[rrmm]

The research is done in terms of a temperature anomaly from a reference that is chosen. The absolute mean temperature is an estimate based on the specific technique chosen using the data as input. The delta values are more important because they are calculated with a consistent method for a given model, whereas from model to model the value may change.

from https://www.pnas.org/content/94/16/8314

""" The two main marine data sets are those of Jones et al. (ref. 9; see also ref. 11) and the U.K. Meteorological Office (UKMO) (12, 13). These two data sets have overlapping primary source material but differ in the way that they are corrected for instrumentation changes. """

https://pubs.giss.nasa.gov/docs/2007/2007_Hansen_ha09210n.pd... (mentions the particular model in section 1, along with a discussion of the data sources).

You basically want to be looking at the temperature anomaly not the absolute temperature.

[busymom0]

That doesn't answer the sudden change from 15C claims to 14C for the 1950-1980 period.

For example, in the following data, for 2016, they say the average global temperature as 14.8C:

https://www.ncdc.noaa.gov/sotc/global/201613

If the average global temp used to be claimed to be 15C but since 2000, they claim it was 14C, then somebody is making false claims about the avg temp.

[rrmm]

The larger point I'm trying to make is that average global temperature is something that is calculated as a result of the specific method the people doing the study use. It's not a simple average of a list of numbers. It tries to account for missing and spotty data, instrument biases, location biases, etc.

So if they find a better way to do it the calculated value will change.

[busymom0]

That's a fair point and others have mentioned that before. However, all the sources I have used in my original comment are from the same person - Dr James Hansen who served as the director of NASA Goddard institute for 35 years. He used the 15C claim for many years until he suddenly decided to switch to 14C. In any scientific claim, especially when such small changes could impact major things, a scientist shouldn't simply be allowed to change their data claims without any explanation, especially when it suddenly doesn't fit their claim.

Also in this source:

https://books.google.ca/books?id=VyFpAwAAQBAJ&pg=PA62&lpg=PA...

in 1997, the scientific community knew that the temperature was actually going down and not up:

> "Global Temperature Down Slightly"

[rrmm]

The claims Hansen made are contained in his research papers. So you'd have to cite them, though in most of the papers I've read they tend not quote absolute numbers.

In https://pubs.giss.nasa.gov/docs/2010/2010_Hansen_ha00510u.pd... paragraph 6:

""" 6] One consequence of working only with temperature change is that our analysis does not produce estimates of absolute temperature. For the sake of users who require anabsolute 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. """

The 2007 paper is https://pubs.giss.nasa.gov/docs/2007/2007_Hansen_ha09210n.pd...

Jones' paper https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999...

Section 6 discusses the anomaly vs absolute temp.

https://core.ac.uk/download/pdf/19595636.pdf Introduction gives a brief history and list of the historically calculated values.

So he was probably using the best value they had at the time. I don't know why any specific number was printed in the popular press at any given time though.

The 90's "pause" is still reflected in the data (depending on how you long you want to average over), then temps started increasing again.

[pjc50]

Fortunately there are an awful lot of other proxy variables which can be used to show a long-term change in climate. One example of thousands is the Japanese cherry blossom season: https://www.economist.com/graphic-detail/2017/04/07/japans-c...

And yes, there have been reversals in some years. But overall you can see the industrialisation "hockey stick" quite clearly in that calendar data.

[busymom0]

That graph seems to have opposing data for the global Medieval Warm Period and Little Ice Age. Medieval Warm Period (MWP) from about 900 A.D. to 1300 A.D. and the Little Ice Age (LIA) from about 1300 A.D. to 1915 A.D.

[pjc50]

The MWP was a local phenomenon; per wikipedia

> The Medieval Warm Period (MWP) also known as the Medieval Climate Optimum, or Medieval Climatic Anomaly was a time of warm climate in the North Atlantic region lasting from c. 950 to c. 1250.[1] It was likely[2] related to warming elsewhere[3][4][5] while some other regions were colder, such as the tropical Pacific

.. which implies that Japan may have been slightly colder at the time.

(Neatly illustrates that computing global average temperature from a set of local measurements is not as simple as it sounds, because there may also be local climate phenomena)

[empath75]

> If the average global temp used to be claimed to be 15C but since 2000, they claim it was 14C, then somebody is making false claims about the avg temp.

You get different baseline body temperatures depending on whether you use an oral thermometer or one of those infrared thermometers you put in the ear, but you'll both be able to detect a fever -- it's higher than it would normally be with that thermometer.

There are many ways of measuring the average temperature of the planet, and they will return different results today, tomorrow and yesterday -- but if you use the same method across time, all of them should agree on whether the planet is warming or cooling over all. One might say it was 15 in the 60s and 16 today, another might say that it was 14 in the 60s and 15 today -- they both agree the planet has warmed by a degree, though.

It's easy to go through several decades of stories on climate and cherry pick measurements from different news stories to make it look like the numbers are all over the place, but you have to compare measurements from a single source using a single method.

[jansan]

Your analogy with the thermometer does not work. If you use rectal or oral measurement you will be able to detect fever, because temperature fluctuation at these locations is low if the person is healthy. With global temperatures this is different. Fluctuation is very high (for example there was snow in Cairo in 2013, but we did not have an ice age then).

Therefore you need to cover many locations, and that's the problem. Global temperature data is not very good for the first half of the 20th century.

[rrmm]

Sorry I don't see 14.8 in the URL you gave?

[busymom0]

> The average global temperature across land and ocean surface areas for 2016 was 0.94°C (1.69°F) above the 20th century average of 13.9°C (57.0°F)