揽瓜阁俱乐部第三期 Day9 2020.07.28
【自然科学-宇宙】 How Exploding Stars May Have Shaped Earth's History （931字 精读 必做篇）
The aftermath of a star's death can rival the events of a creation myth. When a star explodes in a supernova, hurling pieces of itself into the cosmos, it seeds new stars and new worlds with the raw materials required for life. In death, stars are reborn. But like all creation tales, this one has a dark side. Supernovae can rain radiation and death onto living worlds that already exist. And they might be able to change the course of natural history.
One such change might have happened on Earth sometime between 1.7 and 3.2 million years ago. A star about nine times the mass of the sun blew up, and the night sky glowed a bright blue for weeks, during which the supernova outshone the full moon. Long after the darkness returned, lightning set off by cosmic rays would have arced from the sky to the ground, and the planet's climate may have changed. Animals on land and in the shallow sea would have been doused with waves of radiation. Over time, the influx of particles could have sparked mutations in DNA, making small alterations that could have shifted the course of evolution.
From our vantage point on Earth, supernovae appear suddenly; their name comes from the word for “new star.” Their brilliant, visible shine fades away within a few days or weeks, but they continue firing a stupendous surge of x-rays, gamma rays and speedy, energetic particles for much longer. Only recently have astronomers brought these supernovae down to Earth, by wondering how they might have interfered with the planet's climate, and the evolutionary processes that were playing out on its surface.
Adrian Melott, a physicist at the University of Kansas, wondered about the timing of the more recent supernova. Its date range includes the timeline of a minor extinction event at the endstart of the Pleistocene, about 2.59 million years ago, one that was long thought to be caused, in part, by a cooling climate and dramatic regional changes in Africa and central America. Melott and others had wondered whether a supernova could shower enough particles and radiation on Earth to cause mass extinctions. Thanks to the new research on supernova history, they could now look into it in earnest.
Melott ran computer simulations that suggested that even mild star-explosions would shower Earth with radiation for hundreds of thousands of years, provided they were local. They would also ionize the atmosphere to a level eight times higher than normal, which would trigger an increase in cloud-to-ground lightning.
“I really expected to conclude that there wasn't much chance of an effect, because of the distance, but it turned out to be more substantial than I expected,” Melott says.
While Melott and his coauthors were working on this paper, which appears today in The Astrophysical Journal Letters, another supernova archaeology team was refining the most recent research on the two local supernovae from 1.5 and 3.2 million and 6.5 to 8.7 million years ago. Brian Fields, Brian Fry and John Ellis argue that those supernovae were closer than scientists thought — maybe only 150 light years distant, not 325. If that's true, the radiation effect would be even stronger, Melott says.
The immediate radiation dose wouldn't be terrible, roughly comparable to the amount you'd receive in a CT scan. But it wouldn't be a one-shot deal. Instead, the radiation would rain down for hundreds of thousands of years. Melott says the particles would largely include muons, which are a sister particle of the electron and have more energy, so they can penetrate deeper, including into the oceans. They would also add up to a bigger effect on large animals, like mammoths, maybe, or humans. All told, supernova radiation could triple the everyday background radiation from cosmic rays.
“It would trivially increase your chances of cancer, but if you do it to every organism on Earth, for hundreds of thousands of years, there might be something you could see,” Melott says. “If you could have good enough statistics to look for bone cancer in fossils, for instance, you might be able to do that.” Melott said he had worked on research into whether this was actually possible.
Radiation is known to cause mutations in DNA in living organisms and in their sex cells, which leads to mutations and possible physical changes in later generations. A tweak in DNA here, a shift in chromosomes there can add up to substantive changes over time, altering the process of evolution.
To Melott, the real surprise was the increase in lightning. A major spike in atmospheric ionization would increase cloud-to-ground lightning, which might affect the weather, or at the very least might spark more wildfires. “That is one of the things we want to investigate, whether there is any evidence of increase in wildfire in the geological record,” he says. But he notes he's not a climatologist, and it would be up to climate scientists to study how ionizing the atmosphere would affect the climate.
Of course, it would take a great deal of substantial evidence to tie a specific supernova to climate changes and mass extinction. That's why it's the modeling work that's most interesting about this research. If a medium-sized explosion a few hundred light-years away would almost certainly do something—what, then, of huge explosions?
“This is not a major event as far as the Earth is concerned. Such a thing should come along on average every couple of million years,” Melott says. “But it means that the really nearby ones, that come along every couple hundred million years, could be quite devastating.”
Source: The Atlantic
【自然科学-环境】 Forests Getting Younger and Shorter （311字 2分22秒 精听 必做篇）
先做精听再核对原文哦~
They give us paper and fuel, as well as vital ecological services—like cleaning the air, storing carbon and providing habitat. We’re talking about trees, of course. But changes in the environment largely caused by humans appear to be causing profound transformations in trees around the world.
In a new study, scientists reviewed global research on trends in tree birth, growth and death. They combined those data with an analysis of deforestation. And they found that worldwide, older trees are dying at a higher rate than in the past due to factors like rising air temperature, wildfires, drought and pathogens.
“And most of the drivers of that decrease in large, old trees are increasing themselves, such as temperature going up, droughts are more severe, wildfires, windstorms and deforestation are all—although variable across the globe—they’re generally increasing. And so both the loss has already occurred, but we expect more continued loss of big, old trees.”
Nate McDowell, an earth scientist at Pacific Northwest National Lab, who was one of the study’s authors.
“So if we have an increasing rate of death, particularly of the larger, older trees, what’s left are the younger trees. So that’s why, on average, through the loss of bigger, older trees, our forests are becoming inherently younger and shorter.”
This trend is a problem, because old trees are vitally important.
“For sure, the increase in death does limit the carbon storage of an ecosystem and can force the system to become a carbon source to the atmosphere. The second reason we care is from a biodiversity perspective: old growth trees tend to house a higher biodiversity than young forests do. And the third reason is aesthetic: As a society, we care about these trees. We have national parks named after these big trees. So there’s a personal reason for people to care about this as well.”
Source: Scientific American
【笔记格式要求】
精读笔记格式要求： 1.总结文章中心大意 2.总结分论点或每段段落大意 3.摘抄印象深刻或者觉得优美的句子 4.总结文章中的生词 5.记录阅读时间、总结时间、总时间
精听笔记格式要求： 1.逐句听写整篇文章 2.对照原文修改听写稿，标记出错原因 3.总结文章中心大意 4.总结精听过程中的生词 5.记录听写时间、总结时间、总时间
这里也给大家两点学习小建议哦~ 精读：如遇到读不懂的复杂句，建议找出句子主干，分析句子成分，也可以尝试翻译句子来帮助理解~ 精听：建议每句不要反复纠结听，如果听 5 遍都没听出来，那就跳过，等完成后再回听总结原因，时间宝贵，不要过于执着哦~
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发表于 2020-7-27 22:21:52
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