Why did the value for "average surface temperature of Earth" change from 15°C to 14°C in some scientific literature concerning climate change?

Question

I recently stumbled across an article from a global warming skeptic stating that the "average surface temperature of Earth" changed from 15°C to 14°C in some scientific literature concerning climate change.

From the article Fourteen Is the New Fifteen!:

According to the leaders of the global warming doomsday cult, the average surface temperature of Earth is 14 degrees Celsius (57.2 degrees Fahrenheit), but this is a new value which has quietly replaced the original average of 15 degrees Celsius (59 degrees Fahrenheit).

Now, I've read enough to be quite sure that global warming is real, yet this article is interesting because it provides a lot of quotes and links to its sources.

There might be a reason why this "average temperature" was revisited, but due to my nonexistent knowledge in climatology, I am currently unable to provide a counter-argument to these claims.

Can someone with the appropriate knowledge and sources explain why this temperature was changed?

Answer 1

The article quotes a scientific paper as saying:

Hansen and colleagues have estimated that Earth's actual average surface air temperature between 1951 and 1980 was approximately 287 K (14 degrees Celsius) (Hansen et al. 2010).

The quoted paper Hansen et al. 2010 is confirming a study done back in 1999.

For the sake of users who require an absolute global mean temperature, we have estimated the 1951–1980 global mean surface air temperature as 14°C with uncertainty several tenths of a degree Celsius. That value was obtained by using a global climate model [Hansen et al., 2007] to fill in temperatures at grid points without observations, but it is consistent with results of Jones et al. [1999] based on observational data. The review paper of Jones et al. [1999] includes maps of absolute temperature as well as extensive background information on studies of both absolute temperature and surface temperature change.

In fact, Jones et al. are confirming an even earlier study from 1970:

The climatology indicates that the annual average surface temperature of the world is 14.0°C (14.6°C in the Northern Hemisphere (NH) and 13.4°C for the Southern Hemisphere) ... In the Crutcher and Meserve [1970] and Taljaard et al. [1969] atlas climatologies the global average is 14.1°C (NH, 14.9°C; SH, 13.3°C).

Jones, P. D., M. New, D. E. Parker, S. Martin, and I. G. Rigor (1999), Surface air temperature and its variations over the last 150 years, Rev. Geophys., 37, 173–199, doi:10.1029/1999RG900002.

To conclude, around 1970 it was estimated that the global average surface air temperature was 14.1°C. In 1999 this was adjusted to 14°C and in 2010 it was confirmed as 14°C.

The article claims 15°C "was the accepted long-term global average temperature" but provides not a single piece of evidence that this was based on scientific measurements. What the author is quoting is not part of the actual models, but crude estimates used for introductory material or to draw figures in early studies. For example, they quote James E. Hansen making an estimate in the first paragraph of a scientific article, but this is for the purpose of illustrating that the greenhouse effect will add to this rough estimate, not to actually build his model:

The mean surface temperature is Ts ~ 288 K.

That ~ means "roughly".

Actual climate models are not derived from global average temperature, because local average temperatures differ from place to place, even in areas as massive as the northern and southern hemispheres. However, rough estimates of global temperature are sometimes helpful to prove a basic point, for example, that the greenhouse effect is based on a thermodynamic equation that is worth modeling.

Just to show how this works, here is how an illustration was created for an early study, "Sensitivity of a Global Climate Model to an Increase of CO2 Concentration in the Atmosphere" (1980), which was and is widely cited.

The temperature of this isothermal atmosphere ocean system is chosen to be 280 K.

That is to say, for the purposes of making a figure, the authors arbitrarily decided that they would model a system with an average temperature of 6.85°C. This was not based on any data.

Of course it is better to make models and figures based on close-to-real-world averages, so the mistake was eventually corrected.

Answer 2

To add to Avery's excellent answer, I'd like to note that the absolute surface temperature of Earth isn't actually used to measure changes in climate, and that estimates of it changing over a few decades wouldn't indicate that scientists aren't sure of the changes in the Earth's temperature.

NASA has a FAQ discussing how their GISTEMP measure of temperature is calculated:

The GISTEMP analysis concerns only temperature anomalies, not absolute temperature. Temperature anomalies are computed relative to the base period 1951-1980. The reason to work with anomalies, rather than absolute temperature is that absolute temperature varies markedly in short distances, while monthly or annual temperature anomalies are representative of a much larger region.

They calculate temperature by, for each climate station, subtracting the average for that climate station from 1951 to 1980, so each value is now a difference from the average temperature for that station from 1951 to 1980. This is a 'temperature anomaly' value. Then, those anomalies are averaged, across all stations. This works because temperature anomalies are correlated over large distances - if it's unusually cold in Chicago it's probably unusually cold in the entirety of Illinois. Obviously if it's 15c in Chicago it's probably not 15c in the entirety of Illinois.

This allows NASA and the other groups providing global surface temperature records, to provide a much more accurate measure of change in surface temperature, as discussed in this post on Realclimate:

That means you need fewer data points to make a good estimate of the global value. The 2σ uncertainty in the global mean anomaly on a yearly basis are (with the current network of stations) is around 0.1ºC in contrast that to the estimated uncertainty in the absolute temperature of about 0.5ºC (Jones et al, 1999).

This is because you can average more stations together, essentially.

When absolute surface temperatures are reported in the press, like the example quoted in your article:

This year the Earth's average temperature was 14.64C, compared with the long-term average of 14C, said James Hansen, of NASA's Goddard Institute for Space Studies, who analyses the data collected from thousands of weather stations around the world.

This value is calculated by adding the anomaly value for the current year to an estimated preindustrial absolute surface temperature, not by taking the absolute temperature today and subtracting the preindustrial absolute temperature. From the NASA FAQ:

Q. What do I do if I need absolute SATs, not anomalies? A. In 99.9% of the cases you'll find that anomalies are exactly what you need, not absolute temperatures. In the remaining cases, you have to pick one of the available climatologies and add the anomalies (with respect to the proper base period) to it. For the global mean, the most trusted models produce a value of roughly 14°C, i.e. 57.2°F, but it may easily be anywhere between 56 and 58°F and regionally, let alone locally, the situation is even worse.

The implication in the American Thinker blog post that scientists cannot be sure of global warming because they are not confident of absolute surface temperature is very much misunderstanding how temperatures are measured. To make a strained metaphor, if you were measuring a child's growth by getting them to stand up against a wall and drawing a line over their head, you wouldn't say you were unsure how much taller they were getting because your estimated height above sea level isn't precise to a metre.

Some more discussion of this aspect of climate science can be found in this discussion from NOAA, and this Realclimate blog post.

Answer 3

The problem is that the "average surface temperature of Earth" do not exist, it is not a physical measurement, in other words there is not a hole somewhere in the surface of Earth in which you can insert a thermometer ...

More than that, measured T° are not homogenous, some come from old airport where the nearest town has grown over the years, for the ocean some comes from a bucket thrown into the water in the 1800s, now from tankers from the water 15 meters below the surface for engine cooling. Satellites for 15 years measure radiation from which T° is deduced ...

So, an average of not homogenous local T° is only a concept, through calculations you have a final number, in that or that way through "sophisticated" input factors and / or mathematical numerical model, but that is not physical science.