A year is 12 months long. It is also the period of time between January and December, inclusively. But you can use that first definition (we do it all the time) when appropriate. So, we can ask the question, how does the last 12 months, ending at the end of January 2015, compare to previous 12 month time periods in terms of global surface temperature?
We can do this using a moving average. A moving average for a series of values is the average of a certain number of values in sequence, calculated to correspond to each value. So a one year (12 month) moving average of temperature would be calculated by taking the average of the 12 months that end in January 2015, then the 12 months that end in December 2014, then November 2014, etc, going backwards in time.
In some ways this is a preferable measure than taking each year’s value. There are two reasons a 12 month moving average is good. First, it is 12 months long so the variation that happens across a year in surface temperature values is included in the average, so that relatively unimportant squiggling up and down of the data is dampened. Second, it lets us see the march of temperature change over time.
I used the NASA GISS data base, which just updated its value for January, to calculate a 12 month moving average for the entire record, which begins in 1880. January, as you will recall, was the second warmest January in the entire record. (For those keeping track, February of 2015 promises to be pretty warm too, and will without a doubt be warmer than February 2014, because that was an oddly cool month.) The temperature anomaly value for the last 12 months (up through January 2014) is about 68 (1/100ths of a degree C, the standard number) above the baseline used by NASA. That is the highest value ever for a 12 month period, so just as 2014 was the warmest year on record in that database, the last 12 months were also the warmest year (defined as a 12 month period) in that record.
There is nothing surprising here. Global warming is happening. We’ll continue to have many 12 month periods which are the warmest ever, along with the occasional 12 month period which is not, because the temperature squiggles up and down as it trends upward.
Also covered HERE.