In 28 August 1981, the mean surface temperature is 288K = 15 degree C:

http://climate-dynamics.org/wp-content/uploads/2016/06/hanse...

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In 1988, it was claimed that the global avg temperature was 59 deg F = 15 degree C

Multiple sources:

https://www.nytimes.com/1988/03/29/science/temperature-for-w...

https://news.google.com/newspapers?nid=1915&dat=19880705&id=...

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IPCC's First Assessment Report in 1990, table on page xxxvii of the report listed the "Observed Surface Temperature" of Earth as 15 degrees Celsius:

https://www.ipcc.ch/site/assets/uploads/2018/03/ipcc_far_wg_...

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In Jan 12 1992, it's 15 degree C:

https://news.google.com/newspapers?id=S2xGAAAAIBAJ&sjid=7ugM...

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############## SOMETHING CHANGED IN 1997:

"Global Temperature Down Slightly":

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

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December 14, 2002: They magically started using 14C as the long term average instead of 15C without any explanation:

https://www.smh.com.au/world/global-warming-blamed-for-heat-...

> This year the Earth's average temperature was 14.64C, compared with the long-term average of 14C, said James Hansen, of NASA's Goddard Institute for Space Studies, who analyses the data collected from thousands of weather stations around the world. The meteorological year runs from December to November. During that period, 2001 temperatures were 14.51C. The record remains with 1998, when global temperature rose to 14.67C - the highest since records were first compiled in the late 1800s. The warm temperatures of 2001 and 2002 are especially significant when they are considered in the light of El Nino weather patterns that alter global climate, Mr Hansen said. "The fact that 2002 is almost as warm as the unusual warmth of 1998 is confirmation that the underlying global warming trend is continuing," Mr Hansen said.

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August 25, 2011:

> But Hansen and colleagues have estimated that Earth's actual average surface air temperature between 1951 and 1980 was approximately 287 K (14 degrees Celsius) (Hansen et al. 2010). The difference in temperature is attributed to greenhouse gases that trap thermal radiation, warming Earth as depicted in figure 2.1.

https://www.gao.gov/assets/330/322216.html

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Sometime between 1997 and 1998, they magically started claiming the avg to be 14 degree C instead of 15.

18 January 1998's "Vital Signs 1998: The Environmental Trends that are Shaping Our Future" magazine made this magical change with a small footnote:

https://books.google.ca/books?id=EfZRAwAAQBAJ&pg=PA174&lpg=P...

> In earlier versions of Vital Signs, Worldwatch added the temperature change reported by the Goddard Institute to an estimated global temperature of 15 degrees Celsius, but the institute has since informed Worldwatch that a better base number would be 14 degrees Celsius. James Hansen, Goddard Institute for Space Studies, New York, email to author, 18 January 1998.”

Can someone please explain this unexplained change on how avg surface temperature between 1951 and 1980 went from claims that it was 15C to now being claimed it was 14C?

Edited: changed magic to unexplained change