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One New Hampshire spring tradition is declaration of “ice out” on lakes, of which the most famous is on Lake Winnipesaukee, the state’s biggest body of fresh water. This year it was declared on April 5, which is pretty early.

The announcement led a reader to ask me: Is climate change making it come earlier? Since there’s an online record of ice-out dates on Winnipesaukee since 1887, it was fairly easy to check.

The answer is yes, particularly in the past decade, but with a big proviso: “Ice-out” is a subjective call. Right now it’s when one guy thinks the MS Mount Washington ship can safely get to its five ports of call, but that hasn’t always been the guideline. Comparing results over time is questionable.

With that in mind, a look at ice-out records on shows:

  • The three earliest calls (March 18, 23 and 24) have all come in the past decade (2016, 2012, 2010).
  • There hasn’t been a late ice-out – i.e., happening in May – since 2001 and that was an outlier: Before that it hadn’t happened since 1972.

Averaging by decade:

The average ice-out during the 2010’s was April 6, by far the earliest of any decade.

2000’s – April 17.

1990’s – April 15.

1980’s – April 17

1970’s – April 26.

1960’s – April 21.

Going back further there isn’t much change by decade. Fifty years earlier in the 1910’s, for example, it was April 19 and in the 1890’s (the first full decade) it was April 25.

So the decade-by-decade measure seems to show a significant shift since the 1970’s, which if true would be consistent with many other measurements. The 1980’s are when the weather effects of accumulated greenhouses gases began to be noticeable.

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