Attribution certainty differences between AR4 and AR5

One of the conclusions from the AR5 Summary for Policymakers is the higher degree of certainty of the human attribution on the average temperature increase since the 1950s. It was said it went from 90% certainty in 2007 to 95% in the current report. Journalists took this on face value and creatively put this in their articles. It makes me wonder if they even have read the summary report. It makes me wonder even more if they read the previous report and took the time to compare the two statements.

So what did the IPCC stated in those two reports?

AR4 (SPM p. 10)
Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG concentrations.

AR5 (SPM p. 12)
It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.

Indeed we see the word very likely (according to AR4 this is 90%) and extremely likely (according to AR4 this is 95%). But there seems to be several subtle rewordings. It is interesting to see the difference between the two:


Which temperatures Global average temperatures Global average surface temperature
Period of attribution Since the mid-20th century From 1951 to 2010
Cause Due to the observed increase in anthropogenic GHG concentrations Caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together
Attribution Most More than half
Which temperatures
In AR5 they specified global average surface temperature. Does this mean they exclude the satellite record? As far as I know satellites don’t record surface temperatures, only the temperature of the different layers of the atmosphere. Does it mean they only rely for the conclusion of attribution on surface measurement series like Giss or HadCrut?
Period of attribution
This surprised me a bit. Why suddenly use a specific endpoint (2010) when in previous report it was just till the year of the report was created? Maybe they just wanted to have whole decades? But to me it looks a bit strange why they didn’t/couldn’t state this for the last three years. Did something changed in the last 3 years so they couldn’t state this anymore?
The cause
AR4 only talked about anthropogenic greenhouse gases. AR5 includes also other anthropogenic forcings. It would be interesting to read more about that in the final report. For whatever reason the observed is omitted. Is this attribution made by something else than observations/measurements? Maybe partly by models?

Both are not sharply defined quantifications. At first glance if you look at them in a pure logical way, they are equivalent. “More than half” is from the first unit of one half (51% or 50,1% or …), so in theory it could span to the last unit before 100 (99% or 99.9% or…). The same with “Most”. It is definitely higher than 50% and definitely smaller than 100%.

What bothers me is this: obviously someone had the bright idea to change the wording. Why change one vaguely defined quantification into another vaguely defined quantification?

If one looks at it from a pure logical standpoint, “More than half” and “Most” are identical. But does everybody looks at it in a logical way? I recognized the discussion. A couple weeks ago I had this discussion about the difference between “Most” and “More than half”. Intuitively I would place “More than half” somewhat above 50% and “Most” more to the end of the spectrum. If I would have to use them both, than “More than half” would be lower than “Most”.

In my search on the difference between the two, I found this was actually a field of study for linguists and psychologists. They study for example how those words are used in our daily language. In On measurement and quantification: The case of most and more than half I found a graph that shows the occurrence of the two terms where also a quantifier is used in Corpus Of Contemporary American English (COCA).

Occurrence of the two terms where also a quantifier was used

Occurrence of “Most” and “More than half” where also a quantifier was used Source: gwz-berlin

It shows that “More than half” is mostly used in cases of 50-60%, and a low occurrence in the range 60-75%. “Most” is used in the full range, but the highest occurrence are in the range of 80-95%. Personally, as a simple member of the public, I would use “More than half” as 50-60% and “Most” as “70-90%”. In that sense, to me it feels like they are more sure about a lower occurrence of human attribution, or put differently: that they are more sure that there is less attribution than previously stated. That looks like a step back, cleverly disguised as a step forward. The press probably didn’t look at the differences in both reports and some of them even gave it a personal interpretation!

I do not know what the IPCC really intended with the changed quantification, but jumping from one vague quantifier to another could well come to haunt them in the long run. This is a report for policymakers, they won’t necessarily look at it in a pure logical way and some of them might want to compare both reports.

There seem to be a lot of subtle changes between the two reports, which doesn’t make it easy to compare both. Maybe even make it impossible. It is not the same period they talk about, not the same temperatures, not the same cause and not the same quantification. This makes them not directly comparable.


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