Much of the discussion of climate change focuses on slow changes in average temperatures and precipitation over time. But this focus masks the larger changes in weather variability and extreme weather events that will accompany modest changes in averages. Damages aren’t typically associated with average rainfall events or gradual increases in temperatures but are driven by extreme flooding events, periods of extended drought or prolonged, intense heat waves.
Extreme weather events have always been an important part of our climate history. The Dust Bowl drought of the 1930s, the 1927 Mississippi River flood, and the 1980 Heat Wave that blanketed much of the Midwest are just a few examples of extreme events that are etched in our nation’s history. By its very nature our climate system produces variable weather including an occasional extreme event. By increasing greenhouse gases in the atmosphere, however, we are loading the dice toward more favorable climate conditions for extreme weather and are very likely to experience more frequent extreme events over time.
A large body of scientific evidence suggests that droughts, floods and heat waves are likely to become more frequent and/or intense. Recent data suggests we are experiencing this trend already. For example, the amount of rain that falls during intense precipitation events has increased by 20 percent in the United States over the last century. Over the past decade, record high temperatures now occur about twice as often as record lows; the ratio was about one-to-one in the 1950s (Figure 1). Recent floods in Tennessee, the lower Mississippi and North Dakota, droughts and wildfires in the southwest, and intense, humid heat waves in the Midwest, illustrate the high costs of extreme weather events. These types of changes are fully consistent with what scientists have long warned would be the consequences of increasing greenhouse gas concentrations in our atmosphere. A large body of scientific evidence makes it clear that the risk of such events has increased and should be expected to continue to increase as the climate warms.
The events we have experienced in recent years provide important information about our vulnerability to extreme weather, the human and economic costs that could result, and most importantly, actions we can take today to minimize the risks of more frequent extreme weather events. This information is useful regardless of why any particular event happened and whether climate change made it worse or not. The often asked question about whether climate change caused a particular weather event cannot be answered definitively. Individual events are caused by the interaction of many factors and efforts to isolate the role of climate change will not be resolved cleanly in the years to come. What virtually all climate scientists agree on, however, is that the climate is already changing, that all weather events now form under different conditions than they used to, and that this change is increasing the probability of extreme weather events happening. Moreover, scientists agree that severe heat and heavy downpours are already more frequent and intense than they used to be. Since the rising risk of extreme weather is well established, it makes sense to learn what we can from actual events and avoid getting caught up in an irresolvable debate about why a particular event happened.