Unsettling the settled: Simple musings on the complex climatic system

Demetris Koutsoyiannis^*George Tsakalias

• National Technical University of Athens, Athens, Greece

Our revisit of fundamental issues of climate challenges the notion and term of the "greenhouse effect", and attempts a scientific reevaluation using minimal assumptions, such as Newton's laws, maximum entropy and gas spectroscopy. It replaces terms like "greenhouse gas" with "radiatively active gas" (RAG) and "greenhouse effect" with "atmospheric radiative effect" (ARE). While ARE exists in several planets' atmospheres, on Earth it is primarily driven by water vapor and clouds, with CO₂ playing a minor role (especially anthropogenic CO₂ which represents 4% of total emissions). Equilibrium thermodynamics, via entropy maximization or molecular collision simulation, leads to an isothermal atmosphere at about 250 K (the average temperature of the troposphere and stratosphere) irrespective of RAG presence or not. It is the troposphere's 6.5 K/km temperature gradient (lapse rate), partly shaped by moist adiabatic processes, that drives the atmosphere away from this equilibrium and warms the surface to about 288 K on average, with ARE (mainly water vapor and clouds) contributing to the warming, but only when this gradient exists. The temperature gradient varies spatially and temporally and, since 1950, has weakened in the tropics and grown in the polar areas, resulting in a decrease of the surface equator-to-pole gradient, as expected in global warming conditions.