For the past three decades, Europe has been experiencing its warmest summers since the days of the Roman Empire, according to a study published in the Environmental Research Letters Journal. The study, compiled by 40 academics, concluded that average summer temperatures have been 1.3 degrees Celsius hotter than they were 2,000 years ago. Heat waves also occur more often and last longer. The temperature figures were calculated by analyzing the tree ring analysis of three pine species found in Austria, Sweden, and Finland, as well as climate modeling and historical documents. The report says that summers were particularly warm between Roman times and the third century, before cooling until the 7th century. Temperatures warmed up again during medieval times, then dropped again from the 14th to 19th centuries. The recent warming, however, is unprecedented and cannot be explained by natural variability, but is directly related to manmade climate changes, the scientists said.