Let's Get 7 in IELTS

 ScienceDaily (June 12, 2008) — East Siberia's permafrost contains about 500 Gigatons (1100 trillion pounds) of frozen carbon deposits that are highly susceptible to disturbances as the climate warms.

Called the Yedoma, this permafrost has not undergone much alteration by soil microorganisms since its formation, which took place between 20,000 and 40,000 years ago. To investigate how easily this huge carbon stock could be degassed in future warming scenarios, Khvorostyanov et al. use a model of heat transfer and soil organic matter decomposition in frozen soils and find that specific conditions trigger the irreversible thawing of Yedoma, which is maintained by heat production by soil microbial activity.

Once started, irreversible thawing could release 4.4-6.2 trillion pounds of carbon per year into the atmosphere between the years 2300 and 2400, transforming 74 percent of the initial carbon stock into carbon dioxide and methane.

Further investigations reveal that the faster the planet's surface warms, the sooner fast deep-soil decomposition will start, although the tipping point above which soil carbon starts irreversible mobilization due to permafrost thawing increases slightly with larger external warming rates.

alteration : n  改變
decomposition : n  分解
degas : vt  脫氣
disturbance : n  騷動
formation : n  形成
initial : a  最初
investigate : vt  調查
irreversible : a  不能翻轉的;不能取消的;不能倒置的
methane : n  甲烷
microbial : a  微生物
microorganism : n  微生物
mobilization : n  動員
reveal : vt  展現,顯露出
scenario : n  腳本
susceptible : a  易受影響
undergo : vt  經歷