
With more and more of the world's population moving into urban centers, the number of such areas with more than 10 million people has exploded in the last quarter century.
In 1990 there were only 10 of these "megacities," with about seven percent of the global population. Now there are 28, according to the UN report on World Urbanization. The total number of people living in megacities has grown from 153 million to 453 million during that period, says the UN report. About 12 percent of the world's population lives in one, while 54 percent lives in an urban area, expected to grow to 66 percent by 2050. By 2030, the world is projected to have 41 such megacities.
Of those 28 megacities, sixteen are in Asia, four are in Latin America, three each are in Africa and Europe, and two are in Northern America. Tokyo leads the list with more than 38 million people living in its metro area. Delhi is a distant second with 25 million. However, the population of Tokyo is projected to shrink somewhat while Delhi is expected to grow rapidly until the two cities are nearly equal in population. Shanghai, Mexico City, Sao Paolo and Osaka follow with over 20 million apiece. Beijing has almost 20 million.
Meanwhile, the rural population is expected to shrink. It's currently at 3.4 billion and is expected to decline to 3.1 billion by 2050. And a majority of Earth's 3.9 million urban residents live in smaller cities, although many of those are among the fastest growing.
This urbanization has some positive implications for the environment and climate change. The report emphasizes that a urban planning will be essential for economic and human development, with attention to issues like health care, education, public transportation, housing, electricity, water and sanitation. It says that providing such services for a for a dense urban population is typically cheaper and less environmentally damaging than providing such services to a dispersed rural population.
“Managing urban areas has become one of the most important development challenges of the 21st century,"said John Wilmoth, director of UN Department of Economic and Social Affairs Population Division. "Our success or failure in building sustainable cities will be a major factor in the success of the post-2015 UN development agenda."
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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:katherine.kornei@gmail.com" 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>Earth's ice is melting 57 percent faster than in the 1990s and the world has lost more than 28 trillion tons of ice since 1994, research published Monday in The Cryosphere shows.
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