ambiguities in defining extreme events and difficulties in combining different analyses from different sites also complicated attempts to determine, on a global scale, whether extreme events are changing in frequency. A Workshop on weather and climate extremes was held in 1997 to examine what needed to be done to improve data sets and analyses for extreme weather monitoring (Karl et al., 1999), inspired by the inability of the IPCC SAR to determine how extreme events had been changing globally. The Workshop noted that the ‘first step in the detection/attribution of climate change is the assembly of high-quality time-series of key variables’. This Workshop led to a series of Workshops in Asia (Manton and Nicholls, 2000; Nicholls and Manton, 2005; Peterson, 2005; Klein Tank et al., 2006), the Caribbean region (Peterson et al., 2002), Africa (Easterling et al., 2003; Mokssit, 2003; New et al., 2006); South and Central America (Vincent et al., 2005; Aguilar et al., 2005; Haylock et al., 2006), and the Middle East (Zhang et al., 2005; Sensoy et al., 2006). These Workshops used a common approach to select high-quality stations, perform quality control, and investigate trends in extreme events (eg, Manton et al., 2001; Griffiths et al., 2005; Nicholls et al., 2005), and proposed increased efforts to rescue and digitize data stored on paper in many countries (Page et al., 2004). The collation and analyses of daily data sets has not been a simple task. One reason is that few countries have the capacity or mandate to freely distribute daily data. Another reason is that data need to undergo rigorous quality control before being used in any extremes analysis since values are likely to show up erroneously as extreme when incorrectly recorded. Since the IPCC Third Assessment (TAR) in 2001, the World Meteorological Organization (WMO) Expert Team on Climate Change Detection, Monitoring and Indices (ETCCDMI) has overseen the development of a standard soft-

ware package that not only quality controls data but provides researchers with the opportunity to exchange and compare results. The main purpose of the quality control procedure is to identify errors in data processing such as negative precipitation or daily minimum temperatures greater than daily maximum temperatures. In addition ‘outliers’ are identified in daily temperatures, ie, values outside a given number of standard deviations of the climatological mean value for that day. These can then be manually checked and removed or corrected as necessary. The software, RClimDex, developed by the Climate Research Branch of the Meteorological Service of Canada (http://cccma.seos.uvic. ca/ETCCDMI/software.html) also calculates a standard set of 27 extremes indices, derived from daily temperature and precipitation and chosen to reflect changes in intensity, frequency and duration of events. While the quality controlled daily data are rarely exchanged, there have been few obstacles to exchanging the climate extremes data, especially given that the software has been made freely available to the international research community. In addition to these simple quality control measures, perhaps an even more important aspect of the study of extremes is to remove inconsistencies or ‘inhomogeneities’ (that is artificial changes which cannot be explained by changes in climate) from the daily data prior to analysis. Inhomogeneities can be introduced into climate data, for example, by the relocation of an observing site to a more shaded or exposed location or the implementation of more accurate recording instrumentation. However, the identification, removal or indeed correction of these types of errors is extremely complex and difficult to do well (Aguilar et al., 2003). Recently there have been many statistical techniques put forward (eg, Wijngaard et al., 2003; Wang, 2003; Menne and Williams, 2005) to identify these step changes in climate series. The ETCCDMI have therefore also coordinated the development of other standard software,


