James Hansen: 2C Temperature Rise Would Be 'Disastrous'
By Tim Radford
Governments have set the wrong target to limit climate change. The goal at present—to limit global warming to a maximum of two degree Celsius higher than the average for most of human history—“would have consequences that can be described as disastrous,” say 18 scientists in a review paper in the journal PLOS One.
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With a two degree Celsius increase, “sea level rise of several meters could be expected,” they say. “Increased climate extremes, already apparent at 0.8 degrees Celsuis warming, would be more severe. Coral reefs and associated species, already stressed with current conditions, would be decimated by increased acidification, temperature and sea level rise.
The paper’s lead author is James Hansen, now at Columbia University, NY, and the former NASA scientist who in 1988 put global warming on the world’s front pages by telling a U.S. government committee that, “It's time to stop waffling so much and say the evidence is pretty strong that the greenhouse effect is here.”
Hansen’s fellow authors include the economist Jeffrey Sachs of Columbia University and the biologist Camille Parmesan, of the University of Plymouth in the UK and the University of Texas at Austin.
Their argument is that humanity and nature—“the modern world as we know it”—is adapted to what scientists call the Holocene climate that has existed for more than 10,000 years—since the end of the Ice Age, the beginnings of agriculture and the first settlement of the cities.
Warming of one degree Celsius relative to 1880–1920 keeps global temperature close to the Holocene range, but warming of two degree Celsius, could cause “major dislocations for civilization.”
The scientists study, uncompromisingly entitled Assessing ‘Dangerous Climate Change’: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature differs from many such climate analyses because it sets out its argument with remarkable directness and clarity, and serves as a useful briefing document for anyone—politicians, journalists and lay audiences—anxious to better understand the machinery of climate, and the forces that seem to be about to dictate climate change.
Its critics will point out that it is also remarkably short on the usual circumlocutions, caveats, disclaimers and equivocations that tend to characterise most scientific papers. Hansen and his co-authors are however quite open about the major areas of uncertainty: their implicit argument is that if the worst outcomes turn out to be true, the consequences for humankind could be catastrophic.
The scientists case is that most political debate addresses the questions of reducing greenhouse gas emissions, but does not and perhaps cannot factor in the all potentially dangerous unknowns—the slow feedbacks that will follow the thawing of the Arctic, the release of frozen reserves of methane and carbon dioxide in the permafrost and the melting of polar ice into the oceans. They point out that 170 nations have agreed on the need to limit fossil fuel emissions to avoid dangerous human-made climate change.
“However the stark reality is that global emissions have accelerated, and new efforts are underway to massively expand fossil fuel extractions by drilling to increasing ocean depths and into the Arctic, squeezing oil from tar sands and tar shale, hydro-fracking to expand extraction of natural gas, developing exploitation of methane hydrates and mining of coal via mountain-top removal and mechanized long wall-mining,” the scientists write.
The scientists argue that swift and drastic action to limit global greenhouse gas emissions and contain warming to around one degree Celsius would have two useful consequences. One is that it would not be far from the climate variations experienced as normal during the last 10,000 years, and secondly that it would make it more likely that the biosphere, and the soil, would be able to sequester a substantial proportion of the carbon dioxide (CO2) released by human industrial civilization.
Trees are, in essence, captive carbon dioxide. But the warmer the world becomes, the more likely it is that existing forests—the Amazon, for example—will start to release more CO2 than they absorb, making the planet progressively even warmer.
Therefore the scientists make a case for limiting overall global carbon emissions to 500 giga-tons rather than the 1,000 billion tons in the two degree Celsius rise scenario.
“Although there is merit in simply chronicling what is happening, there is still opportunity for humanity to exercise free will,” says Hansen.
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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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