CLIMATE CHANGE is affecting all parts of the globe. But no place is feeling the heat quite like the Arctic. Although average global temperatures have risen by about 1°C since pre-industrial times, warming in the Arctic is happening two to three times as fast. The area of ice covering the Arctic ocean is at a record low, according to America’s National Snow and Ice Data Centre. Over the past 30 years, the minimum coverage of summer ice has fallen by half.

Scientists know the Arctic has less ice than it than it once did, but precise measurements are limited. Satellite images of the region, the best source of data available, go back less than 40 years. And even though they are useful for measuring the area of sea ice, they provide no information about its depth. To fill in the blanks, researchers have turned to ships’ logs from the past. Old Weather, a crowdsourcing project, has enlisted the help of thousands of volunteers to transcribe entries from 20th-century ship logbooks (some written by hand) to help scientists compile historical climate data to feed into their models.

In a new paper, researchers from the University of Washington used these data to reconstruct conditions in the Arctic dating back to the early twentieth century. They found that an earlier period of Arctic warming, between 1920 and 1940, led to a significant decline in the volume of sea ice, mostly on the Atlantic side of the ice-cap. The recent decline of Arctic sea ice, largely driven by human activities, has been about six times faster. It has also affected the entirety of the Arctic ocean, especially the East Siberian Sea. The authors reckon that, between 1979 and 2010, the volume of sea ice in the Arctic in September declined by between 55% and 65%.

Roald Amundsen, a Norwegian explorer, led the first successful crossing of the Northwest Passage—connecting the Atlantic with the Pacific by skirting Canada—in 1903. Today the Arctic is starkly different. As the sea ice continues to shrink, new shipping routes may open up. A journey that once took Mr Amundsen three years may soon take as little as a couple of weeks.