In 1896, Swedish chemist Svante Arrhenius made a prescient calculation that showed the vast quantities of carbon dioxide released into the atmosphere by burning coal and other fossil fuels would eventually cause the planet to get warmer. Little did he realize that the effect he described was … being dutifully recorded by … trees. Now, scientists have tapped into that record and demonstrated that the human effect on Earth’s climate can be traced back to the turn of the last century, when it began leaving its indelible mark on the growth patterns of tree rings. …John Pomeroy, a University of Saskatchewan hydrologist who directs Canada’s largest research program on water risk and climate change, said the NASA study does a good job of incorporating other climate records together with the tree-ring data and matches the occurrences of drought that were experienced in Western Canada around the time of the First World War and again in the 1930s.