Global warming is typically measured at the surface, with data from thermometers all across the land areas and sea surface temperatures combined. That isn't the whole story, of course. Much of the added heat, an effect of human generated greenhouse gas pollution, goes into the upper 2,000 meters or so of the ocean. But we use the surface measurements to track global warming because we have the data for a long period of time, and those data in turn have been linked to longer running but less precise paleo data.
Almost every month for way over a year now has been warm, and April 2015 is no exception. The official NASA GISS surface temperature data is out for the month. April came in at an anomaly value of 75. That's 0.75 degrees C above the base period for that data set.
That makes April the fifteenth warmest month in the database, which starts in 1880. March was warmer (fifth) and I'm pretty sure May is going to be warmer too (maybe as warm as March, we'll see).
If we start in April and work backwards 12 months, we can get a one year long average. And, we can go back to the beginning of the data set to look at a 12-month running average for the entire thing. When we do that, we see that the current 12 month period, the one just ending, was the warmest ever in the data set, which makes it likely the warmest one year period in thousands and thousands of years. We have had several record breaking 12-month periods in a row. Here's what that looks like:
Another way of looking at this is for the calendar year only. This is a graph of the average temperature for the first four months of the year for the entire NASA GISS data set, to give you an idea of where our Year-To-Date stands:
Will 2015 end up being a very warm year? Almost certainly. Unlike last year, we will see significant enhancements of surface heat from El Nino. This is probably already happening. Using only the year to date data to estimate 2015, it is a good sight warmer than any other calendar year on record. As noted, May will be warm (over 0.80) and subsequent months are likely to be even warmer if El Nino does pan out.
The following graphic indicates where the remaining months of the year will have to fall (on or above the horizontal red line) for 2015 to break the record set by 2014.
"That’s 0.75 degrees C above the 20th century mean for that data set".
Actually, it's 0.75 degrees C above the mean temperature for the years 1951 to 1980
[GACK! Thanks for pointing that out. I was thinking of some other data set. -gtl]
The April anomaly is 0.77 Celsius above the 20th century mean. See here: http://data.giss.nasa.gov/cgi-bin/gistemp/nmaps.cgi?sat=4&sst=3&type=an…