TEMPERATURE AND HEAT WAVE TRENDS IN MEXICALI, MEXICO

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1 TEMPERATURE AND HEAT WAVE TRENDS IN MEXICALI, MEXICO Polioptro Martinez-Austria Carlos Villegas-Zeron Department of Civil and Environmental Engineering, Universidad de las Américas Puebla, Mexico. Abstract Increase in climate extremes is one of the main expected impacts of climate change, and one of the first signs of its occurrence. Nevertheless, the results emerging from General Circulation Models, while sufficient for large scales, are not enough for forecasting local trends and, hence, the IPCC has called for local studies based on on-site data. Indeed, it is expected that climate extremes will be detected much earlier than changes in climate averages. Heat waves are among the most important and least studied climate extremes, and their very definition is controversial. This paper discusses the concept of heat waves and their expected frequency changes due to global warming. Climate records in one of the cities with extreme climate in Mexico, the city of Mexicali in the state of Baja California, are analysed. Trends of increased temperature extremes and heat waves are found. 1. INTRODUCTION Climate change represents one of the major global challenges for human kind in the 21st Century. The global warming that will be produced, which in fact has already begun, will have major health, environmental, economic, and social effects. Modern societies are striving, without much success so far, to mitigate its effects by reducing the greenhouse gas emissions produced by our current civilization, which insatiably consumes oil, gas and carbon. The concentration of greenhouse gases continues to increase. According to the US National Oceanic and Atmospheric Administration (NOOA), in May 2014 it reached ppm in the Mauna Loa observatory ( The increase rate of these concentrations not only has not reduced, but rather continues to grow, reaching in 2013 a value of 2.62 ppm/year. According to studies by the Intergovernmental Panel on Climate Change (IPCC), the regions located in middle and high latitudes will experiment the highest temperature increases. In the course of this century, a mean global temperature increase is expected to occur which might reach up to 4 ºC (IPCC, 2013).

2 These predictions are based on the results of general circulation models. The IPCC has estimated the performance of these models by making a comparison of their results with the climate observed during the period. With respect to temperature, when multimodel results (the average of 23 general circulation models) are analyzed, the estimation error, i.e. the difference between that observed and the model, is rarely greater than 2 ºC, although individual models can show errors close to 3 ºC (Randall & Word, 2007). Nevertheless, the IPCC notes that "the largest-scale features of climate are simulated more accurately than regional- and smaller-scale features." The analysis of climate change impact and vulnerability should therefore be soundly based on observational evidence. Thus, the fourth and fifth IPCC Assessment Reports (IPCC 2007, 2013) are not based solely nor predominantly on the results of general circulation models, but also on observational evidence. However, the latter report notes that there is "a lack of geographic balance in data and literature on observed changes, with marked scarcity in low and middle income countries". Furthermore, the reported trends and the forecasts from general circulation models focus especially on average values (IPCC, 2008). For instance, there are average temperature forecasts for several scenarios, but there are no general forecasts for extreme temperatures. Nevertheless, the IPCC Fifth Assessment Report notes that it is very likely that the number of cold days and nights has decreased globally, and that probably the number of heat waves has increased in Europe and North America (IPCC, 2013). If forecasting mean temperature changes is a difficult task, forecasting extreme temperatures is much more difficult, since it depends almost entirely on observational evidence. Therefore, the analysis of the occurrence of heat waves is relevant, especially in vulnerable areas, such as the Mexican Northwest, where the city of Mexicali, chosen for this study, is located. 2. Heat Waves A heat wave can be defined qualitatively as a period of time, usually lasting several days, of temperatures significantly higher than average. Their importance lies mainly in their effects on human health, producing disorders that cause minor alterations in or even the collapse of the body's capacity for regulating its temperature by means of changes in blood circulation or sweating. In extreme cases, health effects can lead to death. The elderly and small children are particularly vulnerable to heat waves. In the last few decades there have been records of particularly dangerous heat waves that have caused several deaths, even in developed countries with good public health services. Worth noting are the heat wave that hit Chicago in 1995, causing 514 heat-related deaths (Whitman et al, 1997) and the heat wave of 2003 in Europe, which affected mainly France and caused almost 15,000 deaths ( Hémon & Jougla, 2003, cited by Le Tertre et al, 2006). There are different thresholds to determine the presence of a heat wave. For instance, the UK Met Office establishes a regional system with an average threshold of 30 C by day and 15 C overnight

3 for more than two consecutive days. ( The NOOA establishes thresholds considering maximum temperatures and ambient humidity, which comes closer to the measurement of apparent temperature. In Mexico, in recent studies, López Díaz recommended for the city of Veracruz a temperature threshold over the 96th percentile, meaning that that case surpasses the threshold of 34 ºC in at least three consecutive days. Another definition, suggested by Campbell (2009) from the World Meteorological Organization, which has no direct association with health effects, establishes it as the condition in which temperatures higher than the mean maximum temperature of the area by 5 ºC (9 ºF) are recorded for five or more consecutive days (Campell, 2009). For the city of Mexicali, (García-Cueto, Santillán-Soto, Ojeda-Benitez, & Quintero Nuñez, 2012), a study was made of temperature trends in the summer (June September) and in the winter (November February), adjusting the data to different probability distributions. The study used a temperature threshold of the 95th decile. 3. HEAT WAVES IN MEXICALI, BAJA CALIFORNIA The city of Mexicali, located in the state of Baja California, bordering with the United States, has inhabitants. Station 2033 of the National Weather Service is located there and was used for this study. Mexicali is one of the hottest cities in Mexico, for its population size; therefore, the study of the occurrence of temperature maximums is of special importance. Figure 1 shows monthly mean maximum temperatures recorded in Mexicali for the January 1960 May 2011 period. As can be observed, the hottest months are July and August. Figure 1: Monthly mean maximum temperatures in Mexicali for the period.

