Droughts, heavy rain, heat waves, wildfires and intense hurricanes are more likely to affect North America because of global warming's effect on extreme weather, the Bush Administration's Climate Change Science Program said Thursday.
There's high confidence that the frequency and intensity of extreme weather events has already been influenced by global warming, and even greater confidence that more expensive, damaging and deadly weather is to come as temperatures continue to rise. The risk is tied directly to human emissions of heat-trapping greenhouse gas emissions like carbon dioxide from burning fossil fuels.
The findings are familiar, but the report is the most specific and comprehensive to address how climate change affects weather in North America, and it may be the Bush Administration's most clear acknowledgments of the dangers of climate change to date. The report's release coincides with President Bush's visit to the flood-ravaged Mississippi Valley, where thousands of lives have been upended, enough crops have been ruined to drive up world food prices, and the cost to repair damage will run into the billions.
"We know global warming over the last 50 years is primarily due to human-induced increases in greenhouse gases; in conjunction with this increase in global temperatures, we've observed in North America a number of changes in extreme weather and climate events," report co-chair Tom Karl, director of NOAAs National Climate Data Center, told reporters. "We expect these changes to continue into the future as greenhouse gases continue to accumulate in the atmosphere, and the planet continues to warm."
The findings have direct implications for the record flooding in the Midwest. Floods that once would have occurred every 20 years might occur now every five or six years by the end of the century. The last major flood in the region was just 15 years ago. Even while more flooding is expected, drought is expected to be more intense and long-lasting, particularly in the Southwest, where an ongoing drought since 1999 is rivaling the most intense on record.
"Essentially, when it rains it rains harder, and when it's not raining, temperatures are warmer and more water can be evaporated, so droughts can become more intense and last longer," Karl said.
Tropical storms, too, such as hurricanes Dean and Felix rapidly intensifying cyclones that made back-to-back landfall as Category 5 storms in Central America in 2007 are likely to become more frequent or intense. The report's authors did caution that more research is needed to increase the confidence of this finding, reflecting disagreement among scientists.
At a glance, here are the report's major findings:
Abnormally hot days and nights, along with heat waves, are very likely to become more common. Cold nights are very likely to become less common.
Sea ice extent is expected to continue to decrease and may even disappear in the Arctic Ocean in summer in coming decades.
Precipitation, on average, is likely to be less frequent but more intense.
Droughts are likely to become more frequent and severe in some regions.
Hurricanes will likely have increased precipitation and wind.
The strongest cold-season storms in the Atlantic and Pacific are likely to produce stronger winds and higher extreme wave heights.
Many of the changes will occur regardless of what is, or is not, done to reduce greenhouse gas emissions. Carbon dioxide is so long-lived in the upper atmosphere that past pollution has locked in years of additional warming. However, the extent of future changes and therefore the extent of death and destruction that results is dependent on whether and how much the world reduces carbon dioxide pollution.
(Emissions, according to recent research, are increasing at a faster rate than is anticipated in this chart.)
The Bush Administration has steadfastly avoided commitments to reduce greenhouse gas emissions, and China's emissions have grown so fast that they now exceed American pollution by a significant and growing margin. Both candidates for president, however, Barack Obama and John McCain, support legislation to mandate cuts in carbon pollution.
The Bush Administration's parting shot on climate science, after years of manipulating government assessments to downplay the risks, ups the ante for Bush's successor, and shows more clearly than ever the cost of inaction.
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