Using GrADS to calculate monthly global mean temperature and plot the time series:
http://cola.gmu.edu/grads/

Total field: with seasonal cycle
Anomaly field: deviation from climatological seasonal cycle
               (long-term mean seasonal cycle removed)

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Data:
UK Meteorological Office
Kaplan Extended SST V2 (Sea Surface Temperature)
https://www.esrl.noaa.gov/psd/data/gridded/data.kaplan_sst.html
Gridded global SST anomalies from 1856-present 
derived from UK Met Office SST data
Temporal Coverage: 01/1856 - 03/2017
Spatial Coverage: 
5.0 degree latitude by 5.0 degree longitude global grid (72x36)
87.5S - 87.5N, 2.5E - 357.5E

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JMA (Japan Meteorological Agency)
COBE Sea Surface Temperature (SST2)
https://www.esrl.noaa.gov/psd/data/gridded/data.cobe2.html
Monthly Means: 1850/01 to 2015/12
1.0 degree latitude x 1.0 degree longitude global grid (360x180)
89.5N - 89.5S, 0.5E - 359.5E

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Jones (CRU) Air Temperature Anomalies Version 3: CRUTEM3
https://www.esrl.noaa.gov/psd/data/gridded/data.crutem3.html
Monthly anomalies and statistics from 1850/01 to 2014/05
5.0 degree latitude x 5.0 degree longitude global grid (72x36)
87.5S - 87.5N, 177.5W - 177.5E

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US
NOAA Global Surface Temperature (NOAAGlobalTemp)
https://www.esrl.noaa.gov/psd/data/gridded/data.noaaglobaltemp.html
Monthly anomalies and statistics from 1880/01 to present
5.0 degree latitude x 5.0 degree longitude global grid (72x36)
87.5S - 87.5N, 2.5E-357.5E


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US
NOAA Extended Reconstructed Sea Surface Temperature (ERSST) V3b
https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.ersst.v3.html
Monthly values for 1854/01 - present.
2.0 degree latitude x 2.0 degree longitude global grid (89x180)
88.0N - 88.0S, 0.0E - 358.0E