Atmospheric Aerosols and Climate
Although aerosols reduce visibility through the formation of haze layers, these haze layers produce beautifully coloured pink-to-red sunset skies. In addition to colourful sunsets, these haze layers reduce the amount of solar energy transmitted through the atmosphere as the haze reflects some of the sun’s incoming rays back out to space. In addition to forming haze layers, aerosols are essential for the formation of clouds as they provide condensation nuclei for cloud water drops and ice particles to form on. Without these nuclei, there would be no clouds, no precipitation and no hydrological cycle. Further, these clouds provide the most reflecting layers in the atmosphere, also reducing the amount of solar energy transmitted through the atmosphere. These reflecting layers have a net effect of cooling down the planet—without these haze and cloud layers, the global temperature would be of the order of ~10°C higher. Changes in the abundance of these aerosols lead to changes in the reflection, or cooling efficiency, of these haze and cloud layers.
This effect of increased aerosol abundance on increased cloud reflectance is readily visualised over marine stratiform clouds overlying shipping lanes where tracks of higher reflectance in layered clouds produced by the aerosol pollution emitted from the ships stacks occur.
Increased aerosol availability can also influence precipitation and cloud lifetime, depending on the cloud type. For shallow stratiform clouds, an increase in aerosol availability reduces precipitation onset, leading to more persistent clouds, while for deep convective clouds, an increase in aerosol availability can lead to more intense precipitation. At very high aerosol concentrations, the absorbing ‘black carbon’ component of aerosol pollution can cause sufficient heating of the aerosol layer to impact on dynamics and suppress convection and even cloud formation.
The Inter-governmental Panel on Climate Change (IPCC) Assessment Report 4 (AR4) concluded that the aerosol contribution to radiative forcing amounted to a cooling effect that partly off-set the warming induced by the accumulation of greenhouse gases (GHGs) in the atmosphere. This suggests that aerosols have been obscuring the true rate of global warming, or, specifically, the climatetemperature sensitivity to CO2-induced global warming. One would intuitively expect a lower level of brightness, or dimming, if more solar radiation is reflected back out to space. Over the past 40 years, both dimming and brightening trends have been observed, the explanation of which converges towards an aerosol influence on climate.
Regional Climate Model projection of temperature changes and sulphate mass concentrations over Europe into the future using the RCP6.0 emissions scenario projection. Time-slice years are taken for 2006, 2050 and 2100. Colour bar on left-hand side represents temperature for 2006, while upper-right colour bar represents temperature change for 2050 and 2100 relative to 2006. Lower-right colour bar represents sulphate (PM2.5) mass (μg m-3) for time-slice years 2006, 2050 and 2100.
Representative Concentration Pathways: RCP6: Stabilisation without overshoot pathway leading to a radiative forcing of 6 W m-2 (~850 ppm CO2 eq) at stabilisation after 2100; emissions peak at ~11 PgC per year around 2050, reducing to ~4.5 PgC per year by 2100. SO2 emissions reduce almost linearly until 2100 to less than 25% of current emissions (110 Tg SO2 year-1 in 2000 to 25 Tg SO2 year-1 in 2100).