Headline: Global radiative forcing and megacities

Emissions of greenhouse gases and air pollutants from megacities impact the climate. The long-lived greenhouse gases CO 2, CH 4 and N 2O as well as climate-active pollutants such as NO x, VOC and particulate matter (PM) are all emitted from megacities. NO x and VOC contribute to tropospheric ozone formation and affect the lifetime of long-lived greenhouse gases. Anthropogenic aerosols include sulphate, black carbon (BC) and particulate organic matter (POM). Aerosols impact climate directly (absorption, backscattering) and also have indirect (cloud) effects. We assess the climate impact of megacity emissions with the Met Office Hadley Centre Earth System Model HadGEM2 applying an "annihilation" scenario in which the emissions at megacities are entirely removed. Generally, the contribution of megacities to global pollutant emissions is on the order of 2-5% of the total global annual anthropogenic base emission flux. The impact of megacity climate-active pollutants is assessed via an annual mean top-of-atmosphere direct radiative forcing (AMTOA-DRF) from long-lived GHG as well as ozone, methane and aerosols. In this simulations the long-lived component (CO 2, CH 4 and N 2O) contributes a positive TOA-DRF of +120.0, +28.4 and +3.3mWm -2, respectively, under present-day conditions. Climate-active pollutants (NO x, VOC) contribute an AMTOA-DRF of +5.7±0.02mWm -2 from an increase in the ozone burden -1.9±0.04mWm -2, -6.1±0.21mWm -2 from the aerosol AMTOA-DRF in the short-wave spectrum and +1.5±0.01mWm -2 from aerosol in the long-wave spectrum. The combined AMTOA-DRF from all climate-active pollutants is slightly negative at -0.8±0.24mWm -2 and the total AMTOA-DRF amounts to +150.9±0.24mWm -2. Under future conditions (2050s) the total AMTOA-DRF from long-lived GHG is found to profoundly increase to +322.6mWm -2 while the total AMTOA-DRF from climate-active pollutants turns positive and decreases slightly to +0.5±0.09mWm -2 yielding a combined AMTOA-DRF of +323.1±0.09mWm -2 in the future. It is apparent that under the given emission scenarios the radiative forcing from long-lived GHG, particularly CO 2, by far dominates the impact of megacities on climate.