The number of open water days per year — meaning days without sea ice — increases significantly in the Arctic region between 1920 and 2100, according to a new modeling study. (Individual images: Katy Barnhart. Animation: Tom Yulsman) Ice was in the news quite a lot last week. There was, for example, the news that Antarctica could be gaining, not losing, ice, at least for now. Lest you were tempted to breathe a sigh of relief, critics pointed to contrary evidence. And there were also these unsettling findings about the gigantic West Antarctic Ice Sheet. But far to the north, some equally important news unfortunately got less attention. For the long and short of it, see the animation above. I created it to portray in visual form what a study published last week has revealed: Unless we start reining in greenhouse gas emissions, "our projected impact on the climate system will change the face of Arctic sea ice," says Katy Barnhart, who led the study while she was at the University of Colorado's Institute for Arctic and Alpine Research. (Full disclosure: I'm a professor at the University of Colorado.) The animation shows computer model simulations of how Arctic sea ice is likely to respond to continued human-caused warming. More specifically, it models how the number of days of open water change each year from 1920 to 2100 under a "business-as-usual" scenario. By 2100, according to the study, much of the Arctic has greater than 150 additional days of open water as compared with the pre industrial period prior to 1850. But we won't have to wait that long to see significant changes. If greenhouse gas emissions continue to rise, the entire coastline of the Arctic, and most of the Arctic Ocean itself, will see at least 60 additional days a year of open water by mid-century. Some sites will even see 100 additional open water days. Arctic sea ice has already shrunk in geographic extent during the period of modern observations. This has been one of the significant signs and symptoms of human-caused warming. But for people and other species in the Arctic, something else is even more important than an overall, general decrease in ice extent: What's going to happen in their neck of the woods? With that in mind, Katy Barnhart, and her colleagues, sought to produce a detailed forecast of open water patterns in the Arctic. Among the questions they sought to answer were these:

Specifically where will melting sea ice leave behind open water?

For how long during the course of a year will that water actually stay open?

When will these changes become so significant that they'll completely exceed natural variability?

To understand why these are important questions, imagine that you're a polar bear. Seals are your main source of food, and to hunt them you need to perch on sea ice. Your life is already more difficult, thanks to shrinking ice. But what might happen to you and your kin in the future? Scientists could get a better fix on this if they knew where open water is likely to occur in a warming climate, and how long it might stay open each year. And if you happen to be the owner of, say, a shipping company in China, you may be hoping that shrinking Arctic sea ice will allow you to send your ships to Europe along the Northern Sea Route, which runs off Russia's Arctic coast. It offers a much shorter sail to Europe than the long way around through the Suez Canal. Right now, sea ice even in summer usually makes the route impractical, at best. Knowing where and when open water will occur in coming years could be quite useful to your business as you plan for the future.

The Swedish icebreaker Frej rides at anchor just off Longyearbyen in Svalbard. As sea ice shrinks, more open water could lure increased ship traffic to the region. Although there will like be less ice, conditions will still be harsh and prone to sudden local shifts — ironically increasing the need for ships like this. (Photo: © Tom Yulsman) Polar bears and shipping magnates — not to mention phytoplankton and fishermen — might be interested in what Barnhart and her colleagues found:

Throughout the Arctic, the open water season began lengthening in the 1990s, "with ice break-up starting earlier and freeze-up setting in later," Barnhart says. The study also showed that this marked the beginning of a shift away from pre industrial conditions.

Even so, for most locations, the shift remains within the natural range of preindustrial times. And t hat means what we've seen in the Arctic so far has been the result of both the natural, internal variation of the climate system itself plus a push from us through our greenhouse gas emissions.

But the study also shows that by 2040, increases in open water driven by human-caused warming will completely exceed the range of variability seen prior to 1850. As part of the study

, which appeared last week in the journal Nature Climate Change, Barnhart and her colleagues also examined specific places along the Arctic coastline, including Drew Point on Alaska's North Slope. Here, open water once was present for an average of 50 days each year. Since about 2000, both satellite observations and the scientists' model simulations show an increasing open water trend. This, in turn, has caused rapid and accelerating coastal retreat, due to erosion along the seashore. You can watch it happening in this time-lapse video: https://www.youtube.com/watch?v=LaB3nKxPjiU The coastal retreat is happening because sea ice ordinarily fortifies the coastline against wave action and storm surges. With more days of open water, and therefore less protection, erosion has increased — a phenomenon that has posed significant challenges

for some Alaskan North Slope communities. With continuing increases in greenhouse gas emissions, things are likely to get worse. At Drew Point, the simulations show that sea ice is likely to be absent for almost 200 days by 2070. As Barnhart summarizes things

: