Extreme Weather

Within a changing climate system, some of what are now considered to be extreme events will occur more frequently (e.g., heat waves), and some less frequently (e.g., cold snaps) (figure 2). More frequent extreme events occurring over a shorter period reduce the time available for recovery and adaptation. In addition, extreme events often occur in clusters. The cumulative effect of compound or back-to-back extremes has far larger impacts than the same events spread out over a longer period of time. For example, heat waves, droughts, air stagnation, and resulting wildfires often occur concurrently and have more severe impacts than any of these alone.

Figure 1 The blue bars show the number of events per year thatexceed a cost of 1 billion dollars (these are scaled to the left side of the graph). The blue line (actual costs at the time of the event) and the red line (costs adjusted for wealth/inflation) are scaled to the right side of the graph, and depict the annual damage amounts in billions of dollars.

Many extremes and their associated impacts are now changing. For example:

• Most of North America is experiencing more unusually hot days and nights and fewer unusually cold days. The last 10 years have seen fewer severe cold waves than any other 10-year period in the historical record, which dates back to 1895. The number of heat waves has also been increasing since 1950.
• There has been a decrease in frost days and a lengthening of the frost-free season over the past century.
• Extreme precipitation episodes (heavy downpours) have become more frequent and intense and now account for a larger percentage of total precipitation.
• Droughts are becoming more severe in some regions.
• Atlantic tropical storm and hurricane destructive potential has increased substantially since about 1970.
• Storm tracks have shifted northward in both the North Atlantic and North Pacific over the past fifty years. The strongest cold season storms are becoming even stronger in the North Pacific.

Figure 2 Most measurements of temperature (top) will tend to fall within a range close to average, so their probability of occurrence is high. A very few measurements will be considered extreme and these occur very infrequently. Similarly, for rainfall (bottom), there tend to be more days with relatively light precipitation and only very infrequently are there extremely heavy precipitation events, meaning their probability of occurrence is low. The exact threshold for what is classified as an extreme varies from one analysis to another, but would normally be as rare as, or rarer than, the top or bottom 10% of all occurrences.

Why these changes?
As the climate has warmed, primarily due to human activities, we have also seen a variety of changes in extremes. Human-induced warming has likely caused much of the average temperature increase in North America over the past fifty years and, consequently, changes in temperature extremes. For example, the effect of human-induced emissions of greenhouse gases has been associated with the very hot year of 2006 in the U.S.

• Heavy precipitation events averaged over North America have increased over the past 50 years, consistent with the observed increases in atmospheric water vapor, which have been linked to human-induced increases in greenhouse gases.
• It is likely that the human-induced increase in air temperatures, and the associated increase in evaporation potential over land, are already contributing to droughts that are longer and more intense.
• It is very likely that the human-induced increase in greenhouse gases has contributed to the increase in sea surface temperatures in the hurricane formation regions. Because there is a strong statistical connection between Atlantic tropical sea surface temperatures and Atlantic hurricane activity (Figure 3), this suggests a human contribution to recent hurricane activity (a confident assessment will require further study)
• Human influences on changes in sea-level pressure patterns have been detected over the Northern Hemisphere and this affects the location and intensity of cold season storms

What about Future?
Projected continued warming of North America has direct implications for the occurrence of extreme weather and climate events. For example, climate models indicate that many currently rare extreme events will become more commonplace.

• Future changes in extreme temperatures will generally follow changes in average temperature. Abnormally hot days and nights and heat waves are very likely to become more frequent (Figure 4). Cold days and cold nights are very likely to become much less frequent. The number of days with frost is very likely to decrease.
• Sea ice extent is expected to continue to decrease and may even disappear entirely in the Arctic Ocean in summer in the coming decades. This increases extreme coastal erosion in Arctic Alaska and Canada due to the increased exposure of the coastline to strong wave action.
• On average, precipitation is likely to be less frequent but more intense, and precipitation extremes are very likely to increase.
• In the future, droughts are likely to become more frequent and severe in some regions, leading to an increased need to respond to reduced water supplies, increased wildfires, and various ecological impacts. These regions include the U.S. Southwest and parts of Mexico.
• For North Atlantic and North Pacific hurricanes, it is likely that rainfall and wind speeds will increase in response to human-caused warming. Analyses of model simulations suggest that, for each 1°C (1.8°F) increase in tropical sea surface temperatures, core rainfall rates will increase by 6 to 18% and the surface wind speeds of the strongest hurricanes will increase by about 1 to 8%.
• In the future, there are likely to be more frequent strong cold-season storms in both the Atlantic and Pacific basins, with stronger winds and more extreme wave heights

– from www.climatescience.gov

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