版主:黑木崖
    
 · 九阳全新免清洗型豆浆机 全美最低
 
America colonisation ‘cooled E
送交者: 大马里昱 2019月02月01日17:00:59 于 [世界军事论坛] 发送悄悄话
回  答: 明朝灭亡是欧洲人占领美洲害的,因为农耕地都变回森林了 大马里昱 于 2019-02-01 16:57:09

America colonisation ‘cooled Earth's climate’

By Jonathan Amos BBC Science Correspondent


    31 January 2019


Share this with Facebook

Share this with WhatsApp

Share this with Messenger

Share this with Twitter


    Share 


Media captionMark Maslin and Chris Brierley: "A genocide-generated drop in carbon dioxide"


Colonisation of the Americas at the end of the 15th Century killed so many people, it disturbed Earth's climate.


That's the conclusion of scientists from University College London, UK.


The team says the disruption that followed European settlement led to a huge swathe of abandoned agricultural land being reclaimed by fast-growing trees and other vegetation.


This pulled down enough carbon dioxide (CO₂) from the atmosphere to eventually chill the planet.


It's a cooling period often referred to in the history books as the "Little Ice Age" - a time when winters in Europe would see the Thames in London regularly freeze over.


"The Great Dying of the Indigenous Peoples of the Americas led to the abandonment of enough cleared land that the resulting terrestrial carbon uptake had a detectable impact on both atmospheric CO₂ and global surface air temperatures," Alexander Koch and colleagues write in their paper published in Quaternary Science Reviews.


    Welcome to the Meghalayan Age

    New temperature 'starting point' sought


Image copyright Getty Images

Image caption Cool times: A depiction of a 17th Century Frost Fair at Temple Stairs on London's Thames River

What does the study show?


The team reviewed all the population data it could find on how many people were living in the Americas prior to first contact with Europeans in 1492.


It then assessed how the numbers changed in following decades as the continents were ravaged by introduced disease (smallpox, measles, etc), warfare, slavery and societal collapse.


It's the UCL group's estimate that 60 million people were living across the Americas at the end of the 15th Century (about 10% of the world's total population), and that this was reduced to just five or six million within a hundred years.


The scientists calculated how much land previously cultivated by indigenous civilisations would have fallen into disuse, and what the impact would be if this ground was then repossessed by forest and savannah.


The area is in the order of 56 million hectares, close in size to a modern country like France.


This scale of regrowth is figured to have drawn down sufficient CO₂ that the concentration of the gas in the atmosphere eventually fell by 7-10ppm (that is 7-10 molecules of CO₂ in every one million molecules in the air).


"To put that in the modern context - we basically burn (fossil fuels) and produce about 3ppm per year. So, we're talking a large amount of carbon that's being sucked out of the atmosphere," explained co-author Prof Mark Maslin.


"There is a marked cooling around that time (1500s/1600s) which is called the Little Ice Age, and what's interesting is that we can see natural processes giving a little bit of cooling, but actually to get the full cooling - double the natural processes - you have to have this genocide-generated drop in CO₂."

Image copyright Heidi Roop/NSF

Image caption The fall in global carbon dioxide levels is recorded in the air bubbles trapped in ice cores

Where's the support for the connection?


The drop in CO₂ at the time of the Great Dying is evident in the ice core records from Antarctica.


Air bubbles trapped in these frozen samples show a fall in their concentration of carbon dioxide.


The atomic composition of the gas also suggests strongly that the decline is being driven by land processes somewhere on Earth.


In addition, the UCL team says the story fits with the records of charcoal and pollen deposits in the Americas.


These show the sort of perturbation expected from a decline in the use of fire to manage land, and a big grow-back of natural vegetation.


Ed Hawkins, professor of climate science at Reading University, was not inv

0%(0)
0%(0)
标 题 (必选项):
内 容 (选填项):
实用资讯
北美最大最全的折扣机票网站
美国名厂保健品一级代理,花旗参,维他命,鱼油,卵磷脂,30天退货保证.买百免邮.