The white colored rock (approximately 100 ft high) shows the drop in the water level of Lake Mead as a result of the ongoing 10-year drought along the Colorado River. Photo: Guy DeMeo , U.S. Geological Survey
summer temperatures are already occurring more frequently in the United States,
and will become normal by mid-century if the world continues on a business as
usual schedule of emitting greenhouse gases.
analyzing observations and results obtained from climate models, a study led by
Phil Duffy of the Lawrence Livermore National Laboratory showed that previously
rare high summertime—June, July, and August—temperatures are already occurring
more frequently in some regions of the 48 contiguous United States.
“The observed increase in the frequency of previously
rare summertime-average temperatures is more consistent with the consequences
of increasing greenhouse gas concentrations than with the effects of natural
climate variability,” said Duffy, who is the lead author of a report in Climatic Change. “It is extremely
unlikely that the observed increase has happened through chance alone.”
The geographical patterns of increases in extreme summer
temperatures that appear in observations are consistent with those that are
seen in climate model simulations of the 20th century, Duffy said.
Duffy and colleague Claudia Tebaldi, a senior scientist at
the nonprofit news and research group Climate Central, showed that the models
project that previously rare summer temperatures will occur in well more than 50%
of summers by mid-century throughout the lower 48 states.
The team first compared the period 1975 to 2000 to the
preceding 25 years, and found that both observations and results based on 16
global climate models show that summertime-average temperatures that were rare
in the earlier period occurred more often in the later period, in certain
regions. The agreement between observations and models demonstrates that the
models are able to simulate changes in the occurrence of extreme summertime
temperatures, Duffy said.
Duffy and Tebaldi performed a statistical analysis showing
that the increases in rare summer temperatures seen in the later period are
very unlikely to have occurred through chance weather variations.
Next, Duffy and Tebaldi assessed the present period, by
using results obtained from climate models for 1995 to 2024; they found that
summer temperatures that were extreme during 1950 to 1979 occur more often in
the later time period. This supports the conclusion that extreme summertime
temperatures are already occurring more frequently in parts of the lower 48
states. A second statistical analysis showed that this increase also is very
unlikely to be due to chance weather variations alone, such as El Ninos or La
Finally, the team evaluated model results for 2035 to 2064
(representing the middle of this century) and found that extreme summertime
temperatures that were rare during 1950 to 1979 are projected to occur in most
summers throughout the 48-state region in the mid-century period. For the
mid-century, summertime mean temperatures that historically occurred only 5% of
the time are projected to occur at least 70% to the time everywhere in the 48
“The South, Southwest and Northeast are projected to
experience the largest increases in the frequency of unusually hot
summers,” Duffy said. “The strong increase in extremes in the
Southwest and Northeast are explained by strong historical and projected warming
there. This result is based upon assuming a commonly used scenario for future
emissions of carbon dioxide, the main driver of human-caused climate change.
“What was historically a one in 20-year occurrence will
occur with at least a 70 percent chance every year. This work shows an example
of how climate change can affect weather extremes, as well as averages.”