Many of the threats we know are associated with climate change are slow moving. Gradually rising seas, a steady uptick in extreme weather events, and more all mean that change will come gradually to much of the globe. But we also recognize that there can be tipping points, where certain aspects of our climate system shift suddenly to new behaviors.

The challenge with tipping points is that they're often easiest to identify in retrospect. We have some indications that our climate has experienced them in the past, but reconstructing how quickly a system tipped over or the forces that drove the change can be difficult. Now, a team of Norwegian scientists is suggesting it has watched the climate reach a tipping point: the loss of Arctic sea ice has flipped the Barents Sea from acting as a buffer between the Atlantic and Arctic oceans to something closer to an arm of the Atlantic.

Decades of data

The Norwegian work doesn't rely on any new breakthrough in technology. Instead, it's built on the longterm collection of data. The Barents Sea has been monitored for things like temperature, ice cover, and salinity, in some cases extending back over 50 years. This provides a good baseline to pick up longterm changes. And, in the case of the Barents Sea in particular, it's meant we've happened to have been watching as a major change took place.

The Barents Sea lies north of Norway and Russia, bounded by Arctic islands like Svalbard and Franz Josef Land. To its west is the North Atlantic, and the Arctic Ocean is to its north. And data from prior to the year 2000 indicates that the Barents acted as a buffer between the two oceans.

To the north, the Arctic Ocean has been dominated by sea ice, which spreads into the Barents during the winter. The ice acts as a barrier to exchanging heat with the atmosphere and blocks sunlight from reaching the ocean water, helping keep the Arctic colder in the summer. As it melts, the Barents also creates a layer of fresh water that doesn't mix well with the salt water below it, and it is light enough to remain at the surface. The water of the Atlantic is warmer but saltier and better mixed across its depths.

In between, in the Barents, the two influences create a layer of intermediate water. The Arctic surface water and sea ice helps keep the Barents fresher and cool. And while the Barents is warmed from below by the dense, salty Atlantic water, it's not enough to allow the two layers to mix thoroughly. This helps keep the Barents Sea's surface water cold and fresh, encouraging it to freeze over during the winter.

The researchers behind the new work say that this layered structure was "remarkably stable" from 1970 all the way through 2011. But change started coming to the area even as the layers persisted. The atmosphere over the Arctic has warmed faster than any other region on the planet. In part because of that, the amount of ice covering the Arctic Ocean began to decline dramatically. It reached what were then record lows in 2007 and 2008. As a result, the Barents Sea was relatively ice-free in the Arctic summer, decreasing the fresh water present in the surface layer.

Sea-ice drift into the Barents sea dropped enough so that the 2010-2015 average was 40 percent lower than the 1979-2009 mean. The researchers checked precipitation at some islands on the edge of the Barents Sea, and they confirmed that the loss of fresh water at the surface was due to the loss of ice rather than a change in weather patterns.

(For context, the Barents Sea is essentially ice-free at the moment, even though the melt season typically extends through September.)

Triple threat

The loss of ice also means that the surface water in this area is exchanging heat with the atmosphere and absorbing more sunlight during the long Arctic summer days. These two have combined to heat the top 100m of water dramatically. If the mean of its temperature from 1970-1999 is taken as a baseline, the temperatures from 2010-2016 are nearly four standard deviations higher. 2016—the most recent year we have validated data for—was 6.3 standard deviations higher.

This has the effect of heating the intermediate water from above. Meanwhile, the warm Atlantic water will heat it from below. As a result, the cold intermediate water has essentially vanished from the Barents Sea, turning the area into a basin dominated by Atlantic water. The entire water column, from surface to the sea floor, has both warmed and gotten saltier, all starting in the late-2000s.

While dramatic, that in and of itself doesn't make for a tipping point. But the authors argue that the present conditions make it extremely difficult for the sea ice to re-establish itself during the winter: "Increased Atlantic Water inflow has recently enlarged the area where sea ice cannot form, causing reductions in the sea-ice extent." The water both starts out warmer and has increased salt content, making freezing more difficult.

In essence, the authors argue that the entire Barents Sea has started to behave as an arm of the Atlantic. Unless some external factor re-establishes the layer of fresh water on the surface, "the entire region could soon have a warm and well-mixed water-column structure and be part of the Atlantic domain."

Tip of the ice

From a strictly human-centric position, the changes aren't necessarily a terrible thing. In terms of ecosystems, the authors describe the Barents as "divided into two regions with distinct climate regimes—the north having a cold and harsh Arctic climate and ice-associated ecosystem, while the south has a favorable Atlantic climate with a rich ecosystem and lucrative fisheries." The expansion of these fisheries, while coming at the cost of the native ecosystem, could prove a boon for the countries bordering the region.

But the general gist of the study is considerably more ominous: not only have we discovered a climate tipping point, but we've spotted it after the system has probably already flipped into a new regime. It also provides some sense of what to expect from the future. Rather than seeing the entire planet experience a few dramatic changes, we're likely to see lots of regional tipping points that have more of a local effect. The future will be the sum of these events and their interactions, making it a bit harder to predict which changes we should be planning for.

Nature Climate Change, 2018. DOI: 10.1038/s41558-018-0205-y (About DOIs).