Historical climate change in New England: the importance of record length in estimating magnitude of change

Many studies have shown that lake ice-out (break-up) dates in the Northern Hemisphere are useful indicators of late-winter/early-spring climate change. Trends in lake ice-out dates in New England, USA, were analyzed for 25, 50, 75, 100, 125, 150, and 175 year periods through 2008. More than 100 years of ice-out data were available for 19 of the 28 lakes in this study. The magnitude of trends over time depends on the length of the period considered. For the recent 25-year period, there was a mix of earlier and later ice-out dates. Lake ice-outs during the last 50 years became earlier by 1.8 days/decade (median change). This is a much higher rate than for longer historical periods; ice-outs became earlier by 0.6 days/decade during the last 75 years, 0.4 days/decade during the last 100 years, and 0.6 days/decade during the last 125 years. The significance of trends was assessed under the assumption of serial independence of historical ice-out dates and under the assumption of short and long term persistence. Several indices of large scale atmospheric circulation patterns (such as the North Atlantic Oscillation) and sea-surface temperatures were tested for correlation with the historical ice-out dates, but these indices explained 11% or less of the interannual variability of the ice-out dates.