James Aman
Are extreme droughts in the West and Southwest caused by climate change?
The answer in a nutshell is "partially". As is often the case with climate change, there are lots of pieces to fit together to solve this puzzle.
To solve the puzzle, let's start with the easier edge pieces. There are certain things that can be pretty much proved with scientific data. For example, it is a known fact that levels of carbon dioxide (CO2) in the atmosphere have been slowly increasing over the past 250 years and have been increasing more rapidly in the past 50 to 60 years. The CO2 levels are measured in parts-per-million (ppm) and have increased from less than 300 ppm in the 1800s to well over 400 ppm today.
We also know that CO2 is a greenhouse gas, that tends to trap the sun's energy in our atmosphere. Simple physics says this should lead to an increase in temperatures when averaged out around the globe. Day-to-day weather varies a lot, and so sometimes it is hard to see long-term trends, but more and more temperature data is showing a steady warming trend in air temperatures when averaged over the longer term. Another way to see this is in ocean water temperatures, which have shown a very clear pattern of slow but steady warming over the past 70 years.
Another known fact from physics and chemistry is warmer air can hold more moisture. This means that warmer air coming from warmer oceans should lead to increasingly heavy precipitation around the globe. There is more and more evidence that global rainfall is increasing, and the number of extreme rain events are also increasing in the past few decades.
Here's where the puzzle gets more complicated. If there is all this extra moisture and rainfall, why isn't that spread across the entire world? Why isn’t everybody seeing more rain? How is it that a few areas are actually seeing less rain and worsening drought conditions?
It turns out there are a number of factors at work to create this unexpected result. In areas that have always been dry, the warmer air temperatures cause increased evaporation, leading to dried out vegetation and less surface water. Also, warmer temperatures mean that winter precipitation that used to fall as snow in the higher elevations is now falling more as rain. This rain runs off quickly and is gone in a few days. In contrast, cooler weather would allow for snowpack that could last for months, allowing for melting to feed streams and rivers well into the summer.
Unfortunately, these factors are found in the western and southwest U.S. The United States Geological Survey (USGS) says "Since 2000, the western United States is experiencing some of the driest conditions on record. The southwestern U.S., in particular, is going through an unprecedented period of extreme drought." The USGS also says "Climate change has further altered the natural pattern of droughts, making them more frequent, longer and more severe."
Looking online, there are several studies on links between climate change and extreme drought. For example, a report in the journal "Science" used a 1200-year record of tree rings in the western U.S. to generate historical rainfall data. This study found that in the Southwest U.S., the drought in the years 2000–2018 was the second driest 19-year period in the past 1200 years, exceeded only by a late-1500s megadrought. And in an interesting twist, the top 5 megadroughts in the past 1200 years in the Southwest U.S. each lasted 30 to 40 years, so the long-term drought in Arizona and New Mexico could possibly continue in some fashion for at least another decade. This same study then lined up this long record of droughts with computer models of climate and found that 46-percent of the 2000–2018 drought severity in the Southwest U.S. could be attributed to climate change.
Another study in the journal "Nature" showed a clear increase in both extreme rain episodes and extreme drought over the past 20 years. This study showed the primary factor in both was increase in global temperatures over the past few decades.
As more and more pieces are fit into this puzzle, the picture is showing more clearly that climate change is deepening the drought in the West and Southwest U.S.
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