OPINION: Antarctic sea ice took a new course in 2015
Jan 11 (Reuters) - Although the Arctic ice sheet gets all the media attention, noteworthy activity has been brewing in the Antarctic. The Antarctic ice sheet has stopped expanding for the first time in four years, and in fact, sea ice extent has ...
Jan 11 (Reuters) - Although the Arctic ice sheet gets all the media attention, noteworthy activity has been brewing in the Antarctic.
The Antarctic ice sheet has stopped expanding for the first time in four years, and in fact, sea ice extent has dipped below average levels.
The Antarctic ice sheet holds roughly 61 percent of the Earth's fresh water, and despite the mass shrinking of its Arctic cousin over the last decade, the Antarctic has experienced several periods of growth over the same time frame.
At the start of 2015, Antarctic sea ice extent was at all-time high levels for the time of year since records began in 1979, extremely far above the long-term mean. Sea ice remained anomalously high until July, when rapid shrinking began ( http://tmsnrt.rs/1OSKRg3 ).
In August 2015, monthly sea ice extent fell below average levels for the first time since November 2011. For the duration of 2015, sea ice extent hovered very close to average levels.
By Jan. 6, even though sea ice was only a sliver below 30-year averages for the date, it measured at the lowest relative levels in 10 years.
Although global temperatures hit an all-time high in 2015, it is uncertain whether this directly caused the shrinking of the ice sheet, and there is even more uncertainty around upcoming trends for Antarctic sea ice.
NOT WHAT YOU THINK
Oceans are known to be one of the main drivers of global climates, and since Southern Hemispheric oceans were record-warm from May through December last year, a quick retreat of sea ice might appear as the obvious consequence, but it may not be that simple.
Yearly ocean temperature anomalies versus yearly Antarctic sea ice extent from 1979 to 2014 are positively correlated, although weakly. In other words, as oceans in the Southern Hemisphere warm, sea ice extent tends to grow as well, the exact opposite of what seems logical ( http://tmsnrt.rs/1OSMDhm ).
Despite this relationship, it is still difficult to understand how Antarctic sea ice was able to maintain all-time high levels in 2013 and 2014, which until last year were the third-warmest and warmest years to date.
There are some potential theories as to why Antarctic sea ice was able to persevere over the past few years despite the global warmth, such as impacts of the ozone hole and feedback loops due to the ice melt, but these theories stop short of explaining the sudden shift mid-last year.
Surface air temperatures over Antarctica also do not explain why relatively, so much sea ice disappeared in 2015, but did not during the two prior years. Temperatures in 2015 were the coolest of the last three years, further enhancing the mystery of how sea ice seemingly bucked the trend last year.
THE FUTURE IS UNCERTAIN
If the positive correlation theory is taken to be true, then at face value, Antarctic sea ice extent may continue to decrease throughout 2016 if ocean temperatures fall far short of the record-high values observed last year.
It is widely believed that the current El Niño may be headed for a rapid descent into La Niña mid-year, which would drastically cool down a large portion of the Pacific Ocean. However, the El Niño-Southern Oscillation (ENSO) phenomenon accounts for less than 20 percent of the yearly variability of global ocean temperatures.
Year-on-year temperature trends explain roughly 75 percent of this variability, and given the rather convincing upward trend in global ocean temperatures, it would not seem like significantly cooler oceans in 2016 are the safest bet ( http://tmsnrt.rs/1OSL9DG ).
All of this leaves great uncertainty as to what will happen in 2016 and beyond. Did the recent activity in Antarctica truly signal a shift in sea ice patterns? Perhaps that is still left unknown.
The linkages among Antarctic sea ice, the surrounding atmosphere, and impacts for daily human life are much less understood than those in the Arctic, but one thing that is known is that melting of the Antarctic ice sheet would inevitably lead to a rise in ocean levels.
If the entire Antarctic ice sheet were to melt, sea level would rise approximately 190 feet (58 meters), completely eliminating many major world cities, including several small countries.
However, this is absolutely nothing to be worried about anytime soon, as the average sea level rise over the past 20 years has been around 0.13 inch (0.3 cm) per year, but with so much of the world's fresh water supply locked into the Antarctic ice sheet, a relatively small amount of melting could have big impacts.
By Karen Braun
Karen Braun is a Reuters market analyst. Views expressed are her own.