Look carefully, and tree rings can reveal quite a lot – including changes in the soil moisture that the trees are growing in. Now, scientists have collected 600 years of this data to better understand recent climate change, including a weather anomaly that was first noticeable in the middle of the 20th century.
The new records make up the latest edition of the South American Drought Atlas (SADA), showing moisture variability over the past six centuries backed up by other historical records. Meanwhile, periods between intense droughts have been on the rise since the 1930s, with one drought every ten years being measured since the '60s.
What the rings can't tell us is exactly how these extremes came about, but the researchers behind the publication are hoping that the record will be a useful reference point to be used in combination with other sets of data and observations.
"Increasingly extreme hydroclimate events are consistent with the effects of human activities, but the Atlas alone does not provide evidence of how much of the observed changes are due to natural climate variability versus human-induced warming," says palaeoclimatologist Mariano Morales from the National Research Council for Science and Technology in Argentina.
This new edition of SADA highlights changes across Argentina, Chile, Uruguay, Paraguay, most of Bolivia, southern Brazil, and Peru, using data collected in the field from a total of 286 trees.
South America has an "acute vulnerability" to extreme climate events, according to Morales, and indeed recent droughts have led to a dire situation for agriculture in vast areas of the continent. Certain food systems are now under threat of collapse.
The current picture is a varied one though: while parts of Argentina and Chile are suffering one of the worst droughts on record, regions in the southeastern part of the continent are experiencing abnormally wet conditions. If the underlying causes aren't climate change, they certainly look a lot like climate change.
"We don't want to jump off the cliff and say this is all climate change," says palaeoclimatologist Edward Cook from Columbia University. "There is a lot of natural variability that could mimic human-induced climate change."
The researchers identify three key factors at play in the swings of the last sixty years or so: cyclic sea-surface temperature shifts over the Pacific and the Atlantic, a belt of westerly winds around Antarctica called the Southern Annular Mode, and the Hadley cell phenomenon that distributes warm and moist air from the equator.
Shifts that can be attributed to greenhouse gases and the ongoing legacy of ozone-depleting chemicals are interfering with these key factors, the researchers say, and potentially feeding into the swings that are now being seen in South America.
The SADA team hopes that the tales told by the tree rings can give us a better understanding of long-term climate shifts, contemporary conditions, and how to plan for the future. The way that we're currently treating the planet suggests that our weather extremes won't be stopping anytime soon.
"Everything is consistent with the idea that you'll be intensifying both wet and dry events with global warming," says climate scientist Jason Smerdon, from Columbia University.
The research has been published in PNAS.