World's Largest River Floods Five Times More Often Than It Used to
After analyzing 113 years of Amazon River levels in Port of Manaus, Brazil, researchers found that severe floods happened roughly every 20 years in the first part of the 20th century. Now, extreme flooding of the world's largest river occurs every four years on average—or about five times more frequently than it used to.
"With a few minor exceptions, there have been extreme floods in the Amazon basin every year from 2009 to 2015," study lead author, Jonathan Barichivich, environmental scientist at the Universidad Austral de Chile, said in a press release.
This increase in flooding could be disastrous for communities in Brazil, Peru and other Amazonian nations, the researchers pointed out.
"There are catastrophic effects on the lives of the people as the drinking water gets flooded, and the houses get completely destroyed," Barichivich told the Thomson Reuters Foundation.
The study, published Wednesday in the journal Science Advances, also determined that droughts in the Amazon Basin have increased in frequency.
"Our findings unravel the ultimate causes of the recent intensification—wet season getting wetter, and dry season getting drier—of the water cycle of the largest hydrological basin of the planet," Barichivich told Retuers.
The researchers linked the increase in flooding to a strengthening of the Walker circulation, which is induced by the contrast of warm Atlantic waters and the cooler waters of the Pacific.
Flooded area in the center of Manaus in 2009Jochen Schöngart, National Institute for Amazon Research
This ocean-powered air circulation system, which influences weather patterns and rainfall in the tropics and elsewhere, can partly be attributed to shifts in wind belts caused by climate change, as Reuters noted about the study.
"This dramatic increase in floods is caused by changes in the surrounding seas, particularly the Pacific and Atlantic oceans and how they interact. Due to a strong warming of the Atlantic Ocean and cooling of the Pacific over the same period, we see changes in the so-called Walker circulation, which affects Amazon precipitation," study co-author Manuel Gloor, from Britain's University of Leeds, said in the press release. "The effect is more or less the opposite of what happens during an El Niño event. Instead of causing drought, it results in more convection and heavy rainfall in the central and northern parts of the Amazon basin."
With temperatures in the Atlantic expected to continue warming, the scientists expect to see more of these high water levels in the Amazon River.
"We think that it's going to continue for at least a decade," Barichivich told Reuters.
Inundated house along Solimões River (Central Amazonia) during the record-breaking flood in 2012Jochen Schöngart / National Institute for Amazon Research
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By Katherine Kornei
Clear-cutting a forest is relatively easy—just pick a tree and start chopping. But there are benefits to more sophisticated forest management. One technique—which involves repeatedly harvesting smaller trees every 30 or so years but leaving an upper story of larger trees for longer periods (60, 90, or 120 years)—ensures a steady supply of both firewood and construction timber.
A Pattern in the Rings<p>The <a href="https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/coppice-standards-0" target="_blank">coppice-with-standards</a> management practice produces a two-story forest, said <a href="https://www.researchgate.net/profile/Bernhard_Muigg" target="_blank">Bernhard Muigg</a>, a dendrochronologist at the University of Freiburg in Germany. "You have an upper story of single trees that are allowed to grow for several understory generations."</p><p>That arrangement imprints a characteristic tree ring pattern in a forest's upper story trees (the "standards"): thick rings indicative of heavy growth, which show up at regular intervals as the surrounding smaller trees are cut down. "The trees are growing faster," said Muigg. "You can really see it with your naked eye."</p><p>Muigg and his collaborators characterized that <a href="https://ltrr.arizona.edu/about/treerings" target="_blank">dendrochronological pattern</a> in 161 oak trees growing in central Germany, one of the few remaining sites in Europe with actively managed coppice-with-standards forests. They found up to nine cycles of heavy growth in the trees, the oldest of which was planted in 1761. The researchers then turned to a historical data set — more than 2,000 oak <a href="https://eos.org/articles/podcast-discovering-europes-history-through-its-timbers" target="_blank" rel="noopener noreferrer">timbers from buildings and archaeological sites</a> in Germany and France dating from between 300 and 2015 — to look for a similar pattern.</p>
A Gap of 500 Years<p>The team found wood with the characteristic coppice-with-standards tree ring pattern dating to as early as the 6th century. That was a surprise, Muigg and his colleagues concluded, because the first mention of this forest management practice in historical documents occurred only roughly 500 years later, in the 13th century.</p><p>It's probable that forest management practices were not well documented prior to the High Middle Ages (1000–1250), the researchers suggested. "Forests are mainly mentioned in the context of royal hunting interests or donations," said Muigg. Dendrochronological studies are particularly important because they can reveal information not captured by a sparse historical record, he added.</p><p>These results were <a href="https://www.nature.com/articles/s41598-020-78933-8" target="_blank">published in December in <em>Scientific Reports</em></a>.</p><p>"It's nice to see the longevity and the history of coppice-with-standards," said <a href="https://www.teagasc.ie/contact/staff-directory/s/ian-short/" target="_blank">Ian Short</a>, a forestry researcher at Teagasc, the Agriculture and Food Development Authority in Ireland, not involved in the research. This technique is valuable because it promotes conservation and habitat biodiversity, Short said. "In the next 10 or 20 years, I think we'll see more coppice-with-standards coming back into production."</p><p>In the future, Muigg and his collaborators hope to analyze a larger sample of historic timbers to trace how the coppice-with-standards practice spread throughout Europe. It will be interesting to understand where this technique originated and how it propagated, said Muigg, and there are plenty of old pieces of wood waiting to be analyzed. "There [are] tons of dendrochronological data."</p><p><em><a href="mailto:firstname.lastname@example.org" target="_blank" rel="noopener noreferrer">Katherine Kornei</a> is a freelance science journalist covering Earth and space science. Her bylines frequently appear in Eos, Science, and The New York Times. Katherine holds a Ph.D. in astronomy from the University of California, Los Angeles.</em></p><p><em>This story originally appeared in <a href="https://eos.org/articles/tree-rings-reveal-how-ancient-forests-were-managed" target="_blank">Eos</a></em> <em>and is republished here as part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.</em></p>
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