Nature is complicated and often contradictory. Even with such an all-encompassing theory like Anthropogenic Global Warming (AGW) not every result and observation points in the same direction.
It’s these observations that contain new information, especially in the netherlands between AGW forcing and natural climate variability.
The recent decline in Antarctic sea ice is remarkable given that it has been slowly increasing since satellite observations of its extent commenced in 1979.
Things seemed normal until the end of 2016 when over a three month period an additional 4 million square kilometres melted. This is more than the entire accumulation since records began at the average rate of 11,200 +/- 2,100 square kilometres per year. This was followed by a rebound, but not back to the level it was beforehand. Nothing like this has ever been seen since satellite observations began in the late 1970s.
During the last couple of decades, many scientists expressed their surprise that in the era of anthropogenic global warming Antarctic ice was increasing rather than decreasing. A recent paper by Parkinson (2019) detailing the recent developments concluded:
“These increases have been far more puzzling than the Arctic sea ice decreases and have led to a variety of suggested explanations.”
So what could have caused this recent event?
Many suggestions have been made; links to the Ozone Hole, El Nino, the Interdecadal Pacific Oscillation, variations seen in the Amundsen Sea. Parkinson (2019) says that,
“None of these has yet yielded a consensus view of why the long-term Antarctic sea ice increases occurred.”
Of the two greatest masses of ice on our planet Arctic sea ice has been declining, with half of the effect put down to AGW, whilst Antarctic sea ice was expanding for a reason nobody knows. What is more, climate models predicting sea ice extent didn’t get it right as the decrease was far more than they predicted.
Looking at the sea ice data from various regions in the Southern Ocean it’s apparent that the main effect is seen in the Weddell Sea, with lesser effects seen elsewhere.
Being a little more accurate with the dates is illuminating. Looking at the monthly rather than yearly sea ice data it becomes evident that the sea ice extent descended into unknown territory at the end of 2016. A few months earlier the intense 2015/16 El Nino had declined, although the world had remained warm. If the rapid decline is associated with the end of the El Nino, then it is unprecedented since the other two comparable extreme El Nino’s in 1997-98 and 1982-83 did not trigger such a dramatic decline. Then again we know so little about El Nino’s, especially the extreme ones.
Parkinson (2019) acknowledge that there is
“no assurance that the 1979 – 2014 overall positive trend in the Southern Ocean has reversed to a long-term negative trend.”
In short, the Antarctic Ice puzzlement continues.