Scientists Say Climate Change is Contributing to More Extreme Weather Now

Recently, NRDC published a report explaining the potential water-related impacts of climate change on U.S. cities.  The projected impacts include, among other things, more frequent and intense storm events.  Now, some scientists say the effects of climate change are already contributing to more extreme weather events in the U.S. and elsewhere.  Climate scientists from NCAR, Scripps and Weather Underground discussed their findings on a conference call today.  Most of what was covered in the call can be found on the Climate Communication website.

As we have all seen, extreme weather events, such as blizzards, tornadoes, flooding, droughts, heat waves, and tropical storms, have occurred within the past few years both in the U.S. and across the globe.  Scientists say record-breaking extremes are occurring as heat-trapping gases allow the atmosphere to warm and hold more moisture, which provides fuel to these events.  They say the relatively small observed increase in average temperature over the 20th century (about 1°C) is causing large changes in extreme events. 

Because human activity has changed the background environment in which weather events develop, they have concluded that climate change is now a component of all weather events—on the scale of 5 to 10 percent.  For example, analysis indicates that precipitation from Hurricane Katrina in 2005 was greater by 6 to 8 percent due to climate change, which may not seem that much but is equivalent to an additional inch of rain on top of a foot.

Climate change is compounding natural variability to cause record-breaking extreme weather events.  One scientist on today’s call equated greenhouse gases to steroids in the climate system—effectively loading the dice or shifting the odds in favor of more climate extremes. 

Data also show that record high temperatures now outpace record lows. Historically, the ratio of record daily temperature highs to record daily lows has been roughly 1 to 1.  However, in the past decade, this ratio has increased to 2 to 1—the amount of record daily highs being set each year is now twice that of daily lows.  So far in 2011, this ratio is 3 to 1.  Climate models project that this ratio will further increase to 20 to 1 by mid-century and 50 to 1 by 2100 at the rate of current emissions.  Further warming due to greenhouse gases also will lead to additional warming of the oceans and more moisture in the atmosphere and manifest itself in the form of more intense, larger, and longer-lasting storms. 

The linkage between climate change and tornadoes is more complex because of the many factors involved (e.g., high altitude wind patterns, topography, etc.) but the scientists say climate change is leading to more moisture and warm air from the Gulf of Mexico and tropical Atlantic, which fuels the formation of supercell thunderstorms. 

Finally, the observed warming cannot be explained by phenomena other than increased concentrations of greenhouse gases, as there have been no changes in solar activity or clouds that would sufficiently explain observed temperature increases.

The types of extreme weather we have seen this year are certainly consistent with what models project in a changing climate.  Today’s announcement makes a more direct connection and reinforces the notion that how we have treated and continue to treat the planet may be increasingly reflected in how it treats us.

About the Authors

Steve Fleischli

Senior Attorney and Director, Water program

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