The report combined government data about average yearly temperatures with statistics on the number of violent crimes committed between 1950 and 2008. Based on those records, the authors estimate that if the average temperature in the U.S. increases by 8 degrees Fahrenheit (4.4 degrees Celsius), the country's murder and assault rate will jump by about a hundred thousand cases a year._ A 2007 report by the Intergovernmental Panel on Climate Change projected that global surface temperatures could rise by 2 to 11.5 degrees Fahrenheit (1.1 to 6.4 degrees Celsius) by 2100 if no mitigating steps are taken. Higher temperatures can increase human aggression in myriad ways, said study co-author Matthew DeLisi , a sociologist at Iowa State University (See "Global Warming Likely Causing More Heat Waves, Scientists Say"). Brad Bushman , a psychologist at the University of Michigan who specializes in human aggression, agreed. "Hot temperatures make people cranky and irritable," said Bushman, who did not participate in the new research. "Cranky, irritable people are prone to aggression."