I have discussed the difference in the global temperature and the global anomaly before. The most interesting part of that discussion is the observation that different months of the year have different behavior. The Northern Hemisphere (NH) winter months have more variation year to year than the summer months do. That means during the winter there is a bigger difference between mild winters and cold winters. The summer is more consistent year to year.
Looking at the trend for a single month can sometimes show surprising results. For instance, let’s take a look at the month of December. Over the past 150 years it has seen a very large range in temperatures. Here is the December global temperatures from 1850-2009.
There are some interesting pieces of information in this chart. In the 1850’s there were 2 of the warmest Decembers over the entire period. The 1850’s also had some of the coldest Decembers. There is almost a 2.0 °C difference between the winters in the 1850’s. The average temperature was slightly below the zero anomaly, but there were huge swings in temperature.
The moving average dropped almost 0.5 °C into the 1860’s. The big difference was the 1860’s didn’t have any warm Decembers. That was enough to drop the average of the decade 0.15 °C. The 1870’s had some warm months again, just not as warm as the 1850’s. It wasn’t until the 1930’s that the December temperatures again climbed near the warm December averages that the 1850’s experienced. Since the 1930’s didn’t have the cold winters to average out the warmth, the average temperature was 0.25 °C higher. The maximum temperatures were lower, but the average temperature was higher.
Even in the past 10 years what is apparent is not temperatures that are shockingly higher, but there are less very cold temperatures. 1933 was the last year that December had an anomaly temperature that was below -0.5 °C. Over the entire period the average standard deviation for the month of December is 0.34 °C, although the biggest change is that the standard deviation has been shrinking. The month of December may look warmer, but really what has happened is that there as been less very cold Decembers. 3 of the 5 warmest months happened more than 70 years ago, with 2 of the warmest taking place in the 1850’s. Part of that is the nature of the blended data, but as the data gets more accurate the error decreases. Certainly the modern day satellite data is far more accurate than the piecemeal records from the 1850’s.
For the past 160 years the month of December has shown periods with warm winters in the 1850’s, 1930-1940’s and now again since the mid 1990’s. The most unusual difference is the recent lack of very cold Decembers, but that has happened before during the 1930-1940 period when there was a long period of warm Decembers then. It will be interesting to see how this December turns out.