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.
"""
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.
"""
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.
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.
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.
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.
"""
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.
"""
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.
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.
> 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)
> Schematic diagrams of global temperature variations since the Pleistocene on three time scales
> There is giowing evidence that worldwide temperatures were higher than at present during the mid-Holocene (especially 5 000-6 000 BP), at least in summer, though carbon dioxide levels appear to have been quite similar to those of the pre-mdustnal era at this time (Section 1 i Thus parts <si western Euiope China, Japan, the eastern USA were a few degrees warmer in July during the midHolocene than in recent decades (Yoshino and Urushibara, 1978, Webb ct al 1987, Huntley and Prentice, 1988, Zhang and Wang 1990) Parts of Australasia and Chile were also waimei The late tenth to early thirteenth centuries (about AD 950-1250) appear to have been exceptionally warm in western Europe, Iceland and Greenland (Alexandre 1987, Lamb, 1988) This period is known as the Medieval Climatic Optimum China was, however, cold at this time (mainly in winter) but South Japan was warm (Yoshino, 1978) This period of widespread warmth is notable in that there is no evidence that it was accompanied by an increase of greenhouse gases
Sorry about the spelling issues in the copied text, the PDF's OCR isn't the best.
-------
Also here in another report, graph b) on page 2/16 shows the same MWP and little ice age as "Global temperature trend for millennium"
The graph c) also shows that in the past 25,000 years, it first used to be much colder, then got much hotter and then cooler and then warmer. It is pretty much impossible for it to be a local phenomenon if the variation was that much in that many areas.
Dr. David Deming, geologist and geophysicist, College of Earth and Energy, University of Oklahoma, senate testimony from December 6, 2006:
> I had another interesting experience around the time my paper in Science was published. I received an astonishing email from a major researcher in the area of climate change. He said, "We have to get rid of the Medieval Warm Period." The Medieval Warm Period (MWP) was a time of unusually warm weather that began around 1000 AD and persisted until a cold period known as the "Little Ice Age" took hold in the 14th century. Warmer climate brought a remarkable flowering of prosperity, knowledge, and art to Europe during the High Middle Ages. The existence of the MWP had been recognized in the scientific literature for decades. But now it was a major embarrassment to those maintaining that the 20th century warming was truly anomalous. It had to be "gotten rid of."
> The first phase of the LIA began around the thirteenth century in all the regions for
which there is evidence. The glacial phase preceding the MWP seems to have begun
between the seventh and ninth centuries A.D. but is generally less securely dated
and not dated at all in Canada. There are at least some indications of fluctuations
in ice position in the course of the MWP in Norway, Alaska, and perhaps in
extratropical South America and New Zealand, indicating that recession may have
been interrupted by advances, perhaps of limited extent, as in the European Alps.
The available evidence suggests that the MWP was global in extent and not uniform
climatically. The glacial data needs to be considered in relation to that from other
sources, but is of value in obtaining a more complete understanding of both the
environment in the later medieval period and the possible causes of climatic change
on the century time scale.
> The available evidence suggests that the MWP was global in extent and not uniform
climatically.
> The first phase of the LIA began around the thirteenth century in all the regions for
which there is evidence. The glacial phase preceding the MWP seems to have begun
between the seventh and ninth centuries A.D. but is generally less securely dated
and not dated at all in Canada. There are at least some indications of fluctuations
in ice position in the course of the MWP in Norway, Alaska, and perhaps in
extratropical South America and New Zealand, indicating that recession may have
been interrupted by advances, perhaps of limited extent, as in the European Alps.
The available evidence suggests that the MWP was global in extent and not uniform
climatically. The glacial data needs to be considered in relation to that from other
sources, but is of value in obtaining a more complete understanding of both the
environment in the later medieval period and the possible causes of climatic change
on the century time scale.
> The available evidence suggests that the MWP was global in extent and not uniform
climatically.
> Dating of organic material closely associated with moraines in many montane regions has reached the point where it is possible to survey available information concerning the timing of the medieval warm period. The results suggest that it was a global event occurring between about 900 and 1250 A.D., possibly interrupted by a minor readvance of ice between about 1050 and 1150 A.D.
> The findings support the view that the Holocene Thermal Maximum, the Medieval Warm Period, and the Little Ice Age were global events, and they provide a long-term perspective for evaluating the role of ocean heat content in various warming scenarios for the future.
> 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.
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.
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.