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  • Imagine an enormous rainforest teeming with life: trees, insects, pretty little birds.

  • Primates are climbing in the canopy, while crocodiles and turtles swim in the rivers

  • below.

  • Beautiful, isn’t it?

  • Now imagine this lush rainforest ... in the Arctic.

  • There was a time -- and not too long ago -- when the world warmed more than any human has ever

  • seen.

  • So far.

  • This ancient warming took place over the course of just 200,000 years -- the blink of an eye

  • in geologic time -- and it ended much like it began: suddenly and mysteriously.

  • It all started 56 million years ago, at the very end of the Paleocene

  • Epoch.

  • Back then, life was still recovering from the unpleasantness of the Cretaceous-Paleogene

  • extinction event, which wiped out the non-avian dinosaurs.

  • And things were already warm by today’s standards.

  • There were no polar ice caps, which meant sea levels were much higher.

  • And the continents -- which were just beginning to take a familiar shape -- were covered in

  • habitats like temperate forests, and deserts, and a belt of rainforests around the equator.

  • But this environment was about to change.

  • In fewer than 20 thousand years, the global average temperature increased by 5 to 8 degrees

  • Celsius.

  • And the warming was greatest at higher latitudes.

  • So, at the poles, temps on land reached an average of 23 degrees, while the ocean waters

  • got up to a balmy 20 degrees, This means you couldve gone for a comfy swim in the seas

  • around Antarctica!

  • This remarkable and sudden warming event is known as the Paleocene-Eocene Thermal Maximum,

  • or PETM, and it had a massive effect on life on Earth.

  • For one thing, when the PETM reached its peak, rainforests had expanded much farther than

  • they ever had before.

  • Fossils from North America, Europe, and Asia reveal habitats rich in plant life that today

  • are associated with tropical rainforests -- even though these forests were nowhere

  • near the tropics.

  • The fossilized fronds of palm trees have been found as far north as Wyoming, for example.

  • And some places within the Arctic Circle, like Ellesmere Island in Canada, show evidence

  • of ferns, redwoods, and gingkos.

  • So.

  • How was all of this -- any of this -- possible?

  • Our best clues can be found in ancient sediments.

  • Marine sediment samples from Maryland to Antarctica show that, about 56 million years ago, there

  • was a sudden spike in the amount of carbon dioxide and other greenhouse gases in the

  • oceans.

  • And judging by the types of carbon found in these sediments, the gases likely came from

  • organic matter, like plants.

  • See, plants, like most living things, prefer to use the lighter and more common isotope

  • of carbon, carbon-12, as opposed to heavier isotopes, like carbon-13.

  • So, this biogenic carbon -- which weve talked about before -- has a different chemical

  • signature than carbon that’s never been part of a living organism.

  • And, sediments that date to the start of the PETM, show a large and sudden drop in the

  • ratio of carbon-13, compared to carbon-12.

  • This means that a bunch of biogenic carbon must have suddenly been released into the

  • atmosphere, in the form of carbon dioxide, methane, and other gases.

  • But, where did these gases come from?

  • Well, one hypothesis is that there was a rash of massive wildfires that unleashed tons of

  • CO2 that had been locked up in plants.

  • Another model proposes that giant seams of coal were exposed to the heat of volcanic

  • activity, which would have released the carbon from fossilized plants.

  • Or it could be that an otherwise mild warming event triggered the release of greenhouse

  • gases, by melting deposits of a compound known as methane hydrate.

  • Methane hydrate is similar to ice, but it contains molecules of methane trapped by molecules

  • of water.

  • And hydrates are usually stable, as long as theyre under a lot of pressure, like deep

  • in the oceans, or if theyre kept cold, like in permafrost -- the thick layer of frozen

  • soil that forms in cold climates.

  • But if these places warm up, the hydrates melt, releasing bursts of methane, which is

  • an even more potent greenhouse gas than CO2.

  • And of course, the more warming that happens, the more melting there is, which releases

  • even more greenhouses gases, creating a classic positive feedback loop.

  • Now, no matter how it started, it’s worth noting that, during the PETM, carbon was released

  • into the atmosphere at only a fraction of the rate at which it’s being emitted today.

  • One study of marine sediments from the Arctic showed that, at the peak of the PETM, as much

  • as 1.7 billion metric tons of carbon were released into the atmosphere every year, for

  • at least 4,000 years.

  • A similar study of sediments from New Jersey put the figure at about 1.1 billion tons of

  • carbon every year.

  • Now, compare that to the amount of carbon being released today.

  • In 2014 alone, it was 9.8 billion metric tons of carbon.

  • So, 56 million years ago, carbon was being released less quickly than it is now, but

  • those emissions continued for thousands of years.

  • And it was more than enough to create a potent greenhouse effect.

  • With more carbon in the atmosphere than plants could absorb, the planet started to change

  • rapidly.

