U.S. Climate report- Weather and Climate Extremes in a Changing Climate

Although global temperature rise is widely discussed concerning the severity of climate change, a more societally relevant concern is how weather and climate will vary at the local and regional level. Temperature rises over very large spatial scales (e.g., a hemisphere or the globe) do not imply uniform changes of various climatic variables (hurricanes, droughts, storms, etc) but we should expect global inhomogenities in climate as greenhouse gases build up in the atmosphere. For example, changes in global mean precipitation (or evaporation) are not incredibly large in a warmer world; models tend to agree that there is a less than 2% increase per degree C global mean warming. However, changes in horizontal transport and increased precipitation gradients (drier areas getting drier, wetter areas getting wetter, droughts in areas, flooding in others, etc) are a big reason for concern. What’s more, people will inevitably be concerned about the likelihood of extremes, such as the possibility of more anomalous events like the European heatwave of 2003.

The first federal review of research on how global warming may affect extreme climate events in North America is available here. Regions of focus include North America, Hawaii, Caribbean, and U.S. Pacific Islands. A summary of findings below:

— Over most land areas, the last 10 years had lower numbers of severe cold events than any other 10-year period.

— In North America, the 1930’s remain the most severe in terms of unusually hot spells, but there is an increasing trend since mid-century. The ’30s in the U.S. were a very extreme regional warming event, and we’ve not yet approached a point where the global pattern exceeds such extremes, but we’re approaching them. In Mexico, the 1950s and 1994-present were the driest periods.

–Extreme precipitation episodes have become more frequent and more intense in recent decades than at any other time in the historical record

— For increases in area affected by drought, there is no overall average change for North America, but regional changes are evident.

— The observed relationship between increased intensity of Tropical Cyclones and rising ocean temperatures is robust. In terms of the focus of this report, increases are substantial since about 1970 in association with rising Atlantic SST’s. Increasing tendency in West Pacific and decreasing tendency in East Pacific (Mexico West Coast) since 1980.

— Decreased snow cover in United States. Decrease in snowstorms in the South and lower Midwest of the United States, and an increase in snowstorms in the upper Midwest and Northeast.

— It is very likely (IPCC standards) that there will be warmer extreme cold days and nights and fewer frosts, warmer nights (and probably further decrease in the diurnal temperature gradient), more frequent and intense heavy downpours, more frequent heat waves and warm spells, increases in area affected by drought (particularly in the Southwest), and possibly increases in hurricane intensity due to rising SST’s. Changes in precipitation extremes will be pronounced, and more evaporation leading to droughts in other areas.

— In terms of attribution, anthropogenic warming has likely caused much of the average temperature increase in North America over the past 50 years effecting temperature extremes, very likely caused increases in Atlantic SST’s, and also are responsible for increases in water vapor concentration.

Concerning extremes, there may be some benefits, but the report says “But on balance, because systems have adapted to their historical range of extremes, the majority of events outside this range have primarily negative impacts.”

“Actions that lessen the risk from small or moderate events in the short-term, such as construction of levees, can lead to increases in vulnerability to larger extremes in the long-term.”


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