Energetics Responses to Increases in Greenhouse Gas Concentration

Increasing greenhouse gas concentrations warm the troposphere. However, it is not clear whether this implies changes in the energetics. To study the energetics responses to CO₂ increases, changes in the Lorenz energy cycle (LEC) are evaluated using output from the atmosphere–ocean ECHAM5/Max Planck Institute Ocean Model (MPI-OM). Equilibrium 2 × CO₂ experiments and 10-yr transient experiments with 3% increase per year are analyzed. Globally, doubling of CO₂ results in a decrease in the LEC strength—defined as the total conversion of available potential energyPinto kinetic energyK—but also in an increase in the zonal-meanK. These global changes are a consequence of the strengthening of the LEC in the upper troposphere and the weakening of the cycle below. The two opposite responses result from the simulated warming pattern that shows the strongest warming in the upper tropical troposphere and in the lower troposphere at high latitudes. This warming structure causes changes in the horizontal temperature variance and in mean static stability, which increase zonal-meanPin the upper troposphere and decrease it below, triggering the two opposite responses via changes in baroclinic activity. In general, the lower-region weakening is stronger in the Northern Hemisphere, while the upper-region strengthening, and the increase of zonal-meanPandK, is stronger in the Southern Hemisphere. The former is more pronounced in the transient experiments but decreases in the stabilized 2 × CO₂ climate.