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Executive Summary
 In 2006, Americans experienced a summer heat wave that broke records from
coast to coast and killed almost 200 people. The year ended and 2007 began with
the warmest winter on record globally. This unseasonably warm weather is part
of a long-term trend toward rising temperatures and extreme weather events
resulting from global warming.
Global average surface temperatures have increased by more than 1.4°F since the
second half of the 19th century. Earlier this year, the United Nations’ Intergovernmental
Panel on Climate Change (IPCC) concluded that the evidence of global warming is
“unequivocal” and that human activities are responsible for most of the rise in
temperatures.
To examine recent temperature patterns in the United States, we compared temperature data for
the years 2000-2006 from 255 weather stations located in all 50 states and Washington, DC
with temperatures averaged over the 30 years spanning 1971-2000.
Overall, we found that temperatures were above the 30-year average across
the country, indicating pervasive warming.
 SUMMER 2006: RECORD-BREAKING HEAT
A long-lasting summer heat wave hit most of the country in 2006, making it
the second warmest summer on record for the contiguous United States.
Heat waves have serious implications for human health, causing heat stroke,
heat exhaustion, and even death. Our analysis of climate data for June-August 2006
showed:
• During the summer of 2006, the average temperature was at least 0.5°F above
the 30-year average at 82% of the locations studied. In Rapid
City, South Dakota and
Helena, Montana,
average summertime temperatures were 5°F above normal.
• The average maximum temperature — the peak temperature on any given day — was
at least 0.5°F above the 30-year average at two-thirds (67%) of the locations
studied. The Great Plains and Mountain West
suffered some of the most above-normal
summer temperatures in 2006.
• The summer heat wave produced a high number of dangerously hot days at or
above 90°F across the country. Almost three-fourths (71%) of the locations
examined recorded more frequent (compared with the historical average) days
with peak temperatures of at least 90°F. Tupelo,
Mississippi experienced 40 more
90°F or warmer days than normal in 2006.
• The 2006 summer heat wave was marked by above-average minimum temperatures —
the lowest temperatures recorded on a given day, usually at night. The average
minimum temperature was at least 0.5°F above the 30-year average at 81% of the
locations studied and 9.7°F above normal in Reno, Nevada,
the highest in the country. Warmer nighttime temperatures exacerbate the public
health effects of heat waves, since people need cooler nighttime temperatures
to recover from excessive heat exposure during the day.
In April 2007, the IPCC warned that North American cities that currently
experience heat waves are expected to face “an increased number, intensity, and
duration of heat.
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