A look at weather extremes and the big-picture climate connections.

Climate Change and the Phoenix Dust Cloud — What’s the Connection?

On Tuesday night, a massive dust storm rolled into Phoenix, Arizona knocking out power in much of the city, reducing visibility to nearly zero, and grounding flights overnight. Photos of the 100-mile wide dust cloud swallowing the city up circulated yesterday, and the event looked practically apocalyptic. In fact, if the photos weren’t in color, and there weren't YouTube videos of the dust storm, I would have thought I was looking at old-timey images from the 1930’s dust bowl. Now, a couple days later, lingering dust in the air has triggered allergy-like symptoms for many people.

On July 5, a massive dust cloud, or haboob, swept across Phoenix, Arizona, brought by the North American monsoon. Credit: Greg Gorman/flickr. 

This isn’t the worst dust storm Phoenix has endured, mind you; not by a long shot. This kind of immense dust cloud, known as a haboob, can stir up in the Southwest during North America’s monsoon season, and Phoenix can see several of these each year. According to the Wunderblog’s Jeff Masters, last night’s storm was brought on by:

a large complex of thunderstorms known as a mesoscale convective system (MCS) that developed to the east of Phoenix. As the outflow from the MCS hit the ground, large quantities of sand and dust became suspended in the air by 50 - 60 mph winds.

Tuesday's intense monsoon thunderstorm drove heavy winds down across the dry land, and whipped it up into a dust cloud that pushed ahead of rain that accompanied the storm. 

If you’re like me, you might not know much about the North American monsoon, but the Climate Assessment for the Southwest (CLIMAS) has a helpful website on the subject. With help from a couple of scientists at the University of Arizona, the site tackles the tricky question of how climate change might impact the monsoon, which brings beneficial rains to the region. 

As Chris Castro and Dave Gutzler write, there is still a lot of uncertainty about how the monsoon might change, if at all, because “the current generation of global climate models doesn’t come close to any consensus as to what the expectation is for a changed monsoon.” In the past 100 years, there hasn't been a detectable change in how variable the monsoon can be. However, they warn, warmer climates tend to reinforce the monsoon. As global temperatures increase with increasing greenhouse gas emissions, scientists predict the American Southwest will see a corresponding rise in temperatures, by 4-10°F, in the coming decades. 

A more prominent monsoon might sound inviting for the region, where 20-50 percent of average rainfall comes during the stormy summer months. The extra rainfall replenishes reservoirs and moistens farmers' fields temporarily during an otherwise hot, dry season. The intense thunderstorms can also cause flash floods in some areas, ignite wildfires in others, and, as we saw the other day, stir up massive dust clouds that drive ahead of the rain. 

As for the dust, you might recall that just a couple months ago I told you about some new research that suggests increasing temperatures in the Southwest (and specifically along the Colorado Plateau) could create more dust in the region. Hotter average temperatures mean the region could become even drier than it is already, making it harder for perennial grasses and plants to thrive. Without these grasses to keep the soil intact, it’s a lot easier for wind to pick the dust up off the ground. Even though the monsoons bring rainfall that can tamp some of the dust down, clouds like the one that formed on Tuesday stir up ahead of the rain, so drier ground in general could still contribute to these monstrous haboobs. 

In general, scientists predict that the entire Southwest is going to get drier over the next century, which means that, on average, annual precipitation will be lower than it is today. That doesn't mean, however, that the monsoon season could become more prominent and bring more rainfall during the summer — there just isn't any consensus yet on how much the summer rain will offset the overall trend towards warmer and drier conditions.

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