Greenland lost 2 billion tons of ice last Thursday, very unusual
（圖片來源: sciencealert.com）據外媒報導，格陵蘭島冰蓋上周四經歷了一場「不同尋常的融冰」，估計一天之內總融冰量達到20億噸。雖然格陵蘭島到處都是冰塊，但在六月中旬融化這麼多的冰是不同尋常的。格陵蘭島的「融冰期」一般為每年的6月至8月，大部分融冰發生在7月。
Over 40 percent of Greenland experienced melting last Thursday, with total ice loss estimated to be more than 2 gigatons (equal to 2 billion tons) on just that day alone.
While Greenland is a big island filled with lots of ice, it is highly unusual for that much ice to be lost in the middle of June. The average "melt season" for Greenland runs from June to August, with the bulk of the melting occurring in July.
不同尋常的融冰
專門發布格陵蘭島冰蓋和北極海冰監測數據的網站Polar Portal 6月14日發文指出，13日格陵蘭島有超過40%的地區發生了冰雪融化，如此早的大規模融冰「非同尋常」。
（圖片來源：Polar Portal 6月14日發布數據截圖）上圖為Polar Portal 6月14日發布的13日格陵蘭島融冰狀況，地圖上紅色部分為出現冰雪融化的地方。圖片下方的曲線圖顯示了融冰地區面積佔格陵蘭島總面積的比例變化情況，藍色曲線表示今年的融冰程度，深灰色曲線則表示1981年至2010年融冰程度的平均值。淡灰色帶狀部分顯示的是某一日期不同年份之間融冰程度的差異，對於任意一天，該帶狀部分都顯示1981年至2010年期間的所有佔比值，當然，這些數據去除了每個日期的最大值和最小值。
The map released by Polar Portal shows where on the Greenland Ice Sheet there has been melting over the previous day. The red shows areas where melting is occurring.
The curve under the map shows how large a percentage of the total area of the ice sheet has seen melting. The blue curve shows this year’s melt extent while the dark grey curve traces the mean value over the period 1981-2010.
The light grey band shows differences from year to year. For any calendar day, the band shows the range over the 30 years (in the period 1981-2010), however with the lowest and highest values for each day omitted.
（圖片來源：CNN）美國喬治亞大學研究科學家、格陵蘭島氣候研究專家託馬斯·莫特表示，此次突然大規模融冰「非同尋常，但並非沒有先例」。
根據美國科羅拉多大學博爾德分校國家冰雪數據中心的報告，2012年，格陵蘭島融冰量創下紀錄，遠超自1979年有衛星記錄以來的所有年份，整個格陵蘭島冰蓋的97%有融冰的跡象。「今年的融冰量和我們在2012年6月看到的一些融冰峰值相當，」莫特稱。
The sudden spike in melting "is unusual, but not unprecedented," according to Thomas Mote, a research scientist at the University of Georgia who studies Greenland's climate, CNN reported.
"It is comparable to some spikes we saw in June of 2012," Mote said, referring to the record-setting melt year of 2012 that saw almost the entire ice sheet experience melting for the first time in recorded history.
Greenland’s surface melting in 2012 was intense, far in excess of any earlier year in the satellite record since 1979. In July 2012, a very unusual weather event occurred. For a few days, 97% of the entire ice sheet indicated surface melting, according to the National Snow and Ice Data Center (NSIDC).
A bird's-eye view of the Lincoln Memorial, Reflecting Pool, Washington Monument and the National Mall. (File photo from the web)此次融冰量20億噸是什麼概念？據報導，哥倫比亞大學研究人員曾對10億噸冰給出一個類比：「如果你將整個華盛頓國家廣場用冰雪覆蓋，高度達到華盛頓紀念碑（高約169米）的4倍，那麼這就是10億噸冰雪的體量。」
To visualize how much ice that is, imagine filling the National Mall in Washington with enough ice to reach a point in the sky four times higher than the Washington Monument, CNN reported.
格陵蘭島是世界上最大的島嶼，位於北美洲東北方，北冰洋和大西洋之間。格陵蘭島面積200多萬平方公裡，其中被冰蓋覆蓋的面積達180萬平方公裡。該島北岸海域長期覆蓋厚冰，通常夏天都不化。科學家稱這片區域為「最後冰區」，意思是這是北極地區最後一片常年不化的海冰。
Greenland is the world's largest island, located to the northeast of North America, between the Arctic Ocean and the Atlantic Ocean. Greenland covers an area of over 2 million square kilometers, of which about 1.8 million square kilometers are covered by ice sheets.
什麼原因造成了此次融冰？
莫特表示，一種持續存在的天氣模式為當前融冰量達到峰值埋下了伏筆。「春季大部分時間裡，格陵蘭島東部上空一直有一個高壓脊，導致四月份發生了一些融冰現象，這種情況一直在持續。」
A persistent weather pattern has been setting the stage for the current spike in melting, according to Mote.
"We've had a blocking ridge that has been anchored over East Greenland throughout much of the spring, which led to some melting activity in April -- and that pattern has persisted."
