The amazing finding comes from a survey of 39 trees by Brent Helliker and Suzanna Richter from the University of Pennsylvania. The pair looked at both deciduous and evergreen species across the entire North American continent from its frozen north to the steamy south.

It would seem fairly easy to measure the temperature of a leaf directly but the problem is one of scale. You would need to take readings for hundreds of leaves to get a reasonable measurement for a single tree. Instead, Helliker and Richter opted for an unusual and indirect approach – they looked at the ratio of two forms of oxygen, 16O and 18O, in the wood of tree rings.

Working with wood seems like an