Recent summers have been the warmest in Europe in the past two millennia, scientists have said.
Researchers from 13 countries including the UK have used information from tree rings and historical records to reconstruct summer temperatures stretching back 2,100 years.
The data suggests greater natural variation in summer temperatures, seen in warm conditions in Roman times, cooler summers in the 4th to 7th centuries, a warm medieval period and a cold Little Ice Age from the 14th to 19th century, than previously thought.
Average European summer temperatures in the 20th century were around the same as some earlier centuries, including the 1st, 2nd, 8th and 10th centuries, the research published in the journal Environmental Research Letters found.
But the warm summers in the past few decades, including heat waves in 2003, 2010 and 2015, have seen "anomalously high" temperatures, with no evidence that any period in the last 2,000 years has been so warm, the study said.
Researchers said the findings that the past three decades were warmer than any previous natural variation, pointed to the role of human activity in pushing up temperatures.
Professor Neil Roberts from Plymouth University said: "We have always known that temperatures have fluctuated over many centuries, but wanted to test whether the warm weather experienced in recent years was exceptional when viewed over a longer time-scale.
"This study demonstrates that it is, and helps to confirm that human-induced global warming is now a reality, not just a possibility."