Global Mean Temperature Calculator

Global mean surface temperature is a spatially and seasonally weighted average of near-surface air temperature over land and sea surface temperature over ocean. Its absolute value is about 14 to 15 degrees C, but operational datasets report anomalies relative to a fixed baseline such as 1850 to 1900 or 1991 to 2020, because temperature anomalies stay correlated over distances of roughly 1000 km while absolute temperatures vary sharply with elevation, coastline, and station siting. The model used here reproduces the same number from physics rather than thermometers: an effective temperature Te from the Stefan-Boltzmann balance, a surface temperature Ts equal to Te divided by the fourth root of the effective emissivity, and a perturbation dT equal to lambda times dF divided by 3.7. The lambda in that last step is the equilibrium climate sensitivity, the warming reached once all fast feedbacks have played out after a sustained doubling of carbon dioxide. The IPCC Sixth Assessment Report gives a best estimate of 3.0 degrees C, a likely range of 2.5 to 4.0, and a very likely range of 2.0 to 5.0, so entering the canonical 3.7 W per square meter simply returns the sensitivity itself. Two limits matter when reading the output. Equilibrium sensitivity is not what the planet shows on a human timescale, because ocean thermal inertia puts the transient climate response nearer 1.4 to 2.2 degrees C. And a single global number hides the spatial pattern: land warms about 1.5 times faster than ocean, and the Arctic has warmed several times the global rate.

Instrumental temperature records exist for individual European stations back to the seventeenth century, the Central England Temperature series beginning in 1659, but assembling them into a global figure is a twentieth century project. Guy Stewart Callendar, a British steam engineer, published the first serious attempt in 1938, compiling records from 147 stations and linking the resulting warming to rising carbon dioxide. Modern gridded analyses began with James Hansen and colleagues at the NASA Goddard Institute for Space Studies in 1981 and 1987, and with Phil Jones and coworkers at the Climatic Research Unit in the mid 1980s. Four principal records are now maintained: NASA GISTEMP, NOAA NCEI, the joint Met Office Hadley Centre and Climatic Research Unit HadCRUT series, and Berkeley Earth, which was launched in 2010 as an independent reanalysis under Richard Muller and published results in 2013 that matched the existing datasets closely. Much of the technical labour has gone into homogenization, in particular correcting the shift in sea surface measurement from canvas buckets to engine room intakes and later to drifting buoys, a bias of a few tenths of a degree that had to be removed before century-scale trends could be trusted. Satellite microwave sounding of the lower troposphere, running since 1979, provides an independent though differently biased check on the surface records.