The effects of human-caused increased greenhouse gases were predicted as early as the turn of the 20th Century and according to the Ivan Semeniuk of the Canadian Globe and Mail a NASA study of tree rings from the last 120 years is helping to prove out this theory.
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 already under way and being dutifully recorded by a ready-made monitoring system distributed around the globe in the form of trees.
The growth record of trees is recorded in their rings and this growth is highly sensitive to changes in moisture.
Tree rings are among the most direct ways of measuring past climate because trees are sensitive to soil moisture. In drier years, trees grow more slowly and the annual rings that are recorded in their trunks become narrower. By comparing overlapping tree-ring patterns in wood that grew at different times on different continents, scientists have gradually built up “drought atlases” that show changes in moisture distribution dating back to the year 1400 or, in some areas, even earlier.
Drought atlases are nothing new, but using trees to measure the effects of drought across time and region is new science and it’s showing some startling trends.
The scientists found that after centuries of normal variations during which some places alternately became wetter or drier relative to each other, an additional effect on moisture emerged around 1900 that is consistent with climate change. Over all, the data show that much of North America, Australia and the Mediterranean have been getting drier over the past 120 years while parts of Asia, including India and western China, have been getting wetter.
The effect was especially pronounced during the first half of the 20th century, but became more subdued between 1950 and 1975. Since then, it has accelerated. The scientists posit that a huge increase in the release of sulphates and other airborne chemicals in the postwar era served to temporarily counteract the effect of greenhouse gases by deflecting sunlight and promoting cloud formation. This countertrend later subsided after air-quality regulations went into effect in North America and Europe.
The results of this study help confirm that human activity is directly related to global climate change, although trees in the southern hemisphere were not included because their growth patterns are not as seasonally visible.
So it seems that trees are helping to relate the story of climate change in new ways. I’m not surprised. Trees have proven to be one of mankind’s best natural resources and now they’re talking to us in ways we can understand. Whether people will listen is another matter.