Neville Nicholls and Lisa Alexander: Climate progress 1992–2006 RHTest, using the Wang (2003) methodology, which can be used in tandem with RClimDex. However, identifying potential problems is only the first step. On regional scales there has been some limited success in correcting daily temperatures (eg, Vincent et al., 2002) and precipitation (eg, Groisman and Rankova, 2001) for these step change errors, but globally, given the many different climate regimes, this task has proved too problematic and generally suspicious data have not been included in studies (Alexander et al., 2006). These and other multinational efforts to collate and quality control daily weather data meant that, by the time of the IPCC Third Assessment (TAR) in 2001, more could be said about how extreme weather events appeared to be changing. The IPCC TAR concluded (IPCC, 2001) that: • In the mid- and high latitudes of the Northern Hemisphere over the latter half of the twentieth century, it is likely that there had been a 2–4% increase in the frequency of heavy precipitation events. • Since 1950 it is very likely that there had been a reduction in the frequency of extreme low temperatures, with a smaller increase in the frequency of extreme high temperatures. • In some regions, such as parts of Asia and Africa, the frequency and intensity of droughts had been observed to increase in recent decades. • Changes globally in tropical and extratropical storm intensity and frequency were dominated by interdecadal to multidecadal variations, with no significant trends evident over the twentieth century. Conflicting analyses made it difficult to draw definitive conclusions about changes in storm activity, especially in the extratropics. • No systematic changes in the frequency of tornadoes, thunder days, or hail events were evident in the limited areas analysed. The TAR also concluded that it was likely or very likely that continued anthropogenic interference with the atmosphere would lead

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in the future to increased numbers of warm extremes, heavy rainfall events, tropical cyclone peak wind intensities, and droughts, and decreased numbers of cool extremes. The subsequent work of the ETCCDMI and the Alexander et al. (2006) study have allowed the scientific community to fill in some of the gaps identified in the IPCC TAR about observed changes in global extremes. There is now some information about trends in regions where previously there had been large data gaps, eg, Africa and South America. In the tropics, for example, temperature extremes have warmed at least as fast as other extra-tropical regions. Globally, there have been large-scale warming trends in the extremes of temperature, especially minimum temperature, and the evidence suggests that these trends have occurred since the beginning of the twentieth century. Alexander et al. (2006) found that over 70% of the global land area sampled showed a significant decrease in the annual occurrence of cold nights and a significant increase in the annual occurrence of warm nights. Some regions experienced a doubling (or halving) of the occurrence of these indices. This implies a positive shift in the distribution of daily minimum temperature throughout the globe. Daily maximum temperature indices showed similar changes but with smaller magnitudes. Recently, the first attempts to determine whether any changes in extremes could be attributed to human interference with the atmosphere have been reported. Allen (2003) and Stone and Allen (2005) proposed a methodology for making quantitative attribution statements about specific types of climatic events, by expressing the contribution of external forcing to the risk of an event exceeding the observed magnitude. These studies proposed that the concept of the fraction of attributable risk (FAR), an established concept in epidemiological studies, should be applied to the problem of attributing a change in risk of a specific type of event to external forcing. Stott et al. (2004) investigated to what extent climate change could be


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responsible for the high summer temperatures in Europe during the summer of 2003 by applying the FAR concept to mean summer temperatures of a large region of continental Europe and the Mediterranean. They concluded that it is very likely that human influence had more than doubled the risk of a regional scale heatwave of at least the 2003 severity. This was a study of a regional average of summer mean temperatures. The first attempts at attributing the causes of changes in extremes based on daily data (rather than extremes of seasonal means) have also been undertaken. Kiktev et al. (2003) found good evidence that only the inclusion of humaninduced forcings in a climate model could account for observed changes in global temperature extremes. Christidis et al. (2005) analysed a new gridded data set of daily temperature data (Caesar et al., 2006) and detected robust anthropogenic changes in indices of extremely warm nights, although with some indications that the model overestimates the observed warming of warm nights. Human influence on cold days and nights was also detected, although less convincingly. III Some other extremes and how they have changed The discussion thus far has mostly concentrated on trends in extreme temperatures. In this section, current assessments of what we know about changes in other weather/ climate extremes are discussed. For most of these extremes, the situation is less conclusive than is the case with extreme temperatures. In many cases (eg, tropical cyclones, droughts) definitional ambiguities or lack of consistent databases still provide barriers to an effective understanding of how these extremes have changed over time. 1 Tropical cyclones The numbers and proportion of tropical cyclones reaching categories 4 and 5 appear to have increased since 1970, while total numbers of cyclones and cyclone days

