365 Days of Climate Awareness 91 – Global Mean Surface Temperature and Radiative Forcing

Global mean surface temperature (GMST) is an average, drawn from data sets around the world, showing annual to longer (century-plus) trends. The “pre-industrial period” referred to in the Paris Agreement and elsewhere is the 1850-1900 segment of this constructed average. Radiative forcing is a derived measure estimating the warming or cooling effect, in watts per square meter (W/m²), active in the biosphere.

Baselines for any measurable quantity, temperature or otherwise, are useful but generally arbitrary. Statistically, including more samples in a mean value increases its precision and confidence level. In this illustration, six more recent benchmarks are included, valuable to see because they reduce the noise of the annual average and show an increasingly rapid temperature gain.

Radiative forcing is inferred from the global heat budget, as understood from atmospheric, oceanic, and ground temperatures from around the world. When these temperature trends are analyzed, they can be converted into the amount of heat added to or lost from the biosphere. Close to all the heat at the surface of the planet comes from the sun. Insolation—incoming solar radiation—is fairly constant, at the amount of 1,361 W/m². When the climate is at equilibrium outgoing radiation equals incoming. When the planet is warming, incoming radiation is greater than outgoing; when the planet is cooling, outgoing is greater. Any nonzero difference between incoming and outgoing radiation is radiative forcing.

As measured by weather stations around the world, global temperature increases are converted to added heat content (Joules, J) within the biosphere, and then to radiative forcing (1 W = 1 J/s). Increasing radiative forcing of recent years reflects the increasing rate of temperature gain.

Tomorrow: climatic variability.

Be brave, and be well.