Is there such a thing as a global absolute temperature of the earth’s surface? The temperature at any point on the earth’s surface is forever changing, hour to hour, night to day and with the seasons. A global average temperature Tgl is theoretically the area average of all local temperatures integrated over the earth’s surface. I claim that Tgl can really only be defined at one instance in time (maximum one day). In 1993 I was asked to process about twenty 9-track magnetic tapes containing an archive of ECMWF daily forecast data in GRIB Format and then write the results to an optical jukebox. Having finally succeeded, I decided to calculate the ECMWF global average temperatures. These were published were 23 years ago in GRL. Today such archives of weather forecast data are called reanalyses.
As the earth rotates so different fractions of land and ocean are illuminated by the Sun, and the earth’s albedo changes. The highest heating of the earth’s surface is probably at midday over the Pacific Ocean. The absorbed heat is then dispersed through the atmosphere (weather) and by ocean currents. Over land the albedo has been changing over thousands of years due to human activity. Deforestation, agriculture, drainage and urbanisation alter local albedo and weather, while recently man has also increased CO2 levels. Therefore the absolute temperature of the earth’s surface is always changing. Can long term trends be measured directly?
The average daily temperature (Tav) is calculated from the maximum (Tmax) and the minimum (Tmin) recorded temperatures at each station. Tav=(Tmax+Tmin)/2 . Likewise long term temperature series used for climate studies calculate the monthly averages in the same way, where Tmax and Tmin now are the extreme temperatures for any given month. These monthly values also vary from year to year due to fluctuations in weather. The average seasonal variation at one station is calculated over some given 30 years period and are called ‘normal’ values. The 30 year period is called the baseline.
Now suppose you simply calculate the global average temperature for one month or for one year. This is fairly easy to do by preforming an area weighted average of Tav over the earth’s surface and then averaging over one year. Here is the result for land based station in CRUTEM3.
There are obviously some problems here. For example the temperature appears to jump up in 1950, but this is simply because a lot of new stations were suddenly added that year. This demonstrates that there is always a spatial bias due to where you have available measurements and this bias gets worse the further back in time you go. Before 1860 weather stations were mostly confined to Europe and the US , while ocean temperature data was confined to a few shipping lanes.
So we can’t really measure global temperatures much before the satellite era or before ~1980.
The answer to this problem is to use temperature anomalies instead. Anomalies for a given station are defined relative to the monthly ‘normal’ temperatures over the 30-year period. CRU use 1961-1990, GISS use 1959-1980 and NCDC use all the 20th century. The temperature ‘anomaly’ for each month is then Tav-Tnorm. Any sampling bias has not really disappeared but has instead been mostly subtracted. There is still the underlying assumption that all stations react in synchrony to warming (or cooling) as do their near neighbours. In addition it assumes that areas of the world with zero coverage behave similarly to those areas with good coverage. It seems that Guy Callendar was the first person to use temperature anomalies for this purpose back in 1938
So the conclusion is that you can’t measure a global temperature directly, and even if you could it would be changing on a daily and even hourly basis. The only thing you can measure is a global average temperature ‘anomaly’. Spatial biases are reduced but not fully eliminated, plus there remains an overall assumption of regional homogeneity. So when you hear that global temperatures have risen by 1C, it really means that the global average anomaly has risen by 1C. For any given month the temperature where you live could even be colder than ‘normal’. So for example, Europe was colder than normal in October 2015, despite 2015 being itself the ‘warmest’ anomaly year on record.
This post was prompted by Gavin Schmidt : Observations, Reanalyses and the Elusive Absolute Global Mean Temperature