decreased slightly in most basins (Webster et al., 2005). However, data quality and coverage issues, particularly prior to the satellite era, means that there is low confidence, as yet, in this assessment. Nevertheless the numbers of strong tropical cyclones in the North Atlantic (the best observed basin) have been above normal (based on 1981–2000) in nine of the last 11 years, culminating in the record breaking 2005 season. Globally, estimates of the potential destructiveness of hurricanes show a substantial upward trend since the mid-1970s, with a trend toward longer lifetimes and greater storm intensity (Emanuel, 2005). Trends are also apparent in sea surface temperatures (SSTs) and other variables that appear to influence tropical storm development (Hoyos et al., 2006). 2 Droughts Droughts have been widespread in various parts of the world since the 1970s (Dai et al., 2004). Some regions where they have occurred seem to be determined largely by changes in SSTs, especially in Africa and western North America, and through changes in the atmospheric circulation and precipitation in central and southwest Asia. In Australia and Europe, direct relationships to global warming have been inferred through the extreme nature of high temperatures and heatwaves accompanying recent droughts (Nicholls, 2004). More generally, increased temperatures appear to have contributed to increased regions under drought. 3 Heavy rainfall events There appear to have been increases in the number of heavy precipitation events (eg, 95th percentile) across many land regions in the second half of the twentieth century, even in those where there has been a reduction in total precipitation amount. Increases have also been reported for rarer precipitation events (1 in 50-year return period), but only a few regions have sufficient data to assess such trends reliably. However, precipitation extremes do not exhibit as much spatial


Neville Nicholls and Lisa Alexander: Climate progress 1992–2006 coherence as do temperature extremes (Alexander et al., 2006), ie, there are areas where the frequency of heavy precipitation has decreased. In Europe, there is a clear majority of stations with increasing trends in the number of moderate and very wet days (defined as the exceedence of the 75% and 95% percentiles, respectively) during the second half of the twentieth century (Klein Tank and Können, 2003; Haylock and Goodess, 2004). Similarly, for the contiguous United States, Kunkel et al. (2003) and Groisman et al. (2004) found statistically significant increases in heavy (upper 5%) and very heavy (upper 1%) precipitation, by 14% and 20%, respectively. Much of this increase has occurred during the last three decades and it is most apparent over the eastern parts of the country. Also there is new evidence for Europe and the United States that the relative increase in precipitation extremes is larger than the increase in mean precipitation, and this is manifested as an increasing contribution of heavy events to total precipitation (Klein Tank and Können, 2003; Groisman et al., 2004). 4 Mid-latitude winds and synoptic systems Mid-latitude westerly winds appear to have increased in both hemispheres, and this change appears to be related to changes in the so-called ‘annular modes’ (defined as zonal mean pressure differences across midlatitudes, which is related to the zonally averaged mid-latitude westerlies) which have strengthened in most seasons from 1979 to the late 1990s, with poleward displacements of Atlantic and Southern Hemisphere polar front jetstreams. These changes have been accompanied by a tendency toward stronger wintertime polar vortices throughout the troposphere and lower stratosphere. There have been significant decreases in cyclone numbers, and increases in mean cyclone radius and depth, over the southern extra-tropics over the last two or three decades (Simmonds et al., 2003). Geng and Sugi

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(2001) find that cyclone density, deepening rate, central pressure gradient, and translation speed have all been increasing in the winter North Atlantic. However measures of ‘storminess’ over northern Europe (eg, Alexandersson et al., 2000; Bärring and von Storch, 2004) suggest that recent observed changes are not unusual in terms of long-term variability. 5 Small-scale weather extremes Evidence for changes in the number or intensity of tornadoes relies on local reports, and Brooks et al. (2003) and Trapp et al. (2005) question the completeness of the tornado record. In many European countries, the number of tornado reports has increased considerably over the last five years (Snow, 2003; Tyrrell, 2003) but it appears likely that the increase in reports is dominated (if not solely caused) by enhanced detection and reporting efficiency. Doswell et al. (2005) highlight the difficulties encountered when trying to find observational evidence for changes in extreme events on local scales connected to severe thunderstorms. In the light of the very strong spatial variability of small-scale severe weather phenomena, the density of surface meteorological observing stations is too coarse to measure all such events. Moreover, homogeneity of existing station series is questionable. While remote sensing techniques allow detection of thunderstorms even in remote areas, they do not always uniquely identify severe weather events from these storms. IV What needs to be done? All the possible trends in extremes discussed in the previous section are inconclusive because of concerns about the quality, comprehensiveness, and comparability of data over decades. This contrasts with the situation for extreme temperatures, where extensive work and international cooperation over the past 15 years has lead to a clear depiction of increasing warm extremes and decreasing cold extremes (and some studies now attribute


