The Arctic is called by some the canary in the global-warming coal mine. Like that fated bird, carried in cages by pitmen well into the 20th century, it is sensitive to changes which might otherwise not be obvious. Canaries expired in contact with gases such as carbon monoxide and methane, warning miners to leave the area. The Arctic-or, rather, its sea ice-is similarly expiring as the Earth warms up in response to more of another gas, carbon dioxide. The area of the Arctic Ocean covered by ice at the height of summer has been shrinking by 11% a decade for the past 35 years. But the details are obscure-because gathering data in the Arctic Ocean is hard. This year, therefore, a systematic approach to that gathering has begun. The Marginal Ice Zone (miz) programme, paid for by the United States Navy, is seeding the Beaufort Sea-the part of the ocean north of Alaska-with many sensors.
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