By Fred Pearce
How did the Romans manage to grow grapes in northern England when most climate studies suggest the weather was much cooler then? We may now have an answer: it wasn’t that cold at all.
Long-term temperature reconstructions often rely on the width of tree rings: they assume that warmer summers make for wider rings. Using this measure, it seems that global temperatures changed very little over the past two millennia. Such studies are behind the famous “hockey stick” graph, created by Michael Mann of Pennsylvania State University in University Park, which shows stable temperatures for a millennium before the 20th century.
Jan Esper of Johannes Gutenberg University in Mainz, Germany, thinks that at least some of those tree rings actually show something else: a long-term cooling trend that lasted right up until the Industrial Revolution.