Too hot to handle

The many effects of the heat wave in America's Southwest

A tour bus arrives at a visitors’ center that’s difficult to make out as it so sympathetically nestled into the bare rock and sand landscape. The tourists are excited to get to see the famous ghost town, for it is rare that buses can safely travel into the scorching heat. The tour guide explains that visitors must remain under the covered walkway as they make their way from the bus to the museum for the temperature exceeds 120 degrees Fahrenheit (49 degress Celsius). The guide explains that the local people ask that you make a contribution of water by draining the contents of your second water bottle into the cistern just outside the visitor’s center. This water will be used to nurture the world’s last surviving stand of Saguaro cacti. The tour guide explains that they can see the Saguaro stand from the upstairs observation room of the visitors’ center, where they will also be able to see specimens of extinct desert species, like the mountain lion and the chuckwalla, and use telescopes to explore the city ruins. The ground floor of the museum explores how a decades long drought, made extreme by human driven climate change, led to water shortages, heat deaths, power supply problems and the death of almost all wildlife in the region. Human life here became impractical and dangerous. It is the year 2050 and the bus is just outside of Tucson, Arizona. You can hear one boy ask his mom “Did people really used to live here? That seems impossible!”

This narrative may sound extreme, but it is not far fetched. As global warming progresses, different areas will heat more than others and the southwest United States is one such area. It is expected this region will warm by 4 to 10 degrees in the next 50 years, relative to 50 years ago.

This is partly because of global patterns but is also due to local conditions. The Southwest is not only getting hotter, it is also getting drier. This means that the fraction of incoming sunlight going into evaporating water decreases, while the fraction of solar radiation going in raising temperatures increases. So temperatures rise, the soil dries, and plants die. There isthen even less water held by the soil, no shade, and temperatures rise even more. Anthropogenic climate change has already lead to the death of 350 million pinyon trees in the Southwest - disrupting habitats and adding to the fuel load for fires.

You may have seen the headlines recently about the heat wave in the Southwest. Hundreds of daily temperature records (the highest temperature for a given date) have been broken across a broad swath of the west including in Utah, California, Nevada, Arizona, Wyoming, and Montana. 50 million people were issued heat advisories from the National Weather Service last week. There have been record breaking consecutive days of heat as well, and even all time temperature highs recorded. For example on Thursday, June 17th, the temperature in Phoenix reached 118 degrees Fahrenheit (48 degrees Celsius). These temperature records are unusual in three ways: 1. They are very widespread, 2. They are long lasting and 3. They’ve all occurred before summer officially began.

It is not only are temperature records that are being broken, so are water and aridity records. Lake Mead, America’s largest reservoir, is at its lowest level since it was built in 1930. Rivers are so dry that northern California recently moved 17 million salmon to the sea - by truck. There have been headlines stating that this current drought is a once in a millennial event. This is gleaned from the changing widths of the tree rings: the greater the soil moisture in a given year the more growth the tree puts on and hence the wider the tree rings. By compiling 1500 tree ring timelines in the Southwest, scientist have shown that there have been two mega-droughts in the Southwest in the past 1200 years: one in the 1100s and a worse one in the 1500s. During the drought in 1500s, the soil moisture index appears to have been about 60% below the usual minimum of soil moisture - based on a rough eye balling by yours truly of the tree ring graph. Last year, the soil moisture dipped down to match the drastically low levels of the 1500s drought. To qualify as a mega-drought, this extremely low soil moisture must persist for 20 or so years. While the Southwest has already been experiencing a drought for the last 20 years, it hasn’t been severe enough to qualify for a mega-drought. Yet.

Could this low soil moisture found in the Southwest today just be natural climate fluctuations? In short, no. By using a series of models, researchers found that human induced climate change is the only way to reprodce the observed moisture patterns. Anthropogenic climate change must underpin the recent drying and is responsible for nearly 50% of the Southwest’s recent drying out. This means the drying trend is not going away soon and we may well be at the start of a new mega-drought. Climate change is pushing the natural fluctuations in soil moisture from an extended drought into the severe mega-drought category.

Dry soil means dry vegetation or even dead vegetation, in other words, wildfire fuel. During this recent heat wave, 14 fires have broken out in Montana and Wyoming alone. 450 California firefighters have been battling a fire that was 3.7 square miles. And of course, fire season is just starting in many parts of the Southwest, such as in California. You may recall last years dramatic record breaking fire season which lead to 4% of California burning, largely in August.

There are other worrying effects from the aridity and the heat. Heat deaths are the number one cause of weather related deaths in the Southwest, with the elderly and the homeless especially at risk. 40 million people experienced temperatures exceeding 100 degrees Fahrenheit (38 degrees Celsius) in the past week. Air quality is degraded in such high temperatures, with Phoenix recording its worst ever air quality since records started in 1980. The Rio Grande is expected to have half its normal flow this year and New Mexico’s largest reserve is expected to top out at 1/3 of its 30 year average. River ecosystems suffer as will the farms that rely on irrigation with water diverted from the rivers.

And then there’s the implications for the power grid. Last week, Texas and California governments asked residents to conserve energy as much as possible to avoid power outages as air conditioners were expected to be running on overdrive. We all saw the chaos that ensued in Texas last February, when 3 winter storms imposed a big burden on the power grid leading to its failure. This is thought to have been the costliest natural disaster in Texas’ history.

It’s difficult to imagine the Southwest avoiding this long list of consequences from future heat waves, from wildfires, to severe drought, and from power outages to heat stress. But if we act now we might avoid the worst of it. Aside from the critical work of doing our bit to reduce climate change, we can also help reduce water shortages in the Southwest by avoiding the products that drain the rivers and lower the water table. This will help keep the natural vegetation moist.

Did you know that Tucson used to have enough ground water to support the city and the great Colorado River used to have a massive wetland delta of 2 million acres? It sounds like a tale out of Frank Herbert’s Dune. For today, the Tucson water table is severely depleted and the Colorada river doesn’t even reach the sea. Instead, the Colorado’s delta is mostly dried and cracked mud flats. What’s all that water going? 85% of the missing Colorado River water is diverted for use in agriculture. In turn, the main agriculture products in Arizona and Nevada are livestock and dairy, with hay and alfalfa feed for livestock dominating cropland of the Southwest. So here we are again with the mantra that “your impacts are what you eat”. But this time, throw in an added dose of shock and horror over the existence of golf courses in the Southwest. Don’t get me started.