Condensed summary

Observed changes in climate extremes have been documented for both temperature and precipitation in many parts of the globe. These changes include decreases in frost days and a lengthening of the frost-free season, increases in the number of days with temperatures above percentile-based thresholds, and increases in heavy precipitation events. These changes are generally consistent with observed warming in mean annual temperatures, and with observed changes in annual precipitation.

Introduction

This chapter reviews the scientific literature on variability and change in observed climate extremes over the globe. Observed changes in extremes should be considered in light of observed changes in mean quantities, including observed changes in annual average temperature, and changes in maximum and minimum temperatures and the diurnal temperature range (DTR) (Easterling et al., 1997; Vose et al., 2005). The globally averaged annual temperature (Fig. 2.1) shows a linear increase of approximately 0.6 degrees per century since the late 1800s, but the rate of increase since the mid 1970s has itself increased to approximately 2 degrees per century. Seasonally, the strongest increases have occurred in the boreal winter (December–January–February, DJF) and spring (March–April–May, MAM). Figure 2.2 shows that, regionally, the southeastern United States and northern Atlantic continue to show a trend to cooling, but the southeastern US trend appears to be getting smaller with each additional year of data; the largest increases in temperature have occurred in the high latitudes of the Northern Hemisphere.