PNNL

Abstract

Surface air temperature (SAT) is a common metric for evaluating global warming1,2. As shown here, a better metric for linking the warming to climate change and weather extremes is the thermodynamic energy at the surface, which is the sum of thermal energy and latent energy of water vapor. At the surface, the thermodynamic energy is the same as the equivalent potential temperature, Thetae. We use observationally derived SAT and water vapor to determine Thetae. From 1948 to 2018, while SAT increased by 0.9 °C, Thetae increased by 2 °C globally and by as much as 4 °C in the tropics. The increase in water vapor accounts for the factor of 2 difference between SAT and Thetae trends. The tropical amplification of Thetae, which is concentrated over the oceans and the rain forest regions, is as large as the arctic amplification of SAT. This amplification leads to increased trends in convective instability and weather extremes. The data also reveal a belt of negative Thetae trends from sub-tropics to extra tropics including northern China, the Indo-Gangetic plains and north west America, most likely due to the drying effects of pollution aerosols. The next major finding is the north-south differences in the warming trends. Unlike the inter hemispheric asymmetry in SAT trends, the southern-hemisphere Thetae increased by as much as that over the northern-hemisphere. Thus, Thetae is a much better metric for the global nature of climate change and explains better (than SAT trend) why new weather extremes are witnessed almost worldwide. With unchecked emissions, while SAT warming since pre-industrial era can be as large as 4.8 °C by 2100, Thetae can increase by as much as 12.2 °C with more than 16 °C increase in the tropics. Such large increases in Thetae have major implications for future heat waves, droughts, floods and tropical storms.