Feeling the Heat: Global Warming and Rising Temperatures in the United States

7/24/2007

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 percent 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 percent) 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 percent) 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 percent 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 waves,” threatening public health, particularly that of elderly Americans and infants. 

2006: Second Warmest Year on Record

With a scorching summer and mild start to winter, the 2006 average temperature for the contiguous United States was the second warmest on record, according to the National Climatic Data Center. Every state in the Lower 48 experienced above normal temperatures in 2006. Our analysis of 2006 climate data showed:

•           In 2006, the average temperature was at least 0.5°F above the 30-year average at 87 percent of the locations examined. The Upper Midwest and Mountain West in particular experienced warmer-than-normal average temperatures in 2006.

•           The average maximum temperature was at least 0.5°F above the 30-year average at 81 percent of the stations examined. Warmer-than-average days hit Texas and the Great Plains the hardest in 2006, with average peak temperatures soaring more than 5°F above normal in Oklahoma City, Oklahoma.

•           The average minimum temperature was at least 0.5°F above the 30-year average at 80 percent of the stations examined. Minimum temperatures were particularly mild in the Upper Midwest, where temperatures soared almost 5°F above the 30-year average in Minneapolis-St. Paul, Duluth, and Rochester, Minnesota.

2000-2006: Temperatures Rising

The above-average temperatures of 2006 are part of a broader warming trend since 2000.  Our analysis of climate data for 2000-2006 showed:

•           Between 2000 and 2006, the average temperature was at least 0.5°F above the 30-year average at 87 percent of the locations studied. Average temperatures in Alaska were the most anomalous, with Talkeetna near Denali National Park averaging more than 4°F above the 30-year average. 

•           The average maximum temperature was at least 0.5°F above the 30-year average at more than two-thirds (68 percent) of the locations studied. Average maximum temperatures in Pueblo and Alamosa, Colorado were 2.6°F above normal.

•           Overall, temperatures are not dropping at night as much now as they did in the past.  Between 2000 and 2006, the average minimum temperature was at least 0.5°F above the 30-year average at 80 percent of the locations studied. Albuquerque, New Mexico and Sault Ste. Marie, Michigan reported average minimum temperatures of more than 3°F above normal.

Even though the IPCC identified significant risks with continued global warming, the panel also concluded that “many impacts can be avoided, reduced, or delayed” by quickly and significantly reducing global warming pollution. To protect future generations, the United States should:

Cap global warming emissions. The United States should establish mandatory, science-based limits on carbon dioxide and other global warming pollutants that reduce total emissions from today’s levels by the end of the decade, by at least 15-20 percent by 2020, and by at least 80 percent by 2050.