Extreme 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, according to a study released on Wednesday by Lawrence Livermore National Laboratory researchers.

By analyzing observations and results obtained from climate models, the study 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 Phil Duffy, who is the lead author of a report in a recent edition of the journal 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.

Based on model projection, Duffy and colleagues showed that previously rare summer temperatures will occur in well more than 50 percent of summers by mid-century throughout the lower 48 states, with the South, Southwest and Northeast projected to experience the largest increases in the frequency of unusually hot summers.

"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," Duffy said.