Abstract
Using idealized climate model simulations, we investigate the effectiveness of black carbon (BC) aerosols in warming the planet relative to CO2 forcing. We find that a 60-fold increase in the BC aerosol mixing ratio from the present-day levels leads to the same equilibrium global mean surface warming (∼4.1 K) as for a doubling of atmospheric CO2 concentration. However, the radiative forcing is larger (∼5.5 Wm−2) in the BC case relative to the doubled CO2 case (∼3.8 Wm−2) for the same surface warming indicating the efficacy (a metric for measuring the effectiveness) of BC aerosols to be less than CO2. The lower efficacy of BC aerosols is related to the differences in the shortwave (SW) cloud feedback: negative in the BC case but positive in the CO2 case. In the BC case, the negative SW cloud feedback is related to an increase in the tropical low clouds which is associated with a northward shift (∼7°) of the Intertropical Convergence Zone (ITCZ). Further, we show that in the BC case fast precipitation suppression offsets the surface temperature mediated precipitation response and causes ∼8% net decline in the global mean precipitation. Our study suggests that a feedback between the location of ITCZ and the interhemispheric temperature could exist, and the consequent SW cloud feedback could be contributing to the lower efficacy of BC aerosols. Therefore, an improved representation of low clouds in climate models is likely the key to understand the global climate sensitivity to BC aerosols.
Highlights
Black carbon (BC) aerosols, unlike most of the atmospheric aerosols, absorb solar radiation and have a warming effect on the planet
We perform four simulations: (i) a ‘CTL’ simulation, where the atmospheric CO2 concentration is prescribed at pre-industrial level (284.7 ppm), (ii) a ‘2 × CO2’ simulation, which is same as CTL but the CO2 concentration is doubled from the pre-industrial level, (iii) a ‘1 × black carbon (BC)’ simulation with prescribed BC aerosol distribution at 2000 climatological level and CO2 is prescribed at the pre-industrial levels and (iv) a ‘60 × BC’ simulation, which is same as 1 × BC, but the prescribed BC mixing ratio is 60 times the 2000 climatology-spatial distribution is same as the 2000 climatology, only the magnitude is increased
The slow response which is defined as the climate response that is mediated by the change in global mean surface temperature (Bala et al 2010), is calculated as the difference between the total climate response estimated from the slab ocean (SOM) simulations and the fast adjustments estimated from the prescribed-SST simulations (Bala et al 2010)
Summary
Black carbon (BC) aerosols, unlike most of the atmospheric aerosols, absorb solar radiation and have a warming effect on the planet. Only few modeling studies (e.g. Cook and Highwood 2004, Roberts and Jones 2004, Hansen et al 2005, Yoshimori and Broccoli 2008, Stjern et al 2017, Smith et al 2018) have examined the effectiveness of BC aerosols in warming the planet relative to CO2 forcing. These studies find that the BC aerosols are less effective in warming the planet compared to CO2. Efficacy is defined as the ratio of the equilibrium global mean temperature change per unit forcing by that agent to the equilibrium temperature change per unit CO2 forcing from the same initial climate state. It is the ratio of the climate sensitivity to the forcing agent to that due to CO2 forcing
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