What tree rings reveal about America’s megadrought – a visual guide
How we know the American west is experiencing a once-in-a-millennium drought
by Alvin Chang
Last year, wildfires burned through hundreds of thousands of acres of land in the American west.
These fires, extraordinary in size and intensity, are a symptom of a decades-long drought in the region.
Exceptional drought conditions are even more widespread in 2021, according to the US Drought Monitor, causing experts to fear another catastrophic wildfire season.
So just how bad is the western US drought?
One way of answering the question is to compare modern droughts with ancient ones – and scientists can do that by looking at tree rings like this one.
In 2020, that’s exactly what bioclimatologist Park Williams and his team did.
Tree rings tell us how moist the soil was in the past.
For example, in the late 1980s the soil had more moisture. That allowed the tree to grow faster, which made rings grow farther apart.
But since 2000, the soil has been exceedingly dry, so the tree rings have been close together.
Scientists can compare this drought to historical droughts by looking at old trees …
… and matching them up the rings with older trees …
… and even older trees. This helps them create a lineage of tree rings to understand how dry the soil was thousands of years ago.
Williams and his team analyzed more than 1,500 of these tree-ring timelines across the American south-west. They found some ancient droughts that were so bad that they categorized them as megadroughts.
This chart shows the soil moisture levels in the region starting in 800 CE.
One megadrought in the late 1100s lasted almost 40 years.
In the late-1500s there was a megadrought so severe that tree ring experts talk about it as if it were some biblical event.
But if we track the severity of this 1500s megadrought to the current day …
… we can see that the American south-west is currently experiencing a drought almost as bad as the legendary 1500s megadrought.
And this analysis doesn’t even include 2021, which Williams said is “on pace to go down as the worst of the drought years” since the early 1900s.
But here’s the astounding part: Williams and his team also estimated what drought conditions would look like if human activities had not caused global warming …
… and they found that we would still likely be living through a once-in-a-century drought – but human activity accounts for about 46% of the severity of the current megadrought.
And if it continues to be this dry, it could become the most severe megadrought on this entire chart.
“The only reason this drought is lagging behind that 1500s drought is because it’s so young,” Williams said.
Tree ring images courtesy of Daniel Griffin, dendrochronologist at the University of Minnesota