Monday, July 10, 2017

New record for Arctic sea ice minimum?

The planet continues to warm as the US pulls out of climate change accords and popularizes fuzzyheaded thinking about what constitutes knowledge about climate change. I have stated several times that my favorite dataset for tracking warming trends is sea ice levels in the Arctic and Antarctic. The Antarctic has been more dramatic this year, and sea-ice there continues to be at historic lows, but it is the Antarctic winter so the Arctic is where the effect of greenhouse warming should right now be more dramatic. I thought it might be nice to check in with the Arctic sea ice data.

The sea ice in the Arctic has been retreating at around 82 thousand square kilometers a day for the past month or so, and in June averaged 11 million square kilometers. This means that the sea ice retreat is tracing almost the same curve as seen for June and July in 2016, 2014, 2013, 2012 (the current record low year for sea ice), 2011, 2010, and 2007 (Second lowest sea ice year ever) . The geography of the Arctic Sea constrains sea ice dispersal for a short period during the Arctic summer so the tight grouping of sea ice retreat curves is not terribly shocking. However, the fact that these midsummer levels are close [6.4 million square kilometers on 1 August 2012, and the 1981-2010 median minimum is 6.3 million square kilometers on 18 September] to the minimum 1981-2010 median minimum levels (minimums usually occur in late September) is shocking. This is the new normal.

The sea ice retreat traces for each of the past “new normal” years deviated starting around August 1st as the bulk of the Arctic sea ice began to disperse. Factors such as ice-dispersing weather events can cause instability in the measured ice extent when there is a lot more ocean for the ice to be pushed around in. Remember that sea ice is counted as extending over an area when it is covered by as little as 15% actual ice.

Another factor that impacts how the trace will behave after August 1st is, and this is actually why the data is interesting, how warm it has been in the Arctic. There are many agreagate data types designed to describe how warm the Arctic is in any given year. This month the National Snow and Ice Data Center provided an uncharacteristically simple data type. They called it Cumulative Freezing Degree Days (FDD). FDD is just the sum of daily mean temperatures below zero from the same date of the previous year. This year the July 1st (2016) to July 1st (2017) FDD is the lowest ever. This suggests that the post August 1st sea ice retreat trace will fall to a new record low.

Unfortunately data like the FDD are tied to point measurements, and local anomalies can make them less predictive of behaviors in bulk phenomena like sea ice coverage in the entire Arctic. So, while it is likely that we will hit a new low for Arctic sea ice coverage in late September, it is not certain that we will. What looks to be certain, however, is that we will hit a new record minimum some time in the near future.

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