Still-Rising Average Temperatures Fuel Deadly, Unpredictable Weather

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Arizona catches fire.

The drought returns.

The monsoons flicker.

The streams dwindle.

The spring goes bone dry.

Tornadoes touch down on the Rim.

What the heck’s going on around here?

A rush of new climate studies suggest that the steady rise of global temperatures has already ushered in dramatic and unpredictable weather patterns — inflicting a sputtering drought in the Southwest at the same time it floods the Mississippi and sends tornadoes rampaging across the Midwest.

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Photo courtesy of Apache-Sitgreaves National Forests

An air tanker drops a load of flame retardant in a vain effort to keep the Wallow Fire from becoming the largest in Arizona history. Climate experts said rising temperatures will spawn more fires.

The predictions suggest that Rim Country should brace for an unpredictable mixture of drought and massive storms, making the region far more vulnerable to massive wildfires.

The early onset of the fire season this year underscores the point, with half the normal rainfall since January — despite some big winter storms that left a deeper-than-normal snowpack on the Rim.

A recent report by the National Academy of Sciences concluded that “Climate change is occurring, is very likely caused primarily by the emission of greenhouse gases from human activities, and poses significant risks for a range of human and natural systems.”

Most climate experts agree that average global temperatures have risen by 2 degrees in the past 50 years and rainfall has increased by 5 percent. The number and intensity of most major weather systems has risen with the increased energy pumped into the atmosphere by that rise in temperature.

However, the attempt to link that broad average temperature rise with particular weather patterns and regional predictions has yielded mixed results — and revealed the complexity of the feedback systems that either muffle or magnify the effects of global warming.

However, the evidence now suggests the Southwest will likely suffer a big rise in fires and tree-killing insect infestations in coming decades, according to a study published recently in the Proceedings of the National Academy of Sciences. Forests in the Southwest have proven especially vulnerable to climate shifts, because the trees are often living at the edge of their tolerance. The researchers analyzed the results of 1,000 different tree ring studies and concluded that 18 percent of the forests in the Southwest have suffered severe damage from fires or bark beetles in the past 20 years.

That has big consequences for areas like Rim Country, which sits on a delicate ecological edge. Here at the base of the Mogollon Rim, the pinon-juniper habitat meets the dry edge of the world’s largest continuous ponderosa pine forest. Small temperate shifts can make a huge difference for areas perched on that temperature and rainfall boundary between major habitats.

For instance, ongoing temperature changes will likely drive ancient Joshua trees from 90 percent of their range in the next half century — eliminating most of the stands in Arizona, according to a study led by U.S. Geological Study ecologist Ken Cole.

The study used the fossil record to match the range of the giant, yucca-like desert trees to past temperature swings. Turns out, now-extinct giant ground sloths dined on the fruit of the towering, bristly Joshua trees before the last Ice Age. Fossilized ground sloth dung in caves revealed the range of the Joshua trees going back tens of thousands of years. In addition, pack rats readily gathered up Joshua tree fruits and stashed them in their giant middens. Fortunately for scientists, the pack rats used urine to cement the walls of their fortress-like middens, which preserved the plant material.

As a result, the scientists discovered that during a still largely unexplained warming period 12,000 years ago, the range of the Joshua trees shrank dramatically. The Joshua trees managed to recover from that event largely because the ground sloths spread Joshua tree seeds far and wide in their droppings, enabling the trees to spread again once the climate cooled. This time, no large animals ate the Joshua tree fruit. That means the Joshua trees can only shift in response to warming trends by six feet per year on average — not enough to stay ahead of the effects of the projected warming.

Meanwhile, two other studies have demonstrated the close connection between rainfall in the Southwest and surface temperatures in the oceans.

Rising temperatures will likely intensify the effects of the well-known El Niño pattern in the Pacific, which triggers droughts and flooding world-wide as a result of the warming of surface temperatures, according to the study of tree ring growth patterns going back 1,100 years published in May in Nature Climate Change.

The researchers found that the rainfall-related width of tree rings in the Southwest agreed perfectly with other records showing the waxing and waning of the El Niño ocean temperatures. Moreover, the temperature-related variations in certain isotopes in corals in the central Pacific also matched the tree ring testimony in the Southwest.

The researchers concluded that ocean temperatures in the Pacific have long gone through warm and cool phases that can last 50 to 90 years and have a big impact on global weather patterns.

Unfortunately, scientists haven’t yet come up with reliable and consistent climate models that would allow them to predict how a rise in average global temperatures will affect those cycles.

A different set of studies have come to a similar conclusion about a less-well-known water temperature cycle in the Atlantic Ocean, which scientists call the Atlantic Multi-Decadal Oscillation.

Another tree ring study linked the periodic rise in surface temperatures in the western Atlantic with a sharp increase in wildfire in the Southwest over a 500-year period. The researchers discovered that the western Atlantic shifts has warm and cool periods that last for 60 to 80 years at a time. The researchers analyzed nearly 34,000 fire scars dating back five centuries to reach that conclusion.

Generally, as the western Atlantic warms, the number of extended droughts and massive wildfires increases throughout the Southwest.

The researchers noted that the western Atlantic appears to have entered just such a warming period, which will likely be exacerbated as global temperatures rise. The researchers noted that such major climate shifts have a far larger impact on forest fires than shifts in forest management strategies.

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