Lowest amount of sea ice ever seen by humans during...

제목	Lowest amount of sea ice ever seen by humans during...
보낸 사람	The Earth Story
보낸 날짜	2016년 11월 22일 오전 12:00

   

   

Lowest amount of sea ice ever seen by humans during October/November

This week, this remarkable plot began blowing up on social media. The plot tracks global sea ice abundance – a combined measurement of the area covered by ice in both the Arctic and Antarctic. The red line, 2016, shows that Sea Ice extents globally are by far the lowest humans have ever recorded.

Normally at this time of year, Arctic Sea ice is beginning to recover from low volumes reached during the summer, while Antarctic Sea ice is beginning to contract as sunlight falls on that continent. On top of this yearly variation there have been several long-term trends imposed. Most notably, sea ice extents have been decreasing in the Arctic quite regularly for the last 30+ years – the time humans have been actively monitoring sea ice.

The lowest ever extent of sea ice was reached at the end of summer 2012, but that doesn't mean the sea ice has been recovering – instead, years since then have seen record low sea ice values "for specific months". In other words, the long-term trend is gradually decreasing sea ice due to climate change, and records at any time of the year are being broken when a perfect weather pattern hits. One of those weather patterns is hitting in the Arctic right now.

The North Pole right now is already shadowed for the winter. Usually this darkness is associated with plummeting temperatures since no heat from the sun can warm it, but for the past month there have been several times when North Pole temperatures have spiked above the freezing point of water; as much as 15°C above normal for this time of year. These temperatures are being driven by surges of warm air traveling up to the poles from lower latitudes.

Although this effect isn't fully understood yet, warm winter arctic weather combined with surges of colder air traveling south has been a feature of the last several northern hemisphere winters. It has been proposed that these surges are associated with weakening of the jet streams as climate warms – weaken the jet streams, the boundaries between different convection cells in the atmosphere, and it becomes easier for warm air to be pushed all the way up to the poles. So far this fall Siberia has been anomalously cold while the North Pole has been anomalously warm, consistent with this mechanism.

This warm air preventing ice growth also is combining with the low summer sea ice extent. Sea ice is bright and open water is dark, so when sea ice melts the water beneath will store more heat during the summer. 2016 had the 2nd lowest summer sea ice extent observed in the Arctic, so the Arctic Ocean waters are also warmer now than was typical before humans increased greenhouse gas concentrations in the atmosphere.

Meanwhile, the real puzzle this year is the Antarctic. Sea ice in Antarctic has actually been increasing the last few years – a reminder that the Antarctic is a fundamentally different area than the Arctic. In the Antarctic, the presence of the ozone hole has driven more intense winds that have cooled surface waters and helped the formation of Antarctic sea ice. Furthermore, melting on the Antarctic continent has created a supply of fresh water heading into the oceans and fresh water freezes at higher temperature than salty water. Recent work has suggested this could only be a temporary situation – healing of the ozone hole (actually good!) could remove one of the factors protecting Antarctic Sea Ice, and another major factor could be extra warm water at depth. Several recent studies have suggested that Antarctic Sea Ice could be particularly vulnerable to extra warmth in the subsurface – add a bit of extra heat and that ice retreats rapidly.

Right now, there is likely less sea ice on Earth at this time of year than there has been in about a hundred thousand years – going back to before the last cycle of glacial advance. The downward trend, particularly in the Arctic, is extremely likely to continue in future years. It won't always produce an all-time low in September - that takes the perfect weather conditions at the perfect time of year – but when the right weather conditions do show up we'll get record lows for that time of year. Whether this year's rapid sea ice melting in Antarctica will be a one-time event is, as of now, an extremely important question with no good answer.

Sea ice in these areas represents a habitat for many organisms – a stable place where they can rest before heading out into the ocean. As sea ice extents fluctuate, these species are put under increasing threat. Furthermore, the more open ocean there is, the more heat the oceans can take up – so these drops in sea ice can contribute to warming the surrounding oceans and thus put continental ice sheets under even greater stress. Some scientists have been debating whether it is good communication to place all the Earth's sea ice on one chart when both poles are two distinct systems, but the threat to wildlife globally and the effect of warming oceans may represent reasonable worries based on this plot alone.

-JBB

Data credit: NSIDC http://bit.ly/2frYJyk http://bit.ly/1jOd0VZ Read more: http://n.pr/2eBPFLv http://scienceblogs.com/stoat/2016/11/18/sea-ice-wossup/ http://cnn.it/2eRzeKH http://bit.ly/2gnschY

   

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