Ecosystems across the world will dramatically transform as climate change's effects increase, a new study warns. Arizona's forests could retreat with rising temperatures and its deserts could turn hotter and more volatile in the coming century.

The study says human-caused climate change could accelerate changes in vegetation around the globe, filling lush forests with flammable brush and worsening drought conditions where relief is needed most.

Change is nothing new: In the thousands of years since the last ice age, the planet has evolved significantly, gradually turning glacial conditions to grasslands and grasslands to desert landscapes.

It's poised to happen again, the study's authors found, only this time, what took millennia would unfold in less than 200 years, bringing colossal impacts for Arizona's forests, vegetation and water supply.

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The findings are part of a University of Arizona-led report published in the journal Science, which warns that the earth could warm as much as it did in the thousands of years since the last ice age if greenhouse gas emissions are not substantially reduced.

Researchers found changing climates around the globe, but particularly in Arizona and the arid Southwest, where historic drought conditions are showing little signs of relief.

Over the past 20 years, wildfires have singed more than 1.5 million acres of Arizona pine forests. Climate change and drought have made the fires more costly and frequent. Experts say the forests may not come back and, if they do, they won't be the same.

"I think it's ubiquitous," said co-author Jonathan Overpeck, University of Michigan dean of the School for Environment and Sustainability. "But I think Arizona's the canary in the mineshaft."

A poster child for climate change

Although Arizona's drought and shifting landscape provide a case study in climate change, the new report says it's seen everywhere on Earth.

As temperatures increase, ecosystems change along with them. The ecosystem's structure — what makes it a desert, a forest, a tundra or anything else — also changes.

"It's really in the Southwest that we're starting to already see the dramatic changes that we're talking about," Overpeck said.

Research for the new report took place over five years and looked at how increased temperatures after the last ice age correlated to changes in vegetation. They used that analysis to project how much ecosystems could change in the next 100-150 years as the world continues to warm.

Their finding: regions with the largest increases in temperature also had the greatest changes in vegetation.

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By itself, it's not a new finding. In the early 2000s, millions of trees near the Four Corners started dying in piñon-juniper woodlands because of increasingly dry conditions, Overpeck said. Drought weakened the forest and high temperatures didn't help, but packs of bark beetles came in and finished the job.

Those trees are slow growers, so experts at the time predicted it would be years before there were woodlands in the area again. And when something does grow, it's something better suited for the changing climate.

"That was really one of the first poster childs of the forest dieback that we're seeing in the Southwest and around the world," Overpeck said. "Where it's occurred in Arizona, it's essentially grown new vegetation that is in equilibrium with the warming climate."

Researchers examined every continent except Antarctica and say it's the most exhaustive look at the ecological data spanning the time between the last ice age 21,000 years ago and the pre-industrial era.

'We're going to warm it up even more'

Arizona hasn't always been a desert.

After the last ice age, much of what is now considered Arizona's arid desert was piñon-juniper woodlands, like what's found in the Four Corners region, Overpeck said. As temperatures increased over thousands of years, it gradually shifted from woodlands to grassland to desert.

"That's exactly the kind of structural change we're talking about being at risk in the future," he said. "That's a huge change ... now, we're going to warm it up even more. It will be just as dramatic a change."

Only instead of 21,000 years, it will take less than 200, he said. Cramming that much change into so little time will make it more difficult for ecosystems to adapt to the warming climate.

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"It's going to be a lot more chaotic," Overpeck said. "If you're living in this century, you're likely going to see it happen. You've already seen it happen in Arizona."

While researchers can't predict exactly when, where and how these changes will occur, there are some warning signs.

"Let's go down into the southern reaches of the Sonoran Desert and see how that differs from the northern," said Overpeck, a UA researcher until recently. "There's more brush, less of the succulents that you see up in our part of the desert."

That could be the first step in Arizona's desert transformation as climate change shifts vegetation.

As the climate changes, it could open the door to more invasive species of vegetation. Grasses that typically thrive in the Mediterranean and Africa could crop up. The only problem: They're very flammable and could put other desert plants at risk.

"They do well in our desert, but they're introducing wildfire," Overpeck said. "In contrast to the pine forest where you get a lot of fire naturally, you don't get it in the desert. The plants there are not adapted to fire, they don't do so well when you deal with warming."

'Water is a limited resource'

The research comes amid reports that Lake Mead is declining toward a shortage and as water managers work on finalizing a drought contingency plan in time for the next state legislative session.

Losing the forests doesn't just affect the forests.

The new growth is usually less water-sensitive, putting less moisture into the atmosphere as rain and bringing fewer clouds as a result. That means more dust, which contributes to warming and drying effects.

All in all, it doesn't spell good news for ongoing drought conditions.

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"Water is a limited resource and you're going to have less of that," Overpeck said. "This is the drought of record since we've had rain gauges ... the biggest reservoirs in the country are less than half full cumulatively."

The harsh drought sweeping the Southwest may be more than drought — it could be a more permanent aridity, which Overpeck and others call the "new normal." It exacerbates extremes.

"The dry periods will be drier and the wet periods will be less wet," he said. "It's a different way of looking at drought, as the science community is starting to realize."

Is it too late?

Although the new research paints a bleak picture, Overpeck said it's not too late to act.

"We have time to reduce our greenhouse gas emissions, shoot for those Paris targets as much as we can," he said. "If we were able to curb the warming along the lines of what we agreed to in Paris, there would be a significantly lower risk in any part of the world."

The Paris Agreement aimed to keep global temperature increases "well below" 2 degrees Celsius above pre-industrial levels to reduce climate change's impacts.

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Action may not prevent change altogether, but it can help mitigate the extremes, he said.

It's not too late, but a lot in Arizona is at stake, along with most every ecosystem in the world, he said.

"It could really change the desert," he said. "It's not just going to be isolated. It's going to be everywhere if we let climate change go unchecked."

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow The Republic environmental reporting team at OurGrandAZ on Facebook, Twitter and Instagram.

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