TUCSON – Tree rings going back 800 years are giving researchers at the University of Arizona a window into how climate change could expand the planet’s most extreme deserts, including the Sonoran, which extends from the Baja Peninsula into Southern California and much of southern Arizona.
“We see a trend, atmospherically speaking, that the tropical region is moving further north in the Northern Hemisphere,” said Valerie Trouet, a dendrochronologist and associate professor at the Laboratory of Tree-Ring Research at the University of Arizona.
“We can determine how the edge of the tropics has moved over the last 800 years,” Trouet said.
Trouet, who co-authored a study published in October in the journal Nature Geoscience, found that the expansion of the tropics northward from 1568 to 1634 coincided with severe droughts, the collapse of Turkey’s Ottoman empire and the end of China’s Ming Dynasty.
“Our results suggest that climate change was one of the contributing factors to those societal disruptions,” said Trouet in a recent UA News article about the study.
The team’s findings are important because they could help explain how ongoing droughts could change some of the planet’s desert regions, including the Sonoran.
The Sonoran Desert lies on the edge of the tropics, where air-driven atmospheric circulation sinks.
These huge atmospheric circulations are known as Hadley cells and they are the primary driver of the tropics.
The National Weather Service describes Hadley cells as warm air rising at the equator, and sinking around 30 degrees latitude north and south.
UA researchers combined tree-ring data from five mid-latitude regions in the Northern Hemisphere.

California Overcame 1/100 Odds to Beat Its Epic Drought.
Never tell California the odds.
Not only has the state recovered from its record-breaking drought, it did so in record time.
According to a new NOAA study looking at 445 years of climate data, California had a 1 percent chance of breaking the drought in just two years.
“What we see in the historic record is an increase in the likelihood of warm and dry periods, punctuated by wet conditions,” says Noah Diffenbaugh, climate scientist at Stanford’s Woods Institute for the Environment.
“What we used are existing climate reconstructions based on tree ring data, which show river and stream flows,” says Eugene Wahl, the study’s lead author and a paleoclimatologist for NOAA’s National Center for Environmental Information.
To make sure their correlation was sound, Wahl and his co-authors calibrated tree rings to stream flow for over 60 years of data—from 1916 to 1977.
All this legwork was really so Wahl and his team really could figure out the rate at which California recovered from its most recent drought relative to those in the past.
The following year, the bucket gets a little more water.
A little more the following year.