What is global warming

Global warming refers to an increase in the average temperature of the Earth's atmosphere.

Many factors can contribute to climate change and warming — but much attention has been focused in recent years on the role of so-called greenhouse gases stemming from industrial emissions and other human activity. These gases, including carbon dioxide and methane, act something like a greenhouse in that they trap the sun's heat in the atmosphere and cause an increase in temperatures.

Forests in flames

If you've lived in Southern Arizona in recent years, you've witnessed what scientists see as a blazing byproduct of climate change: a huge increase in enormous, devastatingly destructive forest fires.

In 2002, Tucsonans watched as the Bullock Fire ravaged 30,563 acres of the Catalina Mountains. The next year, we saw the Aspen Fire light up the night sky above the city as the blaze consumed 84,750 acres and all but wiped out the mountain community of Summerhaven. Big as they were, those blazes didn't even come close to the state's record wildfire — the Rodeo-Chediski Fire that consumed more than 460,000 acres along the Mogollon Rim in 2002.

Scientists say warming temperatures are likely one cause — but not the only one — behind a sharp increase in the number and size of forest fires.

A recent study found that "6.7 times more area was burned by large forest fires occurring primarily on federal lands in the 11 Western states from 1986 to 2003 as compared to 1970 to 1986," says researcher Thomas Swetnam, director of the UA Laboratory of Tree-Ring Research. "We're also seeing that the fire season has expanded by two months or more in the Southwest — starting in April and extending to September."

What's the connection between global warming and the increase in wildfires?

Swetnam put it this way in a recent report: "My colleagues and I found that increasing spring and summer temperatures were correlated with increasing numbers of large forest fires in the past few decades, with the evidence indicating that earlier snowmelt acts to dry out forest fuels, triggering synchronous, large forest fires throughout the West."

Swetnam is quick to emphasize that other causes, in addition to warming, likely play a role in escalating forest infernos. Among those causes are extensive livestock grazing and a long tradition of fire suppression by land-management agencies — a practice leading to the buildup of fuels that eventually erupt into major conflagrations.

Saguaros in peril

The landscapes of Saguaro National Park — bristling with majestic saguaro cacti and other superstar species of the desert — are in danger of drastic changes brought on by an invasive species called buffelgrass.

And, according to some scientists, it's partly global warming that's allowing the grass to spread like wildfire.

Buffelgrass "could convert desert landscapes into an African grassland" devoid of saguaros, warns Meg Weesner, chief of resources management at Saguaro Park.

Weesner couldn't predict how soon the grassland scenario might come to pass, but other scientists already see signs that buffelgrass is taking over the landscape around Tucson.

"This is happening big time," says Julio Betancourt, a scientist for the Desert Laboratory on Tumamoc Hill. "We're looking at a wholesale transformation of the Sonoran Desert."

How could the African plant — introduced in the Southwest during the 1930s to stabilize soil and feed livestock — so drastically alter the park's landscape?

In at least two ways, Weesner says.

"It grows so thickly and develops such a thick mat that other seeds can't get any sun and access to water," Weesner says. "So, as buffelgrass spreads into an area, it out-competes many of our native species."

The second way buffelgrass might alter the park is by fueling massive, hot-burning wildfires that could wipe out fire-sensitive native plants.

"Native desert plants," Weesner notes, "are widely spaced. So when we have lightning strikes or other causes of fires, the fires stay small and don't spread. But if you get a lightning strike or human-caused fire in thick buffelgrass, it has the potential to spread hundreds and hundreds of acres."

Such fires would devastate desert plants, "which just aren't designed to sustain fire," says Weesner. "Palo verde trees will die with fire because their bark is so thin. Saguaros less than about three feet tall will die.

"And the sad thing is, buffelgrass does not die. It loves fire. It sprouts back and produces another batch — ready for the next match to strike."

How does global warming contribute to the spread of buffelgrass?

Scientists say they have no proof — but many strongly suspect that warmer winter temperatures in the past two decades may have set the stage for a buffelgrass boom.

"When the species was brought to Tucson in the 1930s, reports indicate that it acted like an annual — with the plants almost always getting frozen back almost entirely in the wintertime," says Sue Rutman, the Organ Pipe Monument plant expert. "But sometime in the early- to mid-1980s, Arizona experienced a bump-up in winter temperatures.

"Apparently, that was the foot in the door that buffelgrass needed," she says. "It was no longer freezing off the way it had been. This is just from observations, but it started to behave more like a perennial."

A buffelgrass battle is under way to slow the spread of the species. A group called the Sonoran Desert Weedwackers works diligently to manually remove buffelgrass, while Saguaro Park workers use an herbicide to try to stop it.

A migrating desert

A landmark 2005 study by two UA scientists — called "Is the Sonoran Desert Losing its Cool?" — concluded that "the overall boundary of the Sonoran Desert may contract in the southeast and expand northward, eastward and upward in elevation."

Familiar desert vegetation around Tucson could gradually lose ground to plants more suited to hotter weather. The suspected cause of the shift: human-caused global warming.

Those were the findings of UA scientists Jeremy Weiss and Jonathan Overpeck.

Shrinking Cienega Creek

The Cienega Creek Natural Preserve southeast of Tucson is home to a rare gem in desert country — year-round flowing water.

