Global Surface Air Temperatures (GAST)

Global Surface Air Temperatures (GAST)

A major unanswered question with respect to climate change remains, what is the actual global surface air temperature? It is regularly reported "if it reaches 1.5°C, it will be the tipping point of the climate". I have tried to find out, visiting NOAA, NASA, WMO, ECMWF ECMWF, and the UK Met Office website with no luck.

I cannot find the answer to a very simple question from the main weather/climate institutions. There are plenty of references to temperature differences in respect to a set periods of time (a 30-year climate!), called anomalies.

From the UK Met Office "Absolute temperatures are not used directly to calculate the global-average temperature. They are first converted into 'anomalies', which are the difference in temperature from the 'normal' level. The normal level is calculated for each observation location by taking the long-term average for that area over a base period. For HadCRUT4, this is 1961-1990."


This is what NASA have to say "The term temperature anomaly means a departure from a reference value or long-term average. A positive anomaly indicates that the observed temperature was warmer than the reference value, while a negative anomaly indicates that the observed temperature was cooler than the reference value.
What can the mean global temperature anomaly be used for? This product is a global-scale climate diagnostic tool and provides a big picture overview of average global temperatures compared to a reference value."


In terms of anomalies, the image below is the best I could come up with. It comes from the World Meteorological Organisation (WMO).


The problem with anomalies is, if you use a reference time period to measure against which is cold, as per the WMO chart the reference period is 1850-1900. As such it would be expected to get warmer.

The point here is, using a cold period for a reference point and not the latest 30-year climate period 1981-2010 would produce a much lower rate of clime and these so called "records" may not have been records.

As an example the following might help explain.


Looking at the WMO chart it looks worrying with large increase in temperature. Although you must "keep you eye on the pea" the reference temperature changes (on the left) only range from -0.2-1.2 Deg C.

To obtain a more reasonable depiction, as an example, the average "normal" temperature in the CET record between 1850-1900 was 9.18°C, had the WMO used the latest 30-year (climate) period 1981-2010 the anomaly would have been 10.0°C. If you now subtract 0.82°C from their figures from 1981-presnt it would tell a very different story. Using anomalies from cold reference periods is very misleading.

When fixing set reference periods to obtain anomalies can be misleading. In a small study I show the UKMO CET from 2000-2019 and compare anomalies for 1961-1990, 1971-2000, and 1981-2010.
In this example the difference between using a cold period (1961-1990) as the "normal" and the latest 30-year period (1981-2010) shows a difference of 0.49°C too warm. As observed, with actual temperatures the CET (magenta) shows no statistically significant trend.

Considering the Earth's temperatures naturally rose since the Little ice Age (LIA) ~1645-1750. So what would be "normal".

Absolute temperatures should be used, as per the CET and images should be represented as such, it doesn't look nearly as worrying as those depicted by current anomaly usage. ..

Putting things into perspective, still with the use of anomalies.


"Reconstructed global temperature over the past 420,000 years based on the Vostok ice core from Antarctica (Petit et al. 2001). The record spans over four glacial periods and five inter-glacials, including the present. The horizontal line indicates the modern temperature. The red square to the right indicates the time interval shown in greater detail in the following figure."

"Global monthly average surface air temperature since 1850 according to Hadley CRUT, a cooperative effort between the Hadley Centre for Climate Prediction and Research and the University of East Anglia's Climatic Research Unit (CRU), UK. The blue line represents the monthly values. An introduction to the dataset has been published by Brohan et al. (2005). Base period: 1961-1990. Last month shown December 2010. Last diagram update: 3 January 2011."

Notice the use of base period, again 1961-1990, a modern cool period. As per the prior CET example, on this chart if you change the base line to 1981-2010, and the reduce the data from 1961-2010 by 0.49°C the picture looks very different. As the period 1850-2010 started near the end of the Little Ice Age (LIA) it is quite natural to have an increase in temperatures.

The one place I found an absolute temperature was from Wikipedia, "The average global temperature has increased by 0.9 °C (1.5 °F) compared to the baseline temperature which is about 14 °C. Although a pause has been observed between 1998 and 2013, the global warming continues since at the same pace as before."

Not very scientific "is about 14°C" and 14°C is hardly warm. It also does not mention what the "baseline" temperature is or should be. For the last 360 years we have been recovering (warming) from the effects of the Little Ice Ages (LIA). As such we should expect temperatures to rise. Wiki

Global Surface Air Temperatures (GSAT) a different view:

Using nullschool I recorded the surface temperatures for 62 locations equally spread across the globe at 30° intervals north/south and east/west between 1245-1315 UTC on 23Oct2020 with the following results.

As shown, there were 42 locations over water and 20 over land, which is not far from the water/land mass ratio of the Earth. This study also shows temperatures are colder towards the poles and warmer around the equator, as one would expect. Given the average, excluding the poles the GSAT is 12.9°C. Given that the planet is 510m Km2 and total ice cover is approximately 40m Km2 (7.8%) it would be inappropriate to include the poles themselves into GAT, but what can be said is the GAT would be lower than 12.9°C.