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Thank you, Arnold.
Several readers mentioned that the plot of sea ice anomaly in the Arctic shown in the last post has much greater fluctuation after about 2007. Anomaly values are the difference between a given time’s extent, and the average for that time of year, so to get anomalies you take the original values and subtract the average seasonal cycle. It was correctly concluded that the exaggerated fluctuations after 2007 are because the seasonal cycle itself has changed; subtracting the average seasonal cycle still leaves that difference in place.