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these changes in extremes to human influences on the atmosphere). So, the 15 years of progress regarding monitoring extremes has led to a substantial improvement in our understanding of how extreme temperatures are changing (essentially a shift in the frequency distributions towards warmer values, with fewer cold extremes), but for all the other extremes (droughts, heavy rainfalls, cyclones, tornadoes) the data concerns overwhelm us, still. Is there a way forward? For some extremes the answer is, of course, yes. Tropical cyclones have been observed with satellites since before 1970. Although the satellite technology has changed, along with the methods used to determine the intensity of the systems, it should still be possible to examine the historical satellite pictures to determine whether, for instance, tropical cyclones in the mid-1970s were routinely analysed as moderate rather than intense. This effect would need to be very clear, if it was to explain away the substantial apparent trend towards more frequent intense cyclones (Webster et al., 2005). For smallscale events such as tornadoes, focusing on areas where tornadoes have been monitored for several decades, and where there has been a sufficient population to ensure that systems are not missed, might be the way forward (rather than relying on collating numbers of systems from all areas including those where tornado monitoring is a new concept). For some other extremes (most notably drought) the problem is more definitional – Dai et al. use the Palmer Drought Severity Index (PDSI) to examine changes in droughts. Is this the appropriate index, and how much of an apparent trend is due to the temperature term in this index? There remain many questions to be answered, but another 15 years of improvement as rapid as have been the advances in the past 15 will surely provide definitive answers to the search for trends in all the various extremes. As better data sets for the various extremes are developed and analysed, further work will be required to attribute the likely

causes of any trends in extremes. Thus far, relatively few studies aimed at the formal detection and attribution of the trends in extremes have been completed. Some studies (eg, Hoyos et al., 2006) have used rather simple approaches to detection and attribution, often because credible model simulations of the specific extreme are lacking, as much as problems with data. Christidis et al. (2005) is, thus far, the only formal detection and attribution study that has convincingly associated a trend in extremes with a human influence. Improved historical data sets, and adequate monitoring systems for extremes, can provide the basis for more comprehensive assessments of the likely causes of any trends in extremes. Acknowledgements The development of high-quality databases suitable for determining whether the climate is becoming more extreme has been a multinational effort, involving many people from many institutions, as can be seen from the author lists for the references cited in this paper. We thank the participants in the many workshops at which these databases were prepared, quality controlled, and analysed, and the various organizations that funded these workshops, for their contributions to this research. References Aguilar, E., Auer, I., Brunet, M., Peterson, T.C. and Wieringa, J. 2003: Guidelines on climate metadata and homogenization. WCDMP-no. 53, WMOTD No. 1186. Geneva: World Meteorological Organization, 55 pp. Aguilar, E., Peterson, T.C., Ramírez Obando, P., Frutos, R., Retana, J.A., Solera, M., González Santos, I., Araujo, R.M., Rosa Santos, A., Valle, V.E., Brunet India, M., Aguilar, L., Álvarez, L., Bautista, M., Castañón, C., Herrera, L., Ruano, E., Siani, J.J., Obed, F., Hernández Oviedo, G.I., Salgado, J.E., Vásquez, J.L., Baca, M., Gutiérrez, M., Centella, C., Espinosa, J., Martínez, D., Olmedo, B., Ojeda Espinoza, C.E., Haylock, M., Núnez, R., Benavides, H. and Mayorga, R. 2005: Changes in precipitation and temperature extremes in Central America and Northern South


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