This June was the hottest June ever on record, July was the hottest July ever on record. We just hit a 15-months streak* of record-breaking temperatures [1]! July has even experienced the highest temperature for any month on record [2]. Could we set a new record for August? Check out NOAA’s Global Analysis next month. By the way, who measures global temperature?

Although there is not only one official temperature data, the most cited ones are GISTEMP, GlobalTemp, HadCRUT4, and JMA data, provided by NASA, NOAA, Met office, and JMA, respectively. Each of the groups has their own approach to generate their global temperature data. For example, they have different data sources and data coverages, and adopt different interpolation methods and gridding processes. Owing to the different approaches, their data sometimes show subtle disagreement over time and area. However, it is reasonable to rely on several independent data to achieve one common conclusion: the rise of global temperature.

‘Global temperature spiral’ by, plotted based on HadCRUT4.

In-site measurement, i.e. thermometers at weather stations over land surface, as well as the ones at ships and buoys over sea surface are the main data source to derive those global temperature data. You may wonder why no satellite data are ingested. One reason is that satellite-derived temperature have higher uncertainties compared to the in-site measurements. In fact, that is the favorite data of climate change deniers since the satellite data do not show a clear trend of the increasing temperature unless they are quality-controlled properly.

You already know that satellites can not measure precipitation directly; they measure a proxy variable – cloud temperature. Ditto for temperature. Satellites – more specifically – microwave sensors measure the thermal radiation emitted by oxygen molecules in the atmosphere. And then satellite algorithms calculate surface temperature based on the radiation readings. So it can happen that, for example, clouds or precipitation in the atmosphere contaminate the temperature readings of satellites [3][4]. For this reason, the satellite community regards the in-site measurements as ‘truth’ reference. Indeed, calibration and validation of satellite based observations are one major research part in the community. Even so, please do not blame satellites. Only the satellites enable us to measure temperature/precipitation all over the world, which is essential when it comes to climate research. Therefore, both satellite and in-situ observations are important, since the former provides a worldwide coverage while the latter ensures the accuracy of the measurements.

*According to NOAA’s analysis. NASA said July is the 10th record hot month in a row.