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All of the literature at the heart of the current climate change debate talks about +1.5 degrees or +2.0 degrees above the pre-industrial average.

I understand that, usually, pre-industrial means 1850-1900 (as recorded by the IPCC).

This IPCC Special Report on Global Warming of 1.5°C uses the reference period 1850–1900 to represent pre-industrial temperature. This is the earliest period with near-global observations and is the reference period used as an approximation of pre-industrial temperatures in the IPCC Fifth Assessment Report.

I've tried to find a definition of exactly what temperature the pre-industrial average was in terms of degrees centigrade/Celsius - and exactly how that was determined/measured.

Likewise, what is the current absolute global-average temperature? I can find plenty of deltas from an that unknown baseline, but am curious what the raw data shows.

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    $\begingroup$ FWIW, the whole average-temperature thing is missing the point a bit, and I think it's hurting the case of climate policy. 2.0°C sounds so little, and in a sense it is. The real problem isn't temperature but energy. The temperature rise is small not so much because the greenhouse effect is small, but because Earth is a hugely complex system storing and processing the enourmous energy that the GHE adds. Reassuring oneself that 2.0°C isn't that much is like reassuring yourself that the crane lowering the 16-ton weight onto your car does it very carefully... $\endgroup$
    – leftaroundabout
    Oct 2 at 10:05

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You have asked a very incisive and important question. Nevertheless, the basic core of this question has been answered before, here. In sum, the determination is rather simple. As summarized from NOAA in that answer -

The 2020 global surface temperature was 1.76°F warmer than 57.0°F, which is 58.76°F. This temperature is 2.14°F warmer than the pre-industrial period (1880-1900). Consequently, the pre-industrial period temperature was 58.76°F - 2.14°F, or approximately 56.62°F (13.68°C).

The current average global temperature can be determined for any one year but is usually only an estimate based on the most recently assessed information. The raw data consists of substantially many hundreds of thousands of global sub-hourly/hourly/daily measurements from which the global average temperature is determined. These measurements are taken, recorded, and reported, for many thousands of measurement sites on land, ships at sea, and aerial measurements by aircraft and balloons. These data are adequately summarized for an average annual global surface temperature, and are available on the internet from the IPCC, British Met Office, NOAA, and others. Although there may be slight differences in the time series presented by any particular entity (NOAA, for instance), these time series for the historic global average temperature are quite consistent and comparable.

The pre-industrial temperature record is similarly assessed, based upon established meteorological sites and reported readings during the pre-1900 time period back to about 1850. Keep in mind that as time recedes before 1900, land-based measurement sites become increasingly few and reported measurements increasingly sparse. Hence, the average temperature for the pre-industrial period is established by statistical comparisons and adjustments for concurrent and comparable records later in the early 20th century. Although the time series for these individual records forms the basis for determining the pre-industrial average temperature, the key word is average. Consequently, as an average, the estimate is reasonably good because variations away from the average affect the outcome by a very small margin.

As you have asked -

What is the baseline temperature for the climate-change debate...

the answer is 13.68°C.

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  • $\begingroup$ Thanks for the reply... but "Do the maths" is hardly a definitive answer for one of the most critical numbers in the climate change debate. This MUST be DEFINED somewhere? $\endgroup$
    – Andrew
    Oct 2 at 6:45
  • $\begingroup$ Constants are defined. Temperatures are measured. If a new way to measure historical global temperature averages were discovered and proven to mostly match our current datasets while providing better theoretical accuracy and/or precision, what good would your definition be? $\endgroup$
    – user121330
    Oct 2 at 7:31
  • $\begingroup$ @Andrew: You could search. NOAA and ECMWF for example have the data ready for download. $\endgroup$
    – user29219
    Oct 2 at 9:02
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    $\begingroup$ ...in other words, the best way to estimate the 1880 global average temperature is, counterintuitively, to compute all the local differences between 1880 and 2020, average them, and subtract that average difference from the 2020 global average temperature. $\endgroup$
    – leftaroundabout
    Oct 2 at 9:48
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    $\begingroup$ @Novice_Developer: That merits a proper question. Pls. be sure to read the "how to ask a good question" link when writing :-) $\endgroup$
    – user29219
    Oct 2 at 14:18

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