About 45 years ago, the “consensus” in climate science (as summarized by Williamson, 1975) was quite different than today’s version.
1. The Medieval Warm Period was about 1°C warmer than present overall …….Con.//
2. The island of Spitsbergen, …….warmed by 8°C between 1900 and 1940, ….Cont.//
3. Central England temperatures dropped -0.5°C between the 1930s to the 1950s.
4. Pack-ice off northern and eastern Iceland returned to its 1880s extent between 1958 and 1975.
5. In the 1960s, polar bears ….Cont.//

One of the most profound mistakes is the claim, that the warming at Spitsbergen commenced 1900. The Arctic Warming started at the end of 1918. This date is important as it was the year when a devastation naval war in Northern European waters ended after four years.

See also the next graphic (temperatures since 1850.

https://1ocean-1climate.com/arctic-warming-100-years-ago-due-to-naval-war-in-europe-1914-1918/

As of the mid-1970s, the “consensus” among climate scientists was the globe had warmed by +0.6°C from 1880 to 1940, and then cooled by -0.3°C (to -0.4°C) from 1940 to 1970.

The notion that warming started 1880 is extreme superficial. The graphic shown, indicates for the Northern Hemisphere a ‘balanced’ temperature level since 1850. On a global base, the date 1880 ignores the Krakatoa outbreak in 1883, which cooled the global temperatures over the next few years. 

https://1ocean-1climate.com/krakatoa-northern-hemisphere-winter-warming-after-volcanic-eruptions/

As recently as 1980, it was still acceptable to publish scientific papers saying the Northern Hemisphere alone had warmed by 1°C between 1880 and 1940 and then cooled by nearly the same amount during the next 3 to 4 decades.

Speaking about a warming from 1880 to 1940, demonstrates incompetence, or unwillingness for details. There had been only a dramatic warming form 1919 to 1939, which started and affected the Northern Hemisphere in the Atlantic sector of the Arctic Ocean.
https://1ocean-1climate.com/the-early-20th-century-warming-still-not-understood/

During the 2000s, climate scientists with a keen interest in shaping global temperature data sets (Tom Wigley, Phil Jones, Michael Mann, Gavin Schmidt, Stefan Rahmstorf) exchanged e-mails about “correcting” the temperature data by removing warming from the 1940s “blip” – which they said would be “good” and significant for the global mean because the 1940s were “too warm”.

Speaking about a “1940s blip” is objective cheating. The presumably most pronounced dramatic shift started in the first war winter of World War Two and in Europe, immediately followed by the next two war winters.  https://1ocean-1climate.com/winter-201718-record-cold-regions-not-in-europe/

Even James Hansen acknowledges this fact.

https://oceansgovernclimate.com/712-2/

Phil Jones, overseer of HadCRUT temperature data, admitted the pre-1980s sea surface temperatures in the Southern Hemisphere are “mostly made up” due to insufficient coverage. He also corresponded with a colleague about “inventing” monthly temperature anomalies – which apparently was “fun” to do.

Phil Jones even wrote to his colleagues about changing temperature data so as to support the “argument” that the cooling decades coincided with the timing of a rise in anthropogenic aerosol emissions.

Phil Jones seems to use date a he thinks fit best. An example is his well-known graph about the Arctic,  which show little changes prior WWI and a dramatic jump after 1918, and decrease  at the moment WWI started in autumn 1939.

Thoroughly documented and discussed at:

www.seaclimate.com , particularly in Chapter B and C

Apparently as a consequence of “corrections” to the temperature data, the amplitude of the 1880 to 1940 warming trend has been slashed from +0.6°C (1970s) to +0.1°C today.
The 1940 to 1970 global cooling has been transformed into a -0.05°C hiatus.

Science seems unwilling to understand that data collection during the two World Wars was very high.  WWI was restricted to the sea off the shores of Northern Europe, and the warming started in the North of the continent, around Spitsbergen.

Discussed in the book: http://www.arctic-heats-up.com/

Apparently over 80% of the amplitude of an inconvenient warming or cooling trend can be eliminated from temperature data sets 50-80 years after the original temperatures were recorded.

A thorough investigation of both, the warming of the Northern Hemisphere after WWI, stating in 1918, and the sudden commencement of a severe global cooling in war winter 1939/40, would show, that anthropogenic “climatic changes”, can easily happen. Paying no interest in these periods, but manipulation weather data instead, should be regarded as threat to humanity.