4 Figures 2 and 3 show the number of days with temperatures exceeding the monthly mean maximum temperature in the period, in July and August respectively, which are the hottest of every year. Both of them show a clear ascending trend. This means that the number of days with extreme temperatures has increased over time. Figure 2: Number of days exceeding the monthly mean maximum temperature. July. Figure 3: Number of days exceeding the monthly mean maximum temperature. August. This trend towards the increase of extreme maximum temperatures with respect to the monthly mean maximum values in the months of July and August in Mexicali is corroborated by analyzing the absolute maximum temperatures recorded every year, as shown in Figure 4.

5 Figure 4: Maximum temperatures recorded in Mexicali in the period. In order to observe this trend with greater clarity, a statistic trend test can be applied. For this paper, the Sen trend test (Sen, 2012) was used, giving as a result the graph in Figure 5, which clearly shows a trend in temperature increase. Figure 5: Maximum temperature trend in Mexicali. Sen Method (Sen, 2012). If a heat wave is defined as more than five days of temperatures 5 ºC higher than the mean maximum temperature of the corresponding month, those shown in Table 1 will be found. It is worth noting that most of the heat waves occur in the coldest months, which is consistent with

6 what is observed in other similar regions where not only summers are hotter, but also winters are increasingly less cold and consistent also with the results for the winter period in the study by García-Cueto et al (2012). Month-year of occurrence Duration in days Oct-91 7 Nov Mar-69 5 Jan-71 5 Feb-71 5 Sep-71 5 Nov-75 6 Nov-78 6 Oct Nov May Feb-89 6 Oct-89 7 Oct Oct-96 9 Nov-97 5 Oct-04 8 Nov-10 6 Table 1: Heat waves recorded in Mexicali in the period. If, on the other hand, a heat wave is defined as the occurrence of temperatures higher than the 95th percentile, which in this case corresponds to a temperature of 47.5 ºC, a well-defined trend will be found. Figure 6 shows a cumulative frequency graph of heat waves recorded in Mexicali. In the period, a total of 28 heat waves were recorded, with 82% of them occurring in the last two decades, in the period.

7 Figure 6: Heat wave cumulative frequency graph. 4. CONCLUSIONS A temperature analysis in Mexicali, Baja California, Mexico, shows a clear trend towards an increased occurrence of extreme values. If heat waves are considered as an indicator of these extremes, a well-defined trend towards increase is observed, with 82% of them occurring over the last two decades. Since heat waves are particularly hazardous for human health, the development of an early alert system to warn the population about these events is deemed necessary. References Campell, S. (2009, July 5). Retrieved from /CampbellS.pdf García-Cueto, O. R., Santillán-Soto, N., Ojeda-Benitez, S., & Quintero Nuñez, M. (2012). Escenarios de temperaturas extremas en mexicali, México bajo condiciones de cambio climático. 8th International Conference Climate Change. Extreme and Impacts. Salamanca: Asociación Española de Climatología. Hémon, D., & Jougla, E. (2003). Surmortalité liée á la canilucle d aout Rapport détape (1/3). Estimation de la surmortalité et prinicpales caractéristiques épidémiologiques. INSERM, Paris. IPCC. (2007). Cimate 2007: Impacts, Adaptation and Vulnerability. (M. Perry, O. Canziani, J. Palutikof, P. van Del Linden, & H. C., Eds.) Cambridge: Cambridge University Press. IPCC. (2008). Climate Change and water. IPCC Technical Paper VI. Geneva, Switzerland: Intergovernamental Panel for Climate Change.

8 IPCC. (2013). Summary for Policymakers. In Climate Change 2103: The Physical Basis. Contribution of the Working Group I to the fifth AR of the IPCC. (D. Q. Stokcer T.F., Ed.) Cambride, United KIngdom and New York, USA.: Cambridge University Press. Le Tertre, A., Lefranc, A., Eilstein, D., D. C., Medina, S., B. M.,... Ledrans, M. (2006, January). Impact of the 2003 Heatwave on all cause mortallity in 9 French cities. (L. W. Wilkins, Ed.) Epidemiology, 17(1), Randall, D. A., & Word, R. A. (2007). Climate models and their evaluation. In IPCC, Climate 2007: IMpacts, Adaptation and Vulnerability (pp ). Cambridge: IPCC. Sen, S. (2012). Innovative trend analysis methodology. Journal of Hydrologic Engineering, 17, Whitman, S., Good, S., Donoghue, E. R., N, B., Shou, W., & Mou, S. (1997). Mortality in Chicago attributed to the July 1995 heat wave. American Journal of Public Health, 87(9),