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Atlantic confirmed as accomplice in Arctic sea ice loss

Source: Polyakov, I. et al., Greater role for Atlantic inflows on sea-ice loss in the Eurasian Basin of the Arctic Ocean, Science, 10.1126/science.aai8204 (2017)

The changing Arctic

When people talk about climate change, one of the most common and powerful images invoked is that of a lone polar bear drifting in the Arctic Ocean on a tiny, fragmented piece of ice. There is a good reason that this has become a symbol of the impacts of climate change. Yes, polar bears are cute and lovable and likely to grab our attention, but beyond that, the Arctic is the most rapidly changing region on Earth.

Photo of a lone polar bear in the Arctic. From the World Wildlife Fund.

 

Sea ice cover in the Arctic has been shrinking rapidly since satellites began recording sea ice cover. This winter, the maximum Arctic sea ice extent was at a record low for the third year in a row. The eastern Eurasian Basin of the Arctic has been nearly ice free in summer since 2011. Less ice and warmer waters impact Arctic ecosystems, changing the habitat and food availability for many species. Beyond local impacts, the loss of sea ice in the Arctic has a global impact. Sea ice is bright, and reflects sunlight back to space. When sea ice melts, the white sea ice is replaced by dark ocean. Instead of reflecting 80% of sunlight, the ocean absorbs 90% of sunlight causing the ocean to warm and further melt sea ice.

Drivers of change

Understanding the processes in the atmosphere and ocean that drive the rate of Arctic sea ice melt is essential for predicting how the Arctic will continue to change in the future. Most ice loss happens as a result of summer heating of the surface ocean. Summer solar radiation is absorbed in the ocean surface mixed layer, a well-mixed layer of water with uniform density that usually has a large density difference compared to the water below. The heated surface water seeps into  cracks in the ice, melting the ice and freshening the surface water. … This freshening and subsequent decrease in density acts as a barrier, preventing the heat in the relatively warm, salty water originating from the Atlantic Ocean from entering the surface mixed layer from below. Up until now, the input of heat from the ocean below has been thought to be insignificant compared the heating from the sun, but new observations from the Arctic Ocean show changes that indicate warm water from the ocean interior may be more important for melting sea ice than previously thought.

Figure 1 from Polyakov et al. showing the decrease in ice coverage and ice thickness, as well as increase in time with open water in the eastern Eurasion Basin (shown as a black polygon in panel A) from 2003 to 2016.

New insights

A group of scientists, led by Igor Polyakov at the University of Alaska, Fairbanks, collected new measurements in the Eurasian Basin of the Arctic Ocean in 2013-2015 using fixed moorings and ice-tethered profilers, which are instruments that drift with sea ice, collecting vertical profiles of the ocean properties below. The instruments collected detailed measurements of temperature and salinity from the sea surface to several hundreds of meters below.

By comparing these observations to earlier data, the researchers could see how the vertical structure and seasonal variations in the ocean had changed over time. In the past, the eastern Eurasion Basin had a strong density difference between the surface mixed layer and the Atlantic waters below, preventing mixing up of heat from below to drive sea ice melt. In the new observations, this density difference had weakened noticeably, which allowed more mixing with waters below. As well as this, the pool of warm Atlantic water had become shallower, making more heat available below the surface mixed layer. The combination of these two changes resulted in 2-4 times more heat moving into the surface mixed layer from below than in 2007-2008, which could explain at least as much, or possibly more, of sea ice loss as warming from solar radiation.

Figure 4. From Polyakov et al. showing measurements in 2014 and 2015 at four different moorings in the eastern Eurasian basin of water temperature at each depth (left) and heat content for the 60-135 m layer (right). The red and orange lines show the heat content trend in winter of each year, which is equivalent to the amount of upward heat transfer of Atlantic water.

A possible side effect of this change is that more mixing brings up extra nutrients along with heat, which can trigger increased growth of phytoplankton, increasing productivity. As more of the Arctic becomes ice-free for longer in summer, sunlit waters can support more phytoplankton growth and scientists are still working to understand how this change could change Arctic ecosystems.

Figure 5 from Polyakov et al. illustrating the change in the eastern Eurasian basin of the Arctic from the early 2000s to mid-2010s. The mixing in the surface mixed layer (SML) increased (dashed ellipse), and extended to deeper depths in the mid-2010s compared to the early 2000s. Also, the transfer of heat from Antarctic water (AW) into the surface ocean increased (red arrows), increasing the heat transfer to sea ice.

An ice-free future

The discovery that changes in the mixing of heat from water from the Atlantic is an important driver of sea ice loss illustrates the complexity of the Arctic response to climate change, and highlights the need for sustained observations of the Arctic Ocean. However, it is still unclear exactly what triggers these changes, and how much they are affected by climate variability on decadal timescales. As Arctic sea ice continues to melt, the prospect of an ice-free Arctic summer is becoming more imminent, which has enormous implications not only for species and ecosystems, but for international shipping and trade.

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