Past and Future Radiative Forcing by Climate Active Agents in the EMAC Chemistry-Climate Model

Since scientists know that climate will change they search for a way to quantify its strength and the consequences for human life. One metric to show the impact of changes in the human and natural emissions of climate active agents on the Earth's climate system is the concept of radiative forcing (RF). It quantifies the energy imbalance that occurs when an imposed perturbation, for instance by a change in the mixing ratio of a greenhouse gas (GHG), takes place. There are several ways to calculate the radiative forcing, which differ in the included feedback processes. The instantaneous RF is calculated with fixed atmospheric background conditions to get the net change in the radiative flux ”instantaneously”, while the adjusted RF allows the temperature profile to adjust to a new equilibrium in the stratosphere, while the tropospheric temperature profile remains unchanged.

The goal of this study is to derive the RF of the troposphere due to projected future changes of ozone and GHGs by applying the new sub-model RAD in the ECHAM/MESSy Atmospheric Chemistry (EMAC) model. The instantaneous and adjusted RFs as well as the change of temperature tendency have been calculated. The analyses are based on the reference period 1965 (1960-1969) and the RF is derived for every decade until 2095 (2090-2099). The ozone and GHG concentrations, needed as input for the RF calculations, are taken from two transient simulations of the EMAC chemistry-climate model. The simulations extend from 1960 to 2100, and include forcings by GHGs following the specifications of the RCP6.0 and RCP8.5 scenarios, and by ozone depleting substances following the specification of the adjusted A1 scenario. Sea-surface temperatures and sea-ice concentrations were prescribed from the Hadley Centre Global Environment Model (HadGEM) for the RCP6.0 scenario and from the Max Planck Institute ocean model (MPIOM) for the RCP8.5 scenario. By analyzing the adjusted RF at the tropopause, we will examine how future changes in the stratosphere will potentially influence the troposphere.