It is a measure of the land surface temperature. The land surface temperatures from around the globe are averaged together.
And that number is compared to the base period (1951-1980), which gives us the temperature anomaly.
Key: blue is cool and orange/red is warm.
@ JL 2026
(Land-Ocean Temperature Index, L-OTI)
i.e. deviations from the corresponding 1951-1980 means.
Global-mean monthly, seasonal, and annual means, 1880-present https://data.giss.nasa.gov/gistemp/
GISTEMP Team, 2024: GISS Surface Temperature Analysis (GISTEMP), version 4. NASA Goddard Institute for Space Studies. Dataset accessed 20YY-MM-DD at https://data.giss.nasa.gov/gistemp/.
Lenssen, N., G. Schmidt, J. Hansen, M. Menne, A. Persin, R. Ruedy, and D. Zyss, 2019: Improvements in the GISTEMP uncertainty model. J. Geophys. Res. Atmos., 124, no. 12, 6307-6326, doi:10.1029/2018JD029522.