（圖片來源：gizmodo.com）高壓脊將溫暖潮溼的空氣從大西洋中部帶到格陵蘭島的部分地區，導致冰層上的溫度升高。高壓脊也阻止降水形成，並造成天氣晴朗，陽光明媚。
本月早些時候，另一個高壓脊在美國東南部造成了長時間的乾熱天氣，在過去的一兩個星期裡，隨著高壓脊從美國東部移動過來，這個高壓脊變得更加強大了。
That high pressure ridge pulls up warm, humid air from the Central Atlantic into portions of Greenland,.which leads to warmer temperatures over the ice. The high pressure also prevents precipitation from forming and leads to clear, sunny skies.
Over the past week or two, that high pressure ridge got even stronger as another high pressure front moved in from the eastern United States -- the one that caused the prolonged hot and dry period in the Southeast earlier this month.
（圖片來源：The Washington Post）莫特解釋稱，白色的冰雪很明亮，將太陽光線反射回太空，減少了熱量吸收，有助於保持冰蓋低溫，這一過程被稱為「反照率」。
「這些融化事件導致地表反照率發生了變化，」莫特表示，這將使仲夏時期更多太陽的熱量被冰吸收，並導致冰融化。
White snow and ice, which is bright and reflects the sun's rays back into space, reduces the amount of heat that is absorbed and helps to keep the ice sheet cold, a process known as "albedo."
"These melt events result in a changed surface albedo," according to Mote, which will allow more of the mid-summer sun's heat to be absorbed into the ice and melt it.
哥倫比亞大學的研究員馬科·特德斯科也指出，格陵蘭島東部和中部也異常溫暖，「這引發了大範圍冰雪融化，已經達到了冰蓋面積的45%左右。」
Marco Tedesco, an ice researcher at Columbia University, added that it has been unusually warm in east and central Greenland, as well. "This has triggered widespread melting that has reached about 45 percent of the ice sheet," he wrote in an email.
提前了的融冰季，2019年格陵蘭島融冰量或再創記錄
早在一個多月前，Polar Portal網站就發文稱，格陵蘭島的融冰季幾乎提前了一個月。丹麥氣象研究所科學家5月初宣布今年格陵蘭島融冰期於4月30日開始，今年的融冰季開始時間僅晚於2016年4月開始的融冰季，當時非常不尋常的天氣模式導致融冰季於4月中旬就開始了。
Scientists from the Danish Meteorological Institute announced early in May on Polar Portal website that the start of the Greenland melt season this year falls on April 30, second only to 2016, when a very unusual weather pattern caused a very early start to the melt season in mid-April.
「當（格陵蘭島）冰蓋表面融化超過5%並持續3天，則第一天成為當年融冰季的開始，」科學家彼得·蘭根表示，「我們採取非常嚴格的定義，因為我們希望確保這種融化是一種持續現象，而非非季節性天氣造成的曇花一現。」
「平均而言，融冰季在5月26日左右開始，所以今年的融冰季差不多早了一個月，」他說。
「The start of the melt season occurs on the first of three consecutive days where more than 5% of the ice sheet has melt at the surface.」 said scientist Peter Langen. 「We use a pretty strict definition as we want to make sure it is a consistent start to melting and not just a blip due to unseasonal weather」.
「On average, the melt season starts around the 26th May, so we are almost a full month earlier this year」 he continued.
Icebergs float in a Greenland Fjord in June. (Photo: popsci.com)「從2007年開始，我們看到了一系列大規模的融化，這在歷史上是前所未有的。在上世紀90年代末之前，我們從未見過這樣的情況，」莫特說。
"We've seen a sequence of these large melt seasons, starting in 2007, that would have been unprecedented earlier in the record," he said. "We didn't see anything like this prior to the late 1990s.」
今年夏天的此次大幅融冰或是一個不好的跡象，表明2019年格陵蘭島融冰量可能再創新高。
This much melting this early in the summer could be a bad sign, indicating 2019 could once again set records for the amount of Greenland ice loss.
「所有的跡象似乎都表明（格陵蘭島）冰川正在經歷一個大的融化季節，」莫特說。莫特並非唯一一個這麼想的科學家。
丹麥和格陵蘭地質調查局的冰氣候學家傑森·博克斯也持類似觀點，他在5月底預測「2019年將是格陵蘭島融冰大年」。
"All signs seem to be pointing to a large melt season," said Mote and he is far from the only scientist to think so.
Jason Box, an ice climatologist at the Geological Survey of Denmark and Greenland, predicted in late May that "2019 will be a big melt year for Greenland."
如果極端化融冰成為「新常態」，或對全球產生重大影響，尤其是對海平面上升而言。
莫特表示，在過去20年裡，格陵蘭島對全球海平面上升的「貢獻」越來越大，地表冰雪融化和徑流是其中很大一部分因素。」
If these extreme melt seasons are becoming the new normal, it could have significant ramifications around the globe, especially for sea level rise.
"Greenland has been an increasing contributor to global sea level rise over the past two decades," Mote said, "and surface melting and runoff is a large portion of that."
Sources: Science Alert, CNN, Gizmodo and Polar Portal
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Compiled by Zhang Jian