The global time series is produced from the Smith and Reynolds blended land and ocean data set (Smith et al., 2008). Global-average anomalies are calculated on an annual time scale. The global anomalies are provided with respect to the period 1901-2000, the 20th century average.

Land surface temperatures are from the Global Historical Climate Network-Monthly (GHCN-M). Sea surface temperatures are determined using the extended reconstructed sea surface temperature (ERSST) analysis. ERSST uses the most recently available International Comprehensive Ocean-Atmosphere Data Set (ICOADS) and statistical methods that allow stable reconstruction using sparse data. Due to very sparse data, no global averages are computed before 1880. With more observations after 1880, the signal is stronger and more consistent over time.

https://www.ncdc.noaa.gov/monitoring-references/faq/anomalies.php