  • In many places, the climate delivered a combination of humidity and heat that allowed vast rainforests

  • to flourish.

  • And among the animals that thrived in these warm forests were reptiles.

  • Fossils of alligators, crocodiles, and turtles can be found in nearly every fossil bed from

  • the PETM -- even in the polar forests of Canada and Greenland.

  • And these lush forests were also where many early mammal groups diversified -- including

  • our every own lineage, the primates.

  • In fact, the earliest true primates appear in the fossil record just as the PETM was

  • starting to take off, 56 million years ago.

  • They adapted quickly to a world covered in trees, developing things like forward-facing

  • eyes, fingernails instead of claws, and opposable thumbs.

  • These features gave primates such an edge, that by 53 million years ago, they could be

  • found across the northern hemisphere -- from tiny Eosimias in China

  • to Notharctus in Wyoming.

  • But in the oceans, life in hothouse Earth became much harder.

  • In fact, in some places it was almost impossible.

  • At the equator, ocean temperatures were unbearably hot, sometimes reaching as high as 36 degrees,

  • almost as hot as your average hot tub.

  • This was probably too hot for many kinds of plankton, which were -- and are -- the basis

  • for most ocean food webs.

  • But an even more devastating side effect of high CO2 levels was ocean acidification.

  • When ocean water absorbs CO2, it becomes more acidic.

  • And this in turn depletes the water’s concentration of carbonates -- the compounds that many organisms

  • use to build shells and other structures.

  • And this is why one of the clearest effects of the thermal maximum can still be found

  • in core samples from the deep sea.

  • Sediments that date back to before the warming are typically pale in color, because theyre

  • rich with the skeletons of deep sea foraminifera.

  • Also known as forams, these are tiny protozoans that build shells of calcium carbonate.

  • And where forams were abundant, the chalky fossils of their shells turned the ocean bed

  • white.

  • But when the oceans became more acidic, the sediments turned dark.

  • Becausemost of the forams just disappeared.

  • During the PETM, between 30 and 50 percent of all foram species went extinct.

  • The same phenomenon also stunted the growth of hard corals, which need carbonates to build

  • their skeletons, too.

  • So, during the PETM and for millions of years afterward, big, complex coral reefs all but

  • disappeared from the fossil record.

  • All told, the thermal maximum was a mixed bag for life on Earthproving to be an important

  • period for us mammals, but a major loss for some marine life.

  • And, like all dramatic events, the PETM did come to an end.

  • Although, were not sure how, or why.

  • Over the course of the Eocene epoch, the climate slowly began to cool.

  • And although the temperature occasionally spiked again, it never reached the extremes

  • of the maximum.

  • Temperatures kept dropping during the Eocene -- so much so, in fact, that by the end of

  • the epoch, 34 million years ago, polar ice caps had begun to form.

  • But, how did we get from rainforests near the poles to ice caps?

  • Well, the cause of the initial cooling that actually stopped the PETM 53 million years

  • ago remains a mystery.

  • But something allowed that cooling to take hold, and make the world even colder.

  • And the answer here might have to do, again, with plants.

  • Arctic sediments that date back to the early Eocene -- 49 million years ago -- have been found

  • to contain huge swaths of fossilized aquatic ferns known as Azolla.

  • These plants thrived in the lush, warm Arctic.

  • But as the environment changed, they died off.

  • And as they dropped to the seafloor, the thinking goes, they took tons of carbon with them,

  • which caused temperatures to drop even further.

  • Despite how little we know about its end, or its beginning, the Paleocene-Eocene Thermal

  • Maximum shows us just how polarizing climate change can be for life on Earth.

  • For some organisms, like early primates, the warming was a chance to develop new forms

  • and spread to new locations.

  • But for corals, forams, and other marine life, such extreme heat spelled disaster.

  • It gives us, in the midst of our own period of warming, a view of how extreme the effects

  • of climate change can be.

  • And it allows us to make some pretty striking comparisons.

  • Remember when I said that, during the PETM, the globe warmed more than humans had seen

  • so far?

  • Well, keep in mind that, in recent years, the rate of annual carbon emissions have been

  • more than five times greater than they were at the peak of the PETM.

  • As a result, our world is warming faster than it did back in the Eocene.

  • Just over the past hundred years, the average global temperature has increased by about

  • 0.7 degrees Celsius.

  • But that’s just been over the past century.

  • During the PETM, it took perhaps thousands of years for temperatures to rise that much.

  • So the PETM is the closest we can get to understanding the effects of global warming today.

  • And it has a lot to teach us about the extremes that life experienced, on land and in the

  • seas.

  • Yes, rainforests full of primates and insects and reptiles is beautiful.

  • But I think youll agree with me that most of us like them right where they are today.

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上一次全球变暖(The Last Time the Globe Warmed)

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    joey joey 發佈於 2021 年 04 月 30 日
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