Data downloaded 27 Nov 2023 from
https://www.metoffice.gov.uk/hadobs/hadsst4/data/download.html
selecting
HadSST.4.0.1.0_monthly_GLOBE.csv

SST anamoly wrt the 1961 to 1990 average.
Using 249 datapoints from 2003.0 to 2023.833 to avoid the 10 year
recovery period of geodata post Mt Pinatubo eruption.
Using offline javascript curvefitter at statpages.info
and curve types linear (no acceleration), exponential (increasing
acceleration), quadratic (constant acceleration), indicial (falling
acceleration)
For data to Oct 2023
Best fit quadratic again and 6 figures retained for anyone else
repeating/checking this processing
y=0.408525 -0.004675*x +0.000924*x*x
where y is Hadley SST anomaly in deg C and x is year minus 2000
Projected to 2050, +2.4848 deg C
Nov o/p projected to 2100, +9.181 deg C
[ Still rising. Going by comparing NOAA ONI ENSO index and this SST
record, maximum, neutral and minimum, SST may be somewhere between 1 and
6 months after the relevant ONI month.
Last ONI neutral mar-apr 2023, last minimum ONI apr 2022.]

For data to Sep 2023, best fit quadratic
y= 0.407587 -0.003567*x +0.000874*x*x
Projected to 2050, +2.4142 deg C
to 2100, +8.7909 deg C
[ for the indicial curve fit this time,more conservative for comparison
Projected to 2050, +2.217 deg C
to 2100, +7.5826 deg C
still rising in either case ]

For data to Aug 2023 , best curve fit indicial
y= 0.383174 + 0.001021*x^1.895796
where y is Hadley SST anomaly in deg C and x is year minus 2000
Projected to 2050, +2.081129 deg C
to 2100, +6.701729 deg C

previous month processing for up to July 2023 data
Projected to 2050, +1.940949 deg C
to 2100, +5.844458 deg C

previous month processing for up to June 2023 data
y= 0.377054 +0.001667*x ^1.729662
Projected to 2050, 1.824424 deg C
to 2100, 5.177257 deg C

--
Global sea level rise to 2100 from curve-fitted existing altimetry data
<http://diverse.4mg.com/slr.htm>