Using Earth's Entropy Production Rate as a Global Climate Change Metric

Detailed forecasts of local climate variables are essential for adaptation, but international decision-making is often centered on scalar global summary variables instead. The variables we choose constrain which targets can be set and thus influence policies and interventions. Global area-averaged surface temperature is the principal metric of climate state, alongside sea level and radiative forcing.

The complexity of the climate system is unavoidably difficult to capture in such a simplified manner and improvements would support more careful decision-making, particularly around geoengineering interventions. Useful climate change metrics are those with physical significance which are responsive to alterations in the system’s state.

We argue that the global entropy production rate (EPR) offers a useful additional perspective. It is a natural system-summarising variable combining temperature and energy and accounting for the total irreversible activity of the climate system. A limitation in its application has been ambiguity around its definition: here we present a detailed description of the options for constructing a meaningful entropy production rate and suggest a particular interpretation. We demonstrate this EPR’s estimation from simple energy balance models, and its measurement via radiative transfer calculations from GCM and reanalysis products. This EPR is shown to be sensitive to climate changes and, importantly, well-placed to distinguish between climates with identical global average surface temperatures but fundamentally different behaviours.