Federal Report: Warming = More Harmful Climate Extremes

By ANDREW C. REVKIN

 JUNE 19, 2008 2:27 PMJune 19, 2008 2:27 pm

Arizona’s Lake Powell, 2004. (Kevin Moloney for The New York Times)

The first thorough federal review of research on how global warming may affect extreme climate events in North America forecasts more drenching rains, parching droughts (especially in the Southwest), intense heat waves and stronger hurricanes if long-lived greenhouse gases continue building in the atmosphere.

Ninth Street in Brooklyn, 2004. (Seth Wenig for The New York Times)

The report is distinct from last month’s federal review of specific impacts of warming on agriculture, ecosystems, coasts and the like in the United States, focusing instead on how weather patterns will change. It also largely echoes, at the regional level, findings on global impacts from the Intergovernmental Panel on Climate Change last year.

The report, Weather and Climate Extremes in a Changing Climate, is online at climatescience.gov. The biggest impacts of global warming will be from the shifts in the frequency and duration of extreme events, not the slow rise in the average temperature, it concluded.

Plenty of changes driven partly by the growing human climate influence are already apparent, the report said:

For example, in recent decades most of North America has been experiencing more unusually hot days and nights, fewer unusually cold days and nights, and fewer frost days. Heavy downpours have become more frequent and intense. Droughts are becoming more severe in some regions.

The power and frequency of Atlantic hurricanes have increased substantially in recent decades, though North American mainland land-falling hurricanes do not appear to have increased over the past century. Outside the tropics, storm tracks are shifting northward and the strongest storms are becoming even stronger.

It said there is a 90-percent likelihood that the frequency and intensity of heat waves and heavy downpours will rise. While extreme weather can sometimes come with benefits, the report said, “…on balance, because systems have adapted to their historical range of extremes, the majority of the impacts of events outside this range are expected to be negative.”

These strong conclusions have a lot of scientists and policy experts pushing for prompt action, after decades of waffling, both to cut the odds of the worst outcomes by curbing greenhouse gas emissions and by boosting resilience to climate extremes — whatever the cause.