I just saw this post on US temperatures by Pat Michaels and Chip Knappenberger, and I would recommend readers here give it a look. This line in particular is interesting:
But even if the rest of the month is not quite cold enough to push the entire year into negative territory, the 2013 annual temperate will still be markedly colder than last year’s record high, and will be the largest year-over-year decrease in the annual temperature on record, underscoring the “outlier” nature of the 2012 temperatures.
I have to agree that this is true, although, I think it is worth noting, as usual with “unprecedented” events, just how unprecedented they truly are depends in significant part on how you define them. If December averages at what it has averaged so far, the January to December average temperature in the US will drop about 3.12 degrees Fahrenheit, the previous record holder being 1934 to 35, which was a drop of 2.21 degrees Fahrenheit. If December cools down to average 27.6°F, the drop will be about 3.2 degrees: either way it shatters the record for a drop of Jan-Dec average temperatures. Buuuuut….This does not represent the largest drop of an twelve month average from the previous twelve month average. The average temperature from from April 1935-March 1936 was ~3.39°F below the April to March average of 1934-35, which represents the fastest drop of a twelve month average from the previous, in the entire USHCN record going back to 1895. However, Jan-Dec 2013 from Jan-Dec 2012 will represent the largest drop of all twelve month averages from the previous since then, and the third largest drop in the entire record (the second being March 1935-February 1936 from March 1934-February 1935. That’s still pretty far back to have to go (over 70 years) to find any drop larger.
The above represents what I am talking about: in red is the difference of each 12 month average from the previous 12 month average, by end date. In black are the values for January to December periods, and the green and purple dots represent projected values for 2013-2012. All this goes to show that one should never make too big a deal out of one of those warm spikes like 2012 (or a cold spike for that matter). The almost inevitable result is that the next year will cool down (or warm up) in opposition to the spike because it is a transient weather event. And the larger the event, the more dramatic the pendulum swing can be.