But recent studies of the creek brought a disturbing finding: a decline in length of the surface flow.

"The average flowing length annually since 2001 is 3.9 miles, whereas from 1989 to 1999, it was 7.7 miles," says a November 2007 report to the Pima County Regional Flood Control District.

Scientists say warming could be one factor in drought conditions contributing to the reduced flow. But they emphasize that other factors such as groundwater pumping and normal variations in rainfall might also shrink the creek.

Whatever the cause, "the decline in surface flow is troubling — partly because it affects habitat for native fish and frogs," including some endangered species, says Julia Fonseca, environmental program manager for Pima County Natural Resources, Parks and Recreation.

If the length of the surface flow continues to decline, it means less and less habitat for fish and frogs — and less beautiful, rejuvenating terrain for people to enjoy.

Fruit failures

Apple trees need to chill out — literally — if they are to bear abundant fruit.

Continued warming temperatures, which are predicted for Southern Arizona if nothing is done to curb greenhouse gases, could eliminate the winter chill needed to nurture orchards near Willcox.

"If temperatures keep warming, it could reduce the apple crop," says Rob Call, horticulture educator with UA's Agriculture College. "Eventually, it's possible that orchards could no longer be productive because these are temperate fruits — not citrus or tropical fruits."

Call says the critical factor is something known as a "chill unit."

"When fruit trees experience an hour at temperatures of 32 to 45 degrees, that's one chill unit," he says.

Apples require 1,200 to 1,600 units of chill over the wintertime, depending on the variety. "When they don't get enough chill units, they just don't have the flowering or fruiting they normally would," Call says. "There's not as much yield."

So far, warming's impact on orchards is more of a future threat than a current reality. "We have been raising fruit here in Willcox since 1981, and we've not seen any trends or changes in the weather or number of winter chill hours at this point," says Anne Holcomb, owner of Apple Annie's Orchard.

"Last year we had the highest number of chill hours in many years, and this year have accumulated enough for the trees to bloom on schedule and with each variety blooming over a few days to a week," Holcomb says.

Holcomb says warming "potentially could be a problem for some types and varieties of fruit." But she maintains that growers could adapt to a decline in chill hours by focusing on fruits with modest chill requirements.

Birds at risk

Winged inhabitants of Southern Arizona deserts and uplands are in danger of declining as warming temperatures affect their habitats, scientists say.

In particular peril is the endangered Southwestern willow flycatcher.

In a report called "Potential Impacts of Global Climate Change on Bird Communities of the Southwest," three researchers from the U.S. Geological Survey concluded that "one species that would be most greatly impacted would be the endangered Southwestern willow flycatcher."

Flycatchers — brownish-olive to gray-green birds that are a little less than 6 inches long — commonly live in dense thickets along rivers in the West. Surveys put the estimated population at 1,250 territories, with one to two birds in each territory.

Continued warming and resulting drought conditions could increase the survival challenge for flycatchers, says Tice Supplee, director of bird conservation for Audubon Arizona.

"If drought is persistent enough to cause the reduction of surface water flows and thus loss of riparian habitats suitable for the Southwestern willow flycatcher, the species will continue to decline," Supplee says. "Warming temperatures may affect migration patterns, causing birds to arrive too soon for needed food and nesting resources — but we have not seen any effects as yet."

More warming signs

Here's a partial list of some other environmental impacts in Arizona that have been linked to global warming — in most cases through a connection with a drought made more severe by rising temperatures:

• Stands of some species of fir and spruce trees near the top of the Pinaleno Mountains near Safford have declined — due to fires and bark beetle infestations related to warming and drought. "Species at the very tops of the mountains might have no place to go if temperatures keep rising in the Coronado" National Forest, says Craig Wilcox, a tree expert with the U.S. Forest Service.

• Piñon pine trees in parts of Northern Arizona and along the Mogollon Rim have died off in areas where they once thrived.

• Water levels in Lake Powell on the Arizona-Utah border have dropped dramatically — with a recent rebound brought by increased winter precipitation.

• Some neotropical birds, such as thick-billed kingbirds and five-striped sparrows, have spread northward from Mexico into Southern Arizona sites such as Madera Canyon in the past two decades.

What the skeptics say

Robert C. Balling Jr., former director of Arizona State University's Office of Climatology, has been a prominent skeptic of the greenhouse-gas theory behind warming trends, although he has moderated his views.

"It is very likely that Arizona will warm in the future, but at what level, I don't know if anyone can pin that down," he says.

"I'm ready for a surprise. I can see a thousand things that could derail it," Balling adds. "The output of the sun could alter the future of the temperature rise of the planetary system and that could catch us a little bit off-guard and derail the global warming panic a bit. . . . Solar variability could force us into a cooler period."

Another research scientist, Roger A. Pielke Sr. of the University of Colorado, says the Intergovernmental Panel on Climate Change looks at the warming issue too "myopically," by focusing exclusively on the effects of carbon dioxide.

The panel should also look at how land-use changes such as agriculture and urban growth affect climate, says Pielke, an environmental studies professor.

Other potential causes, he says, include natural variation in solar radiation, as well as how small particles known as aerosols scatter and absorb radiation.