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Sea Ice

Pole wars: Episode 1 – The Phantom Sea Ice

A tale of two poles

The Arctic and Antarctic oceans are both covered by a layer of sea ice a few meters thick throughout the winter. Because the ice is white, it acts like a big shield from the sun, reflecting sunlight back to space. When ice melts, the dark ocean below absorbs the heat from the sun and warms, causing more sea ice to melt.

seaice_01_hr_0

Photograph of Arctic Sea Ice. Image from CoreIDRAW photo.

 

A common misconception is that the North and South Poles are very similar, they are both cold and icy, so why not? In fact, sea ice behaves very differently in the two hemispheres. At the north pole, there is an ocean (the Arctic Ocean) surrounded by land, but at the South Pole, there is land (Antarctica) surrounded by the vast Southern Ocean. As the Earth is warming, we might expect sea ice to be melting at both poles. In fact, for the past few decades up until this year, while sea ice has been shrinking rapidly at the North Pole, sea ice in Antarctica hasn’t really changed much, and has even expanded slightly. This counterintuitive observation has caused a lot of confusion, and scientists have focused a lot of effort in recent years on explaining why the sea ice in the Antarctic has expanded due to changing patterns in the winds that drive the movement of sea ice.

Off the charts

But, in 2016, everything is different. During this year, global temperature records have been continually broken as we head toward the warmest year on record, and this is causing some dramatic, and baffling, changes in sea ice at the poles.

Throughout the first half of this year, sea ice cover was lower than average at both poles, but not unlike what we had measured in other years. But since September, the sea ice cover at both poles have been dropping further and further below the average, far below what has been observed since measurements began in the 1970’s.

nsidc_global_area_byyear_b

Global sea ice area in 2016 (red) compared to other years. From Wipneus.

At the North Pole, as winter sets in, the ice should be rapidly expanding, but instead air temperatures up to 20°C (36°F) above average in the Arctic have prevented the ocean from losing enough heat to form ice, and the ice expansion has stagnated.

monthly_ice_11_nh

Average November sea ice extent in the Arctic 1978-2016. From the National Snow and Ice Data Center.

Average air temperature over the North Pole in November 2016. From the NOAA/ESRL Physical Sciences Division.

Average air temperature over the North Pole in November 2016. From the NOAA/ESRL Physical Sciences Division.

At the South Pole, in the summer the ice is melting, but this year weak winds have caused it to melt much faster than normal, and the ice has hit a record low for this time of year. The ice extent is so much lower than normal that it has left scientists stunned and eager to discover the processes behind the observations

monthly_ice_11_sh

Average November sea ice extent in Antarctica 1978-2016. From the National Snow and Ice Data Center.

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The future of ice

So the question is, do we know what is driving this sudden retreat of sea ice, and should we be worried? The unusually warm temperatures over the Arctic ocean are related to changes in the position of the jet stream, and are contrasted by similar cold anomalies over Siberia. The position of the winds vary naturally year to year, and 2016 happens to be a very extreme event. The Antarctic winds are seeing a similar natural extreme event at the same time, and it is not clear yet how the two events are related. A single extreme event like this cannot be attributed to climate change, because extreme events happen naturally. However, scientists can ask whether these type of events are likely to occur more often now or in the future compared with the past, as a result of climate change. Recent research suggests that because the North pole is warming up quicker than elsewhere, the temperature difference between it and mid-latitudes has decreased, causing the jet stream to weaken, and wiggle more. This wiggle means bigger abnormal temperatures in the Arctic, and in Europe and the US. If this trend of a weaker jet stream continues, then we might expect extreme temperatures and low sea ice in the Arctic to happen much more frequently.

 

Image showing the meandering jet stream, with high wind speeds (red). From NASA.

Image showing the meandering jet stream, with high wind speeds (red). From NASA.

The record breaking low ice cover this year may be a hint of what will become normal in the future, and it is now up to scientists to uncover why 2016 was so extreme, so we can better predict future changes in sea ice, and how these changes will affect our climate.

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