南極半島では過去20年間に気温低下、世界は温暖化進行−報告書
Jessica Shankleman
2016年7月21日 12:18 JST
この地域の気温上昇トレンドは1990年代に停止
変化は自然の気候変動と共に進んでいる可能性：研究者ら
地球上で過去50年間に最も急速に温暖化が進んだ場所の一つで、実際には過去20年間気温が低下していたとの調査結果が示された。
英国南極調査所の研究者10人は20日、科学誌ネイチャーに掲載された報告書で、世界の他の地域が記録的な高温となる一方、南極大陸西部の南極半島の気温は低下に転じており、1990年代後半以降、10年ごとに平均でセ氏約0.5度下がっていると結論付けた。
報告書の執筆者らは、自身らが気付いた変化は自然の気候変動の極端な現象の一つである可能性があると説明し、それが地球温暖化をめぐる議論にとって何を意味するかについては結論を提示していない。このことは、南極の気温の方向性と、それが世界にとって何を意味するかを判断するには数年を要し、さらなる調査が必要である可能性を示唆している。気候変動を専門とする科学者らはこの報告書について、慎重に取り扱われるべきだと指摘する。
英レディング大学の大気科学センターのエド・ホーキンス氏は「地球上の非常に小さな地域の気温が短期的に低下していることは特に驚きではない。大気循環と長期的な温暖化傾向とが相互に作用する中での自然の気候変動として予想されるものだ」と指摘した。
原題：Antarctic Peninsula Cooled in Past Two Decades as World Warmed（抜粋）
https://www.bloomberg.co.jp/news/articles/2016-07-21/OAN8QX6JIJVL01
気候科学：自然変動と一致する温暖化していない21世紀の南極半島
John Turner, Hua Lu, Ian White, John C. King, Tony Phillips, J. Scott Hosking, Thomas J. Bracegirdle, Gareth J. Marshall, Robert Mulvaney & Pranab Deb
AffiliationsContributionsCorresponding author
Nature 535, 411–415 (21 July 2016) doi:10.1038/nature18645
Received 05 February 2016 Accepted 06 June 2016 Published online 20 July 2016
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1950年代以降、南極半島の観測基地で、地表付近の気温の南半球における最も大きな上昇が何回か記録されている。こうした温暖化は、氷河の局地的な後退、浮き棚氷の崩壊、さまざまな植物相の範囲の拡大を通した「緑化」に寄与してきた。成層圏のオゾン層破壊、局所的な海氷の減少、西風の増大、低緯度域と高緯度域の大気遠隔相関の強さと位置の変化などのいくつかの連結過程が、温暖化に寄与する過程として示唆されている。本論文では、蓄積された気温の記録を用いて、1990年代後半以降局地的に温暖化していないことを示す。年平均気温は、統計的に有意な速度で低下しており、南半球夏季に最も急速に寒冷化していた。中緯度ジェットの強化に伴ってウェッデル海北部の状態がより低気圧的になったことに起因して、寒冷な東から南東寄りの風の頻度が増えた結果、気温が低下していた。こうした循環の変化は、半島東岸への海氷の移流も増加させ、その効果を増幅している。今回の結果は、南極大陸の1％のみを対象としているものであり、この地域の気温の十年変動は、主に全球の気温変動の駆動因に伴うものではなく、局所的な大気循環内部の極端な自然変動を反映していることを強く示している。
http://www.nature.com/nature/journal/v535/n7612/fp/nature18645_ja.html?lang=ja
DAILY NEWS 20 July 2016
One part of Antarctica has been cooling since 1998 – here’s why
A lenticular cloud formation over the Lemaire Channel on the Antarctic Peninsula in Antarctica.
A cold wind blows
Steve Allen/Getty
By Michael Le Page
The Antarctic Peninsula had been warming since at least the 1950s, when the first weather stations were set up.
But a shift in prevailing winds has resulted in it cooling since 1998 — although this region is still warmer than it was when observations began.
“What we are seeing is natural climatic variability overriding global warming,” says John Turner at the British Antarctic Survey (BAS), who led the study.
The peninsula has cooled by roughly 0.5 °C per decade, which is similar to the rate at which it warmed in the preceding five decades (see graph below). This cooling will be temporary. Even if the winds don’t change again soon, global warming will overwhelm their effect in the coming decades.
Nor does this slight cooling of the air above one part of Antarctica mean we can worry less about the main threat from the continent: sea level rise. That’s because the accelerating loss of ice from Antarctica is driven almost entirely by the warming seas around the continent.
https://d1o50x50snmhul.cloudfront.net/wp-content/uploads/2016/07/20170022/graphic-800x533.jpg
The Antarctic Peninsula has cooled (blue line) since 1998
Robert Mulvaney/British Antarctic Survey
The unstable West Antarctic ice sheet has already passed the point of no return, meaning there’s no way to stop sea level rising at least 5 metres over the coming centuries. And if we don’t do more to limit warming soon, we’ll be committed to a rise of about 20 metres.
“The real threat is ocean warming,” says Andrew Shepherd at the University of Leeds, UK, who studies the poles and was not part of the study. “We should not lose sight of that.”
Overall, global surface temperatures have risen by more than 1 °C since preindustrial times, but there are big variations from place to place. The Arctic has warmed by several degrees, as its heat-reflecting sea ice shrinks and exposes ever more heat-absorbing open water.
Antarctica, by contrast, has barely warmed at all. The warming seas around its edges are nibbling away at ice shelves and glaciers, but have not – yet – penetrated its vast interior.
What’s more, the winds that circle the South Pole act as a shield, keeping out warmer air from higher latitudes. These winds have grown stronger, partly because of the hole in the ozone layer.
The exception to this was the Antarctic Peninsula, which extends far out from the rest of the continent. Up until 1998, the six weather stations on the northern tip of the peninsula – about 1 per cent of Antarctica – had been showing a clear warming trend, driven by warm westerly winds.
Strong wind
Winter temperatures increased by 6 °C at one station, which led to the peninsula being described as the fastest-warming place on Earth. That warming was linked to a decline in the Adélie penguin population and the disintegration of a large portion of the Larsen B ice shelf, which had existed for millennia.
But over the past two decades, cold easterly winds have increased in strength, resulting in a cooling trend. There’s been little change in the rest of Antarctica, says Turner.
What’s most surprising is that Turner’s team conclude that the previous warming seen in the Antarctic Peninsula cannot be clearly attributed to global warming. Data from ice cores suggest it is still – just – within the range of natural variability seen over the past 1000 years.
This aspect of the study is likely to be seized upon by climate deniers. To be clear, Turner’s team isn’t saying that the net warming in the peninsula isn’t due to climate change, just that several more decades of warming will be necessary before we can say so for sure. Climate models are predicting that the region will warm by several degrees this century.
And with 2016 set to be the warmest year by far since records began, the evidence of global warming has never been clearer. The world is approaching the 1.5 °C above preindustrial limit agreed at the UN climate summit in Paris last December alarmingly fast.
“We have not got to that initial Paris number, but it’s fair to say we are dancing with the lower targets,” Gavin Schmidt at NASA’s Goddard Institute of Space Studies said Tuesday as he revealed that this June was the hottest ever recorded.
Journal reference: Nature, DOI: 10.1038/nature18645
Read more: Major Antarctic ice survey reveals dramatic melting
More on these topics:
https://www.newscientist.com/article/2098187-one-part-of-antarctica-has-been-cooling-since-1998-heres-why/
http://www.asyura2.com/15/nature6/msg/385.html
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