 Climate change seems a factor in the rise and fall of the Roman empire, according to a study of ancient tree growth that urges greater awareness of the risks of global warming in the 21st century. Good growth by oak and pine trees in central Europe in the past 2,500 years signalled warm and wet summers and coincided with periods of wealth among farming societies, for instance around the height of the Roman empire or in medieval times. Periods of climate instability overlapped with political turmoil, such as during the decline of the Roman empire, and might even have made Europeans vulnerable to the Black Death or help explain migration to America during the chill 17th century. Climate shifts that affected farm output were factors in "amplifying political, social and economic crises", Ulf Buentgen, of the Swiss Federal Research Institute for Forest, Snow and Landscape Research, told Reuters. He was lead author of the report in Friday's edition of the journal Science. The review, by experts in Germany, Austria, the United States and Switzerland, extended study of tree rings 1,000 years beyond previous analyses. Thick rings indicate good growth conditions while narrow ones mean poor. The study said the evidence, helping back up written records that are sparse in Europe more than 500 years ago, "may challenge recent political and fiscal reluctance" to slow projected climate change in the 21st century. Modern societies seem less vulnerable but "are certainly not immune" to climate change, especially because migration "will not be an option in an increasingly crowded world", they wrote. The U.N. panel of climate experts says that greenhouse gases, mainly from burning fossil fuels, will lead to more droughts, floods, heatwaves and rising sea levels that could swamp low-lying island states. BARBARIANS The study said: "Wet and warm summers occurred during periods of Roman and medieval prosperity. Increased climate variability from AD 250-600 coincided with the demise of the western Roman empire and the turmoil of the migration period." "Distinct drying in the 3rd century paralleled a period of serious crisis in the western Roman empire marked by barbarian invasion, political turmoil and economic dislocation in several provinces of Gaul," it said. Temperatures and rainfall only returned to levels of the Roman period in the early 800s, around the time when new kingdoms consolidated in Europe. The Black Death bubonic plague of the mid-14th century, for instance, was during an unstable, wet period. "From other studies we know that a more humid environment is more supportive fo the dispersal of plague," Buentgen said. Later on, "temperature minima in the early 17th and 19th centuries accompanied sustained settlement abandonment during the Thirty Years' War (1618-48) and the modern migrations from Europe to America", they wrote. He said Europe had the best record of tree rings because of widespread wooden buildings but that the techniques could be applied elsewhere, for instance in China or the Middle East. For Reuters latest environment blogs, click on: blogs.reuters.com/environment/