Hurricane Katrina leveled New Orleans in 2005 and is most often cited as an example of the effects of global warming. The hurricane season in 2005 had the largest number of hurricanes, and the strongest ones, on record. It is of course not possible to ascribe any single local event, or even a season of events, to a slowly changing general condition. It takes a concatenation of unusual local conditions to produce extreme weather. More far-fetched is the linking of the 2009 wildfires in Australia to global warming.12 Yes, the tinder may have been dry, but there have been droughts before, such as that in Southeast Asia in 1998-2003, that in Australia in 2002-2003, and that in Western North America in 1999-2004. Other events named in connection with global warming are the floods in Europe in 2002 and the heat wave there in 2003. Eleven of the 12 warmest years have occurred in the past 12 years. The opposite extremes are never mentioned. The winter of 1962-1963 in Europe was so cold that the Seine froze, and oil deliveries could not reach Paris. The European winter of 2008-2009 was the coldest in 20 years. Does global warm­ing really cause heat waves, cold spells, floods, droughts, fires, and storms?

Fortunately, extreme weather events can, and have been, documented statisti­cally. In many regions of the earth, good temperature and rainfall records have been kept and published. Alexander et al. [15] have compiled these data and produced graphs from which trends can be seen. For example, Fig. 1.20 shows maps and graphs of the occurrence of temperature extremes. The figure requires some expla­nation. At the upper left, the graph below the map in panel (a) shows, for the period 1951-2003, the number of days per year when the night temperature was very cold, when compared with the average number of such days in the period near the center of the graph. We see that the number of cold nights has been decreasing recently. The map above the graph shows where these cold nights occur, averaged over the entire period, with blue showing a lesser number of cold night and red a greater number. By contrast, we can look at the number of warm nights in panel (c) at the bottom left. We see that the number of very warm nights has increased a lot recently. The map shows, for instance, that western Africa and Latin America have suffered from this the most. In panels (b) and (d), the number of unusually cold and unusually hot days is shown. These show the same trend as the nights, but not as strongly. Remember that these data are not about the general warming trend but are about the occurrence of extreme hot and cold spells. These show a trend toward fewer cold spells and more hot spells as we move into the 21st century.

The shift of cold and hot spells can be seen more clearly in the bell-shaped probability curves in Fig. 1.21. The blue curves are for the 1950s-1970s, and the red curves for the recent period. The horizontal axis is the number of days per year that have the probability corresponding to the height of the curve. Thus, the peak of the blue curve in panel (a) says that there was a probability of 0.12 (12%) that there were 11 unusually cold nights in any year in that period. The plots (a) and (c) of Fig. 1.21 show the red curves to the left of the blue ones, meaning that there are

Fig. 1.21 Bell-shaped curves showing the probability of having the number of days per year (plotted on the horizontal axis) with unusually cold nights (a), warm nights (b), cold days (c), and warm days (d) [6]. The blue curves are for 1951-1978, and the red curves are for 1979-2003 [15]

now fewer cold spells; and the plots (b) and (d), with the red curves shifted to the right, show that there are more hot spells in recent years.

The occurrences of unusually heavy rainfall have also been recorded. These extremes are shown in Fig. 1.22. Though there is considerable variation from year to year, a trend toward more rain falling in big storms since 1990 can be seen.