Extreme Events

The state of weather and climate is one of great spatial and temporal variability in which floods, droughts, storms and rapid temperature changes occur. There is no clear definition of an extreme climate event, which is demonstrated in the following definitions. Easterling et al. (2000) define extreme climate events as those climate events causing extraordinary economic and social (loss of life or livelihood) damage. Wisner et al. (2004) define extreme events as natural hazards that have increased in intensity and frequency. We propose a definition combining elements of the two. We define extreme climate events as events or series of events that occur with greater intensity or frequency than most climate events and cause a disproportionate economic and/or social disruption in the area affected. Within the climate research and natural hazards research communities, there is much interest in investigating whether or not climate events are becoming more frequent and intense and how societies are being impacted by them. Regardless of the degree of damage caused by a particular climate event, in order to label that event as “extreme”, it is important to determine if the variability of climate events is increasing in order to build more resilient communities.

Many climate researchers (Karl and Easterling 1999; Groisman et al. 1999; Hegerl et al. 2006; Alexander et al. 2006; Solow 1999) are investigating historical trends in climate events and possible links in extreme events to current knowledge and projections of global warming. The greatest barrier to uncovering changes in trends is the paucity of climate data, both in terms of record length and the distribution of climate stations around the globe. In many countries, the collection of temperature, precipitation, wind and other climate data began only after the Second World War, making it difficult to statistically determine whether a climate event falls within the normal range of variability or is truly an unusual event in magnitude or frequency of recurrence.

The spatial variability of climate extremes due to a number of factors such as elevation, vegetation, proximity to oceans or land use implies that various regions of the world will experience changes in climate events in different manners. Changes in temperature will have significant impacts on agriculture, energy use, and how land can be used. Warmer nights lessen the risk of frost for farmers and can reduce the need to heat homes and businesses. However, increased periods of extremely warm days or heat waves, can lead to crop and livestock losses, more heat related deaths, and energy blackouts as power grids are strained to meet the increased energy demand for cooling. Therefore, investigations into trends in global and regional temperatures are key. For instance, studies (Plummer et al. 1999; Heino et al. 1999; Alexander et al. 2006) investigating changes in temperature extremes have noted strong regional differences, global temperature trends, and seasonal temperature changes. Plummer et al. (1999) found that the number of frost events decreased in Australia and New Zealand and that minimum daily temperatures increased. Globally, Alexander et al. (2006) have noted that in general, the number of warm days and nights are increasing, in some regions faster than in others. Figure 1 displays the results of the global temperature investigation carried out by Alexander et al.

Figure 1: Global temperature trends. Extreme cold nights/days are defined as those falling at or below the 10^th percentile. Extreme warm nights/days fall at or above the 90^th percentile. Alexander et al. (2006).

In addition to searching for trends in temperature changes, scientists, farmers and the insurance industry are interested in  how precipitation regimes are changing. Droughts and extreme precipitation events that can lead to flooding are well within the normal range of climate variability. From the known historical record, some regions are more prone to prolonged droughts than others. Changes to precipitation will have profound effects on all sectors of the economy and on many communities. As with the temperature data sets, records of precipitation are not complete in many locations, but there are enough data to uncover significant annual trends in global precipitation. In general, the number of heavy precipitation days in the south-central United States and some of South America have increased, while the number of consecutive dry days has decreased throughout parts of India (Alexander et al. 2006). Groisman et al. (1999) found regional and seasonal changes in mean precipitation and extreme precipitation events over the last century for Norway, Russia, the United States, China, Australia, Canada, Mexico and Poland. They documented that while there is an increase in the mean summer precipitation for most of these countries, much of the increase can be attributed to a greater number of extreme precipitation events. Figure 2 shows global precipitation trends.

Figure 2: Global extreme precipitation trends. Very wet days are days with precipitation at or above the 95^th percentile. Extremely wet days are days with precipitation at or above the 99^th percentile. Alexander et al. (2006).

As described in this website's section on poverty and vulnerability, some individuals and groups will be more affected by changes in extreme climate events than others. For instance, Hurricane Mitch was an extraordinarily powerful storm that caused over 11,000 deaths throughout Central America. The strength of the storm, combined with poorly constructed houses and land instability due to deforestation, created an extreme situation that severely disrupted the social and economic infrastructure of Honduras and Nicaragua. Increases in extreme precipitation events in low lying areas of Bangladesh or India have the potential to significantly impact many in these two countries. Even in more affluent countries, increasing economic losses associated with extreme events will have an impact. Swiss Re, one of the world's largest reinsurance companies, has noticed an increase in both the number of extreme storms impacting Europe and rising costs associated with damage claims from these storms (Heck et al. 2006). At this point, it is difficult to predict whether there will be a significant increase in the intensity and frequency of extreme climate events and which regions will be most impacted. However, based on observable trends, in certain regions of the world greater temperature and precipitation extremes have been noted. The changes in trends of extreme climate events might be a foreshadowing of an increased number of more intense events. Furthermore, as communities continue to make decisions that increase their vulnerability to climate events, such as allowing settlement in floodplains, the economic and social losses associated with climate events can be expected to increase.

Bibliography

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