The effects of climate change may seem like a contemporary—and future—phenomenon. But a new study of tree rings shows that human-generated atmospheric CO2 and other greenhouse gases have been influencing the climate for well over 100 years, in particular by making droughts longer and more frequent.
Computer models that forecast the impacts of climate change don’t simply look into the future. They also estimate how the climate has changed since humans began releasing large amounts of greenhouse gases into the atmosphere beginning with the Industrial Revolution in the late 1700s. The problem is, researchers don’t have the same quantity or quality of precipitation records from that time period and periods before to assess if and how much the climate has changed.
That's why in the new study, the team used tree rings as a proxy for soil moisture since precipitation levels are reflected in each ring. In wet years, the rings are thicker, while in dry years, they are thinner. Lisa W. Foderaro at National Geographic reports that the tree rings, collected in drought “atlases” covering various parts of the world going back to 1400, fairly accurately reflect computer models that show how climate change should have affected soil moisture over the past 120 years. The research appears in the journal Nature.
“We asked, does the real world look like what the models tell us to expect?” study co-author Benjamin Cook, a climatologist at NASA’s Goddard Institute and Columbia University’s Lamont-Doherty Earth Observatory, says in a press release. “The answer is yes. The big thing we learned is that climate change started affecting global patterns of drought in the early 20th century. We expect this pattern to keep emerging as climate change continues.”
While researchers assumed that climate change has impacted the severity, length and frequency of droughts in the past, there has been little direct evidence. In the last climate report by the Intergovernmental Panel on Climate Change, they still hedged, pointing out that research had not established a link between drought and rising greenhouse gases.
The new study moves the needle toward establishing a link. “As a scientist, you are always surprised whenever anything you do works,” lead author Kate Marvel, also of Goddard and Columbia, tells Foderaro. “There were increased greenhouse gases in the early 1900s and the models say, ‘Hey, you should see a signal.’ But the fact that the signal is really clear in the models and apparent in the tree rings is pretty amazing. We can argue for a detectable human influence.”
The tree rings divide that twentieth century into three distinct periods. The first period, from 1900 to 1949, has the strongest fingerprint. When corrected for other climate factors, the study shows droughts increasing in Australia, central and North America, Europe, western Russia and southeast Asia—likely attributable to climate change. Other parts of the world got noticeably wetter including parts of China and Canada, central Asia, India and Indonesia.
Between 1950 and 1975, the climate change signal gets weaker, with droughts and increased moisture happening more sporadically. The researchers believe that during this period, when huge amounts of industrial aerosols were first released into the atmosphere, the increased smog might have briefly masked the effects of climate change.
Another phase began in the 1970s, as pollution controls leveled off the smog and as greenhouse gas emissions rose steeply, with signs of climate change reappearing in 1981. Since then the signal has remained, growing stronger around 2000. Over the next decade, the signal should be even more noticeable.
“If we don't see it coming in stronger in, say, the next 10 years, we might have to wonder whether we are right,” Marvel says in the release. “But all the models are projecting that you should see unprecedented drying soon, in a lot of places.”
And we likely won’t have to study tree rings to notice it. John Schwartz at The New York Times reports it’s predicted that major droughts will occur over agriculturally important areas of North America and Eurasia in the near future.
The message of the paper is that “climate change is really here and happening now and not something we can afford (in all meanings of that term) to continue to ignore,” as Friederike Otto, acting director of the Environmental Change Institute at the University of Oxford, tells Schwartz.
She says we’re now moving from an era where climate models can tell us simply that climate change is happening, to a period where studies can now help us predict climate impacts on a more local level.