As mentioned in “Chapter Summary”, AR5 has introduced new terms to estimate the radiative effects of clouds and aerosols. Instead of using aerosol direct and indirect radiative effects as in AR4, in this chapter, the radiative effects due to aerosols are separated into four parts – aerosol radiative effect (REari), aerosol rapid responses to climate change, aerosol feedback to climate change and aerosol-cloud interaction.

 

    Here, we first review the AR5 assessment on the first three effects.

 

1. Aerosol Radiative Effect (REari)

 

    REari is the so-called aerosol direct radiative effect in AR4. It measures the variation of the Earth’s radiation flux due to the scattering and absorption of radiation by aerosols (in W m^-2). To compute REari, one needs to know quantities such as spectral-dependent aerosol extinction coefficient, single scattering albedo, and phase function. These values can be derived from the measurements of aerosol size distribution, chemical composition and mixing state, as well as the shape of aerosol particles.

 

   Estimate of REari, however, is not very straightforward. It is spectral dependent and affected by the radiative properties of the surface. The existence of atmospheric trace gases and clouds (even its relative position to the aerosol can also interfere with REari estimate.

 

• Top of atmosphere (TOA) REari

  • Shortwave TOA REari over cloud-free oceans range from -4 to -6 W m^-2 (mainly due to sea spray);

  • Shortwave TOA REari over cloud-free land is difficult to estimate, because of poorly characterized land surface;

  • Longwave TOA REari is generally positive (mainly due to coarse aerosols such as dust and see salt, as well as due to stratospheric aerosols);

  • Currently, TOA REari estimate in cloudy sky condition remains to be challenging. Except for absorbing aerosols, TOA REari is usually weakened by existing of clouds.

 

• REari due to absorbing aerosols

REari due to absorbing aerosols is positive. Its estimation has great uncertainties due to poor knowledge on their aerosol single-scattering albedo and is vertical profile.

 

2. Aerosol Rapid Adjustments

 

    Rapid adjustments of aerosols is corresponding to the aerosol ‘semi-direct radiative effect’ in AR4. This effect is realized mainly through the change in cloud cover. In “Cloud feedbacks & rapid adjustments”, we have discussed how cloud cover can vary in response to a warming climate caused by increased atmospheric CO2.

 

    Existence of absorbing aerosols can induce