Now that NASA has released their data updated through July, we know that in that data set, this July was the hottest July on record with a temperature anomaly of 0.75 deg.C, i.e. it was 0.75 deg.C above “climatology” (which is what’s usual for the given month). It’s not the hottest temperature anomaly in the data set, however; that record still belongs to January 2007, at 0.96 deg.C above climatology.

Yet it does seem that this July, while not the hottest temperature anomaly on record, is the hottest month on record.



Every year, the global average temperature goes through an annual cycle — not just the temperature at a given location. In the northern hemisphere we tend to be hottest in July and coldest in January, but in the southern hemisphere the seasons are reversed, hottest in January and coldest in July. The seasons are definitely hemisphere-dependent.

But what about the global average? My first instinct, many years ago, was that earth would, overall, be hottest in January simply because we’re closer to the sun (at the perihelion of our orbit). But it turns out (as was quickly pointed out by a blog commenter) earth is actually hottest in July. That’s because when the southern hemisphere is tilted toward the sun in January, all that solar heat mainly strikes ocean, which dominates the southern hemisphere rather than land. The thermal inertia of the oceans is much greater than that of the land masses, so it heats up more slowly, and just doesn’t get that hot even at the peak of summer.

But in July, it’s the northern hemisphere that’s tilted toward the sun. The lower thermal inertia of land (mostly in the northern hemisphere rather than the southern) means it can heat up quickly, so the northern hemisphere reaches higher temperatures at its summer peak than the southern hemisphere does at its summer peak.

Why not translate those temperature anomalies into actual temperature estimates?

The catch is, that from thermometer records it’s hard to determine climatology — what the actual absolute temperature is, globally averaged. That’s why the major temperature data providers track earth’s temperature with anomalies; they can be determined more accurately, and give us just as good a picture of how temperature has changed (which is, after all, what we’re mainly interested in).

But there’s one source which seems to give pretty good estimates of absolute temperature — reanalysis data. I don’t think anyone would trust them to be precise enough to rank individual months, but they just might be our best source of data to estimate climatology.

There are two reanalysis products that are well-known and easy to get: an American version from NCAR/NCEP, and a European version called “era-interim.” Let’s use both to define climatology, then add the anomaly values from NASA to estimate actual absolute temperature.

We’ll start with NCAR/NCEP. Here’s what it gives for global temperature:

I first isolated only the complete years — those with all 12 months — so 2015 is out. Then I computed the average temperature for each month separately. That gives me the NCAR/NCEP climatology:

Indeed July is the hottest month on average, January is coldest. The difference is, according to these data, 3.28 deg.C.

Now we can add this estimate of climatology to NASA temperature anomaly data:

We can also “zoom in” to see which month was hottest:

It turns out that yes, this July is the hottest month on record. It’s not the hottest temperature anomaly, but it’s the hottest temperature. In 2nd place is July 2011, coming in 3rd is July 2009.

What about era-interim reanalysis data for climatology? Here’s the data (expressed in Kelvins rather than deg.C):

Here’s the climatology:

July is still the hottest month on average, January still coldest, and the difference using era-interim data is 3.48 K (equivalent to 3.48 deg.C)

Adding NASA anomalies gives this:

And again, we can zoom in to see which is hottest:

In this calculation also, this July is the hottest month on record. Not the hottest temperature anomaly, but the hottest temperature. Only this time, 2nd place is taken by August 2014, while July 2011 has dropped to 3rd and July 2009 to 4th.

The rivals are very close, and the uncertainties are big enough, that of course we can’t be sure which is really the hottest month ever. The leaders are in what is properly called a “statistical tie.” But the fact remains, that July 2015 takes the gold medal, according to both judges. It’s just another sign how much the earth is heating up. Which it is.

Global warming is real. It’s caused by humans. And it’s dangerous. Very dangerous.