me love trees. I also love dendrochronology — literally, “the study of tree time.” This science, which uses data derived from tree growth rings, provides scientists with a wealth of information regarding the conditions under which trees grow.
AAs a graduate student, I became interested in the now famous method of dating of archaeological sites by analysis of tree rings. Later I became interested in the application of forest-ring interpretation to the dating of musical instruments.
Rdecently, with the increasing intensity of anthropogenic climate change, the topic that is getting more attention from me is dendrochlimatology. This fascinating science uses tree rings to reconstruct ancient precipitation, temperature, and other climatic variables. Unlike various weather tracking instruments, tree rings provide explorers with a record going back hundreds or even thousands of years.
That is why tree rings make me nervous: The long-term picture these signs paint of mega-droughts and climate change in the western United States where I live is deeply disturbing.
The Dawn of Modern Megadrought
Cover an area that includes the Four Corners states (Arizona, Colorado, New Mexico, and Utah), the lower reaches of Nevada, southern California, and a large area across northern Mexico, the North American Southwest is famous for its arid climate. For the past 22 years, the region has suffered from severe drought. This period meets the criteria of “mega-dryness”, and it was notable for both its duration and intensity.
For those of us who care about anthropogenic climate change, two questions arise: How anomalous is the current mega-drought compared to previous mega-droughts? And what is the likelihood that these conditions will continue in the future?
meInstrument-based precipitation and temperature records in the United States Southwest date back only to the late 1800s. For decades, the number of instruments was small, and they were widely spread across a vast area. The data they provide is therefore not very good for estimating climate variability across space and over time. Tree-ring data, on the other hand, is very useful for this kind of large image analysis.
Cfirst, dendroclimatology in the western United States is simple. Given the natural aridity of the region, it makes sense that a lack of rainfall may limit tree growth. As such, a dry year will cause many trees to produce a narrow growth ring, while a wet year will lead to a wide growth ring. (In other parts of the world, such as at high altitudes and near the Arctic and Antarctic, trees respond to temperature fluctuations rather than changes in precipitation. In these places, cold years produce narrow rings, and warm years give rise to wide rings.)
meIn practice, however, dendroclimatology can be surprisingly difficult, requiring rigorous fieldwork and testing strategies, detailed sample preparation and analysis techniques, and complex mathematical modeling. All this is for the specialists. But ordinary people can still learn from their research.
mein 2022 paper published in Nature Climate Change, A. Park Williams, of the University of California, Los Angeles, and his colleagues applied tree-ring analysis to constructing a mathematical model of soil moisture levels dating from AD 800 across the North American Southwest. Their study reveals that the 22-year period from 2000 to 2021, our current mega-drought, was the driest and warmest period in the last 1,200 years.
TAgain rings indicate that there was another 22-year mega-drought from 1571 to 1592 that was almost as dry but not as hot as the current conditions. Anthropogenic climate change makes the detrimental effects of today’s mega-drought all the worse, as it is significantly warmer now than it was in the late 16th century. Indeed, Park and colleagues suggest that the current mega-drought would be just about 60 percent equally severe in the absence of anthropogenic climate change.
That makes a huge difference when it comes to soil moisture and sustainability.
A Troubling Twist of Fate
A a related article published this year used tree analysis to examine the history of Colorado, which provides water for tens of millions of people in Las Vegas, Nevada; Los Angeles, California; Phoenix, Arizona; and hundreds of smaller communities. Researchers at the University of Arizona have focused on dendrohydrology, the science of using tree rings to reconstruct ancient hydrological variables such as flow.
THey has analyzed previous publications that used tree rings to reconstruct the flow of the Colorado River at Lees Ferry, Arizona, one of the main measurement stations on the Colorado River. Their review confirmed that the findings of previous research on water supply and demand on the river were reliable and up to modern standards. This is good news for the scientific method, which often requires independent confirmation. This is bad news for Colorado Basin water users.
Politics can replace nature for a short time, but Mother Nature will rule for a long time.
meIt is important to note that nearly 100 years ago, on November 24, 1922, delegates from seven U.S. states met in Santa Fe, New Mexico, to sign the Colorado River Compact — and inadvertently arranged the region for failure.
The delegates believed the average Colorado stream flow was 16.5 million acre feet per year. Do, their agreement divided 7.5 million acres of water annually to provide for growing cities, industrial applications, and irrigation for farming in what they hoped would be a sustainable way. This would allow the remaining 9 million acres a year to preserve Colorado’s ecosystem. Or so they thought.
Tthe instrumental flow data they used to inform the compactor came from a 22-year period between 1900 and 1921. In an almost incredible turnaround, Colorado enjoyed a significantly higher flow during that 22-year period than at any other time. another period in the last 500 years! We now know this from tree ring analyzes.
CCompact signatories have therefore been legally bound to unrealistically large requirements because their input data, unknown to them and accidentally, came from an anomalously wet period in the climate history of the American Southwest. People in the area have been dealing with the consequences ever since.
meIt seems highly unlikely that the compact will be retracted due to the current political climate. Politics can replace nature for a short time, but Mother Nature will rule for a long time.
TIn summary, forest ring analysis shows that: (1) the mega-drought puts the U.S. Southwest in unprecedented and unexplored climatic territory, and (2) the allocation of the area’s largest and most important water source was based on data from an unusually wet period. and therefore would be insufficient to supply this now prosperous region even if it were not in mega-drought. You just can’t do these things.
Beyond the Shadow of Drought
Ppopular media regularly publishes stories of new and drastic water conservation initiatives being implemented in major metropolitan areas such as Phoenix, Las Vegas and Los Angeles. They report on unprecedentedly low water levels in Lake Mead and Lake Powell, two artificial reservoirs in Arizona, Nevada, and Utah that store water for the area. Both are now at about 30 percent capacity, their lowest levels since they were filled decades ago.
This threatens water and water supplies, and thus livelihoods, across the region. Severe conservation measures are being implemented, but they seem to be too little, too late.
AAlthough tree data allows us to look at the past, it does not help us predict the future. We just have no way of knowing whether the current mega-drought will end next year, or whether it will last for decades. I’m not optimistic.
Robert Fulghum, author of Everything I Really Need to Know I Learned in Kindergarten, wrote about the challenges to make choices when at the same time we know too much and not enough: “Too much high-profile information, and I get the existential will.” Based on the high-altitude data of tree rings, I have the existential wills.