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.

A year ago, the active ingredient in Roundup, the nation’s most widely used weed-killing herbicide, was added to California’s official list of chemicals known to cause cancer.
But Roundup’s critics say it’s hypocritical for one state agency to say the herbicide is a likely cancer hazard while another sprays it into a place where drinking water is pulled.
According the state, the IARC “found that glyphosate is an animal carcinogen and probable human carcinogen” based primarily on studies in which “rodents exposed to glyphosate developed tumors at higher rates than rodents not exposed glyphosate.” However, several other government agencies, including the U.S. Environmental Protection Agency, have concluded there is no evidence that glyphosate causes cancer.
Following the international agency’s listing, hundreds of lawsuits alleging glyphosate causes cancer were filed in state and federal courts across the country.
In addition, the custom blend of Roundup the state uses for aquatic weeds “is absorbed by the plants,” Madsen said.
It’s only active when it’s on the foliage of the plants.” Madsen and other scientists say the herbicide treatments will never completely remove the weeds, but they’re critical for keeping water flowing through the Delta’s 60,000-acre spiderweb of sloughs and river channels stretching from south of Sacramento and west of Stockton to the San Francisco Bay.
Chopping up the plants can harm native species while spreading seeds and other debris that can re-establish the plant elsewhere.
Fish habitats Clear channels for boats and water deliveries aren’t the only reasons for the herbicide treatments.
“Instead of providing good rearing habitat (for young fish), we’ve just got predator habitat,” said Sommer, the Department of Water Resources scientist.
“The rates that they’re using are not going to cause any fish kills.

Last year’s deluge of water was significant enough in Southern California to end a six-year drought in the region, but not enough to keep the drought at bay this year.
The U.S. Drought monitor finds most of San Diego County is experiencing moderate drought conditions with much of the region in the Los Angeles basin experiencing severe drought conditions.
The KPBS Drought Tracker follows rainfall and snowpack conditions, with the help of Scripps Institution of Oceanography, and both indicators are significantly below where they should be for this time of year.
Rainfall totals are well outside local norms for this time of year, according to the numbers compiled by Scripps researchers.
“Right now we’ve received about a third of the annual precipitation that we get so, on March 22nd we received about a third.
So normally we would’ve gotten about three-quarters of our yearly precipitation.
We’re quite a bit below that.
And the normal range that about two-thirds of the years fall into, we should have gotten at least half of our precipitation by now,” said David Pierce, Scripps climate researcher.
The Sierra Nevada snowpack is also only at about 57 percent of where it should be, although this week’s storms could push those number up noticeably.

State and regional water managers are considering permanently reinstating some watering bans and conservation programs.
Jerry Brown lifted California’s drought emergency status a year ago, after a wet winter that snapped a historic 2013-2017 drought, and the state ended his 25 percent mandatory conservation order.
Residents of an east Orange County water district used 203 gallons a day.
"I like the lawn," she said.
Residents of lower-income communities — with much less lawn — are some of the heroes when it comes to keeping water use down.
That includes residents of East Los Angeles, who used an average of 42 gallons a day, and people in Huntington Park, who got by on just 34 gallons.
U.S. drought monitors this month declared parts of Southern California back in severe drought, just months after the state emerged from that category of drought.
A winter of shorts and T-shirts, record warm days, and growing worry over water supply in Southern California are leading California’s Water Resources Control Board to consider next week whether to permanently reinstate some bans on water use that were imposed during the drought state of emergency.
Much of what snow does fall melts in place before spring runoff ever reaches the reservoirs that depend on it.
Water reservoirs are full from last winter’s welcome, near-record rain.

Jerry Brown lifted California’s drought emergency status a year ago, after a wet winter that snapped a historic 2013-2017 drought, and the state ended his 25 percent mandatory conservation order.
The average residential user in one Malibu water district, for instance, used 255 gallons a day, according to the state water board — three times the U.S. average of 83 gallons per person per day.
Residents of an east Orange County water district used 203 gallons a day.
“I like the lawn,” she said.
Residents of lower-income communities — with much less lawn — are some of the heroes when it comes to keeping water use down.
That includes residents of East Los Angeles, who used an average of 42 gallons a day, and people in Huntington Park, who got by on just 34 gallons.
U.S. drought monitors this month declared parts of Southern California back in severe drought, just months after the state emerged from that category of drought.
A winter of shorts and T-shirts, record warm days, and growing worry over water supply in Southern California are leading California’s Water Resources Control Board to consider next week whether to permanently reinstate some bans on water use that were imposed during the drought state of emergency.
Much of what snow does fall melts in place before spring runoff ever reaches the reservoirs that depend on it.
Water reservoirs are full from last winter’s welcome, near-record rain.

Forecasters are watching an area of low pressure and several reinforcing shots of atmospheric energy that are sinking south along the coast out of the Pacific Northwest.
Related: Wildfire worries as drought worsens in Southwest, Plains Weather Highlights: An upper-level atmospheric disturbance will bring some rain and snow to parts of Southern California and the Southwest this week Showers, and a few thunderstorms, are expected to move into the central and south coast of California through the day on Monday More significant rainfall – as well as heavier snow at elevation – is expected for the Inland Empire, extreme Southern Sierra, and toward the Four Corners Unsettled conditions and onshore flow continue through midweek Upper-level low expected to bring more significant moisture onshore as it moves in later week, but how far north or south this falls is still up for debate Most long-range model guidance suggests low may be too far south, over Baja, to bring much rain to SoCal, but should bring locally heavy rain to Arizona/Four Corners region Stay on top of active weather | Visit our warnings and alerts page Watch below: Tracking rain and snow this week Play Video Play Mute 0:00 / 0:00 Loaded: 0% Progress: 0% Stream TypeLIVE 0:00 Playback Rate 1x Chapters Chapters Descriptions descriptions off, selected Subtitles undefined settings, opens undefined settings dialog captions and subtitles off, selected Audio Track Fullscreen This is a modal window.
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February is typically one of the wettest months in California, with Los Angeles collecting an average of 3.8 inches and San Francisco 4.45 inches.
As of February 8th, the Southern Sierra was at only 20 percent of its normal snowpack for the date, and a paltry 13 percent of its usual April end-of-season average.
Thunderstorms may also boost amounts over some areas, raising the concern somewhat for the possibility of mud and debris flows.
Stay with us here at The Weather Network, and follow us on Twitter and Facebook as we continue to update this story as it develops.
With files from The Weather Network meteorologist Mario Picazo.

