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CGD Climate Highlights: Climate Change

Why do we believe the climate is changing?

Changes in Extremes

For any change in mean climate, there is likely to be an amplified change in extremes. The wide range of natural variability associated with day-to-day weather means that we are unlikely to notice most small climate changes except for changes in the occurrence of extremes. Extreme events, such as heat waves, floods and droughts, are exceedingly important to both natural systems and human systems and infrastructure. We are adapted to a range of natural weather variations, but it is the extremes of weather and climate that exceed tolerances.

In several regions of the world indications of a change in various types of extreme weather and climate events have been found. So far, the most prominent indication of a change in extremes is the evidence of increases in moderate to heavy precipitation events over the middle latitudes in the last 50 years, even for regions where annual precipitation totals are decreasing. Further indications of a robust change include the observed trend to fewer frost days associated with the average warming in most middle latitude regions. Results for temperature-related daily extremes are also relatively coherent for some measures. Many regions show increased numbers of warm days/nights (and lengthening of heat waves) and even more reductions in the number of cold days/nights, but changes are not ubiquitous.

Trends in tropical storm frequency and intensity are masked by large natural variability on multiple timescales. Increases may be occurring in recent years, but apart from the North Atlantic basin, most measures only begin in the 1950s or 1960s and have likely missed some events in the early decades. Numbers of hurricanes in the North Atlantic have been above normal in 8 of the last 10 years.