Climate data and scenarios: synthesis of recent observation and modelling results, chapter 3.3

3.3 Extremes

For many climate change impacts, changes in the frequency and magnitude of extreme events are more important than changes in mean values. There are many extremes that have been analyzed in the climate science literature, but by way of illustration we focus here on two basic quantities: changes in annual maximum temperature (i.e., the hottest temperature of the year) and changes in annual maximum 24-hour precipitation. Because global climate models operate with time steps of roughly half an hour, daily minimum, maximum, and mean values can be computed and the projected changes provide an indication of changes that might be anticipated in the future. An important caveat, especially for precipitation, is that the spatial resolution of global climate models remains relatively coarse (typically 100-250 km), and so the precipitation extremes in a model represent averages over an area of several thousand square kilometres. Additionally, climate models may not have all of the physical processes that produce local intense rainstorms. These limitations must be kept in mind when making comparisons to individual meteorological station measurements.

A common way to illustrate changes in climate extremes is to compute the ‘return period’ of events of a particular magnitude for different time periods. The return period is the long-term average interval between recurrences of extreme values. Figure 11 shows projected return periods for annual maximum temperature and the annual maximum amount of precipitation within a 24-hour period. These plots indicate that the recurrence time, or return period, for these extremes is projected to decrease, for both quantities, in the future. That is, extremes of a particular magnitude will become more frequent. For example, the lower right panel of Figure 11 indicates that, under the RCP8.5 forcing scenario, an annual maximum daily temperature that would currently be attained once every 10 years, on average, will become an annual event by the end of the century.

Figure 11 - Projected return periods (in years) for -twentieth century 10-, 20-, and 50-year return values of annual maximum 24-hour precipitation (upper panel) and annual maximum temperature (lower panel) over Canada as simulated by GCMs contributing to the CMIP5 for three RCPs (RCP2.6, left; RCP4.5, middle; RCP8.5, right). Values are computed based on Kharin et al., 2013.

As with mean temperature and precipitation, changes in climate extremes are not uniform across the globe, or even across Canada. Figure 12 shows projected changes in precipitation extremes for different regions of Canada, along with estimates of the uncertainty range around the projected return periods.

Figure 12 - Projected changes (in %) in 20-year return values of annual maximum 24-hour precipitation rates (i.e., precipitation extremes). The bar plots show results for regionally-averaged projections for three time horizons: 2016-2035, 2046-2065, and 2081-2100, as compared to the 1986-2005 baseline period. The blue, green, and red bars represent results for RCP2.6, RCP4.5, and RCP8.5, respectively. Projections are based on GCMs contributing to CMIP5 and the analysis is described in Kharin et al., 2013.