PHILLIPS STATION, Calif. (AP) — California’s brief escape from severe drought ended Thursday after scientists declared more than 40 percent of the state in moderate drought and water officials confirmed lower-than-normal snowpack in the Sierra Nevada.
Los Angeles, Ventura and Santa Barbara counties, which hold nearly a quarter of the state’s population, were rated in severe drought.
During a week of rainless skies and some record-high temperatures in Southern California, water officials also trekked into the Sierra Nevada on Thursday and manually measured the vital snowpack, which stood at less than a third of normal for the date.
In Ventura and Santa Barbara counties, which are about 100 miles north of Los Angeles, the lack of rain and dry vegetation fueled a December wildfire that grew to be the largest recorded in state history.
“I know we need rain, but another mudslide would be awful,” said Santa Barbara restaurant hostess Cayla Stretz.
Survivors in her area are still digging out homes, many beaches and roads are closed by mud, and business is down in the beach town, Stretz said.
Last year’s rain has most of the state’s reservoirs higher than usual, a bright spot, said Doug Carlson, a state Department of Water Resources spokesman.
During the peak of the state’s dry spell, 99.9 percent of California was in some stage of drought and nearly half fell into the very highest category.
The years of disappointing winters and competition for remaining water supplies devastated native species ranging from Chinook salmon to pine forests, dried many household wells in the state’s middle, and compelled farmers to plunge other wells deep into the earth in search of irrigation water.
In California’s Central Valley, the nation’s richest agricultural producer, government officials had to install water systems during and after the five-year drought for small towns such as East Porterville, after household wells dried.

PHILLIPS STATION, Calif. — California’s brief escape from severe drought has ended after scientists declared more than 40 percent of the state in moderate drought and water officials confirmed lower-than-normal snowpack in the Sierra Nevada.
Los Angeles, Ventura and Santa Barbara counties, which hold nearly a quarter of the state’s population, were rated in severe drought.
During a week of rainless skies and some record-high temperatures in Southern California, water officials also trekked into the Sierra Nevada on Thursday and manually measured the vital snowpack, which stood at less than a third of normal for the date.
“It’s not nearly where we’d like to be,” Frank Gehrke, a state official, said of the snow, which supplies water to millions of Californians in a good, wet year.
‘We need rain’ In Ventura and Santa Barbara counties, which are about 100 miles north of Los Angeles, the lack of rain and dry vegetation fueled a December wildfire that grew to be the largest recorded in state history.
“I know we need rain, but another mudslide would be awful,” said Santa Barbara restaurant hostess Cayla Stretz.
Survivors in her area are still digging out homes, many beaches and roads are closed by mud, and business is down in the beach town, Stretz said.
During the peak of the state’s dry spell, 99.9 percent of California was in some stage of drought and nearly half fell into the very highest category.
Cutbacks at the peak of the five-year state drought mandated 25 percent conservation by cities and towns.
In California’s Central Valley, the nation’s richest agricultural producer, government officials had to install water systems during and after the five-year drought for small towns such as East Porterville, after household wells dried.

California is rapidly plunging back into drought, with severe conditions now existing in Santa Barbara, Ventura and Los Angeles counties — home to one-fourth of the state’s population, a national drought monitor said Thursday. The weekly report released by the U.S. Drought Monitor, a project of government agencies and other partners, also shows 44 percent of the state is now considered to be in a moderate drought. It’s a dramatic jump from just last week, when the figure was 13 percent. “It’s not nearly where we’d like to be,” Frank Gehrke, a state official, acknowledged after separately carrying out manual measurements of winter snowfall in the Sierra Nevada mountains, which supplies water to millions of Californians in a good, wet year. Overall, the vital snowpack Thursday stood at less than a third of normal for the date. California lifted a drought state of emergency less than a year ago, ending cutbacks that at the peak of the drought mandated 25 percent conservation by cities and towns, devastated generations of native salmon and other wildlife, made household wells run dry in the state’s…

California is rapidly plunging back into drought, with severe conditions now existing in Santa Barbara, Ventura and Los Angeles counties — home to one-fourth of the state’s population, a national drought monitor said Thursday.
The new figures from national drought monitors came amid growing concern among state officials about another dry winter.
The region is now seeing record-setting heat.
The readings detailed Thursday show the drought has worsened to the severe category in 5 percent of the state.
However, Thursday’s figures were far better than those during the peak of the state’s epic dry spell, when 99.9 percent of California was in some stage of drought, and nearly half in the highest category.
But the drought never really seemed to lift in some Southern California areas, Daniel Swain, a climate scientist at University of California, Los Angeles, noted this week.
When it finally rained, the scorched earth turned into mudslides that sent earth, water and boulders roaring through neighborhoods.
In California’s Central Valley, the nation’s richest agricultural producer, government officials had to install water systems during and after the five-year drought for small towns such as East Porterville after household wells ran dry.
"it never ended," she said of the drought in her area.
Electronic sensors showed statewide snow levels at 27 percent of normal.