Environment

Western US may be headed for a once-in-500-years "megadrought"

Western US may be headed for a...
A Columbia University study of historic drought markers indicates the western US may be in a "megadrought"
A Columbia University study of historic drought markers indicates the western US may be in a "megadrought"
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1,200 years of soil moisture data used in the study. The red fluctuating lines indicate tree ring data, while the blue are from modern records. Red-shaded areas indicate droughts, while green indicates higher rainfall. The bottom blue line represents the mean from 2000-2018.
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1,200 years of soil moisture data used in the study. The red fluctuating lines indicate tree ring data, while the blue are from modern records. Red-shaded areas indicate droughts, while green indicates higher rainfall. The bottom blue line represents the mean from 2000-2018.
The map of the area studied. Darker areas represent more intense drought conditions since 2000
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The map of the area studied. Darker areas represent more intense drought conditions since 2000
A Columbia University study of historic drought markers indicates the western US may be in a "megadrought"
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A Columbia University study of historic drought markers indicates the western US may be in a "megadrought"
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The western United States and parts of northern Mexico have been suffering through drought conditions on and off since the year 2000 – and unfortunately it may not let up any time soon. A new study has examined extreme droughts in the region dating back 1,200 years, and found that the current conditions have the makings of a “megadrought” that could last decades.

The study, conducted by researchers at Columbia University, combined data on rainfall and soil moisture from several sources. Modern weather observations date back to around 1900, but tree rings can preserve this information from centuries ago. Generally, trees grow faster during times when water is plentiful, forming wider rings. Conversely, narrow rings indicate periods of drought.

By examining ring data from thousands of trees, the team managed to track drought patterns all the way back to the year 800 CE, in an area encompassing nine US states (Oregon, Idaho, Wyoming, California, Nevada, Utah, Colorado, Arizona and New Mexico) as well as parts of northern Mexico.

The map of the area studied. Darker areas represent more intense drought conditions since 2000
The map of the area studied. Darker areas represent more intense drought conditions since 2000

Droughts of course are cyclical, and the team spotted dozens of droughts in that 1,200-year period. But among them were four that stood out as megadroughts, with arid conditions lasting decades and in one case, almost a century. These four megadroughts took place in the late 800s, mid-1100s, most of the 1200s, and the late 1500s.

Next, the team compared these large-scale historic events to the one that’s currently unfolding. They took soil moisture records between 2000 and 2018, and compared that 19-year period to the worst 19-year stretches of each of the other droughts. The 21st-century drought, it turns out, ranks worse than the earliest three megadroughts, and about on par with the 16th-century event.

The researchers say that all four previous megadroughts began with patterns similar to what’s happening now, indicating this one might stick around for a while longer yet.

But perhaps the most interesting aspect of the study is the role that climate change is playing. The current drought seems to be affecting a wider area more consistently than the earlier events, which is a hallmark of climate change. In fact, the researchers calculate that this drought would have been pretty moderate otherwise, ranking 11th in severity in the 1,200-year period. Instead, climate change has created higher underlying temperatures which boosted it to the top of the charts.

1,200 years of soil moisture data used in the study. The red fluctuating lines indicate tree ring data, while the blue are from modern records. Red-shaded areas indicate droughts, while green indicates higher rainfall. The bottom blue line represents the mean from 2000-2018.
1,200 years of soil moisture data used in the study. The red fluctuating lines indicate tree ring data, while the blue are from modern records. Red-shaded areas indicate droughts, while green indicates higher rainfall. The bottom blue line represents the mean from 2000-2018.

While there’s always an element of natural variation, the team says that climate change is shifting events like droughts towards the more extreme end of the scale more regularly.

“Because the background is getting warmer, the dice are increasingly loaded toward longer and more severe droughts,” says Park Williams, lead author of the study. “We may get lucky, and natural variability will bring more precipitation for a while. But going forward, we’ll need more and more good luck to break out of drought, and less and less bad luck to go back into drought.”

The research was published in the journal Science.

Source: Columbia University

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11 comments
Gemaeden
How can the mean be below the lowest point between 2000 and 2018. Something's surely wrong.
buzzclick
There may have been other droughts in the past records, but nothing has faced our ecosphere like the present. People need to be cognizant that all this human activity is changing the odds. Perhaps the Corona virus is a wake-up call. It's not too late to turn things around.
guzmanchinky
I live in Newport Beach and we have had record rainfall these last two seasons.
Karmudjun
Very nice article, and well written. I especially like the graph included from the Science article that shows historical data from 800 AD to 2000 AD and includes the mean calculated from the 2000-2018 data across the old historical data. It does show our current soil moisture mean - an 18 year mean - is much lower than the 1200 year mean and was only touched by the previous historical droughts. So Gemaeden has a point that needs to be hammered home - THERE SURELY IS SOMETHING WRONG. And what is wrong is that this current drought has a MEAN, not a nadir that is slightly higher than previous severe droughts nadir. Are dust bowl conditions ahead?
f8lee
Interesting. Years back, Scientific American had an article talking about the 200-year cycle megaflood that was discovered by taking core samples from various lake bottoms around the state.

https://www.scientificamerican.com/article/atmospheric-rivers-california-megaflood-lessons-from-forgotten-catastrophe/

I wonder how this all fits together?
McDesign
I just read an article in Discover - the whole country has been in a rain glut for all of 2019 - particularly the SE and west. What's really up? https://www.discovermagazine.com/environment/2019-broke-multiple-records-for-the-wettest-year-ever
Douglas Bennett Rogers
On the plus side, a drought provides an escape path for infrared radiation!
aksdad
"The western United States and parts of northern Mexico have been suffering through drought conditions on and off going back hundreds or thousands of years". Fixed it. There is nothing unique about droughts after 2000. That's just when the study analyzed soil moisture records. If you look at the Palmer Drought Severity Index going back to 1900, the American Southwest goes through cycles of drought conditions similar to the present. See https://www.ncdc.noaa.gov/temp-and-precip/drought/historical-palmers/psi/190001-202003. The study has been criticized for making a claim suggesting we're in a megadrought when many scientists say you need at least 30 years of drought conditions to prove that. 19 years isn't enough. And in that 19 year period there have been several periods of no drought at all: 2005, 2010-2011, 2017, and 2019 is shaping up to be a pretty good year. Then there's this speculation: "the team says that climate change is shifting events like droughts towards the more extreme end of the scale". Droughts have nothing to do with global warming. Droughts are caused by persistent regional high pressure systems typically over certain arid regions influenced by their location in relation to prevailing ocean and atmospheric circulation like Northern Africa and the American Southwest. There is no data that shows droughts are increasing in duration and severity over those regions since approximately 1950. In fact, IPCC AR5 (2013), the most recent compendium of global warming studies, shows decreasing drought over most of North and Central America, with "spatially varying trends". Essentially what the report shows, globally, is a large amount of natural variation with no global trend of increasing drought. The team can speculate all they want, but unless it's supported by data (it isn't), it's just speculation.
bwana4swahili
The Anasazi civilization fell apart during a past drought. There will other people displaced by droughts in the future, just Nature's way.
Nelson Hyde Chick
My brother lives in Michigan, and he is always bitching about the state of Detroit. I tell him not to worry about it because once climate change has made the Southwest uninhabitable the residents will need to move someplace with water and Detroit has access to 20% of the Earth's fresh water.