The University of Arizona

Dry Heat and Other Climate Extremes

July 5, 2011
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Even before this weekend’s record heat in Arizona, climate and weather extremes have been in the news a lot. Unusually severe drought in the Southwest, stretching through Texas, started it off. The resulting parched landscapes contributed to record wildfires in Arizona and New Mexico. Elsewhere, tornados compelled our attention, killing hundreds and leaving swaths of destruction. Adding to the chaos, we saw epic flooding in the Mississippi and Missouri river watersheds. And, let’s not lose track of what’s going on elsewhere in the world – for example, another extreme drought and heat wave in Europe may be afoot. What’s going on? What does this unusual succession of climate and weather extremes tell us about global warming and our future?

Scientists and non-scientists alike have been quick to offer up all kinds of interpretations, but behind it all has been a strengthening scientific consensus that global warming will bring more climate extremes to many parts of the world. But, that’s too sweeping to be useful. What matters is what kind of extreme, when and where.

In the Southwest, episodic drought will be a continued part of our future, and climate change will stack the odds towards hotter and hotter, more and more harmful, drought. As we’ve seen only too well in the last decade, this means more, and bigger, wildfires. No big surprise, but depressing just the same.

More interestingly, precipitation intensity – the amount of rainfall for a given period of time (like inches of rain per hour) – is rising. More of our precipitation is coming in fewer events. Climate theory has long predicted this, and it’s happening across much of the globe, including in the U.S. Even worse, this change may also be happening more quickly than the climate models predicted – see the recent Min et al. (2011) in Nature.

More intense precipitation events, combined with more snow falling as rain and earlier snow melt, will likely lead to increased flooding in the Southwest.

What’s strange is that the increase in precipitation intensity isn’t yet causing a noticeable increase in flooding. Stay tuned, however, since it seems to me that this outcome is bound to happen. Add in the observed earlier melting of snow in the spring, plus the increasing tendency for spring precipitation to fall as rain rather than snow, (also a result of warming), and an increased risk of spring flooding seems inevitable, at least in mountainous regions. On the coasts, rising sea level clearly will exacerbate flood risk. Any way you look at it, more flooding in many parts of the globe, the U.S., and certainly the Southwest, is a solid bet. We just need to let some time go by before it becomes really obvious, even to skeptics.

But, let’s say you’re a betting person and want to know where to put your money on changing climate and weather extremes. What’s the sure bet? Well, look at Phoenix over the July 4th weekend, where a record of 118°F blasted past the old record of 116°F for July 2, set 10 years ago. Yuma and Tucson also experienced record heat. Moreover, we’re now breaking heat records twice as often as cold ones. If the globe weren’t warming, we’d be breaking both warm and cold records at about the same clip. But now, it’s a sure bet that we’re going to see more and more record high temperatures. Not in any given year or even decade, but the impacts of climate will become clear over the long haul. The projections are for hotter and hotter temperature extremes. You can bet on it.

But, as solid as that sounds, it’s still a bit academic. What does this mean to someone in Phoenix, Tucson, Yuma, Las Vegas, or any other Southwest locale? It means take a really hot day – let’s say 118°F on July 2, 2011 – and add 5°F to get a sense of what will be the norm on that date by 2050, or add 10°F by 2100. That means nearly 130°F in early July in Phoenix. That’s obscenely hot in just about anyone’s book. You could also muse on how early in the year Phoenix will reach 100°F and just stay hot for months on end.

What does this mean? Record hot temperatures, as well as record heat waves, were projected long ago by climate scientists. Now we are observing this projection coming true. Sure, it’s dry heat, but at some point it will just get to be too much for most folks. It means more air conditioning and more energy consumption. It means growing urban heat island effects will only make it worse for those in cities, and it’ll make the droughts worse everywhere. That’s the face of global warming in the Southwest – hot and getting hotter.

Now, bring on the monsoon and some relief! (Just watch out for floods!)

Interesting point about the

Interesting point about the increased flood risk. Increased wildfire frequency could also exacerbate flooding in the future--I was just reading about the worry over floods around the Las Conchas fire in New Mexico.

Absolutely, wilfirefires

Absolutely, wilfirefires often limit - at least until vegetation recovers - the ability of ecosystems to contro precipatationl runoff and flooding. Large amounts of soil and sediment can be mobilized following fires. One interesting impact is that reservoirs can load up with sediment when wildfires occur in their watersheds, reducing the water holding capacity of the reservoir.

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