Greenhouse Gases and The Widespread Heatwave

Greenhouse Gases and The Widespread Heatwave

Aug. 2007  - Greenhouse gases probably 
accounted for more than half of the widespread heatwave felt across the 
continental United States in 2006, according to a new study that will be 
published September 5 in "Geophysical Research Letters," a publication of 
the American Geophysical Union. 
Led by meteorologist Martin Hoerling at the National Oceanic And 
Atmospheric Administration's Earth System Research Lab in Boulder, 
Colorado, the scientists estimate that there is only a 16 percent chance 
that 2007 will bring record-breaking heat.

 
They also found that greenhouse gas increases in Earth's atmosphere 
enhanced the probability of U.S. temperatures breaking a record in 2006 by 
approximately 15-fold compared to pre-industrial times. 
Last year's average temperature was the second highest since recordkeeping 
began in 1895. The researchers found that it was very unlikely that the 
2006 El Nino warming pattern in the eastern tropical Pacific Ocean played 
any role in the high temperatures, though they said other natural factors 
may have contributed to the near-record warmth. 
When average annual temperature in the United States broke records in 
1998, a powerful El Nino was affecting climate around the globe, and many 
scientists attributed the unusual warmth in the United States to the 
influence of this El Nino. 
"We wanted to find out whether it was pure coincidence that the two 
warmest years on record both coincided with El Nino events," Hoerling 
said. "We decided to quantify the impact of El Nino and compare it to the 
human influence on temperatures through greenhouse gases." 
The annual average temperature in 2006 was 2.1 F (1.1°C) above the 20th 
century average and marked the ninth consecutive year of above-normal U.S. 
temperatures.

The near-record warm summer featured a heatwave that peaked during the 
last half of July. All-time records were set in a number of locations 
across the central and western United States, breaking records that had 
stood for decades in many places. 
Each of the lower 48 states reported above-normal annual temperatures, and 
for the majority of states, 2006 ranked among the 10 hottest years since 
1895. 
Using data from 10 past El Nino events observed since 1965, the authors 
examined the impact of El Nino on average annual U.S. surface 
temperatures. 
They found a slight cooling across the country. 
To overcome uncertainties in the data analysis, the team also studied the 
El Nino influence using two atmospheric climate models. The scientists 
conducted two sets of 50 year simulations of U.S. climate, with and 
without the influence of El Nino sea-surface warming. They again found a 
slight cooling across the nation when El Nino was present. 
To assess the role of greenhouse gases in the 2006 warmth, the researchers 
analyzed 42 simulations of Earth's climate from 18 climate models provided 
for the latest assessment by the Intergovernmental Panel on Climate 
Change. The models included greenhouse gas emissions and airborne 
particles in Earth's atmosphere since the late 19th century and computed 
their influence on average temperatures through 2006. 
The results of the analysis showed that greenhouse gases produced warmth 
over the entire United States in the model projections, much like the 
warming pattern that was observed last year across the country. 
For a final check, the scientists compared the observed 2006 pattern of 
abnormal surface temperatures to the projected effects of greenhouse-gas 
warming and El Nino temperature responses. The U.S. temperature pattern of 
widespread warming was completely inconsistent with the pattern expected 
from El Nino, but it closely matched the expected effects of greenhouse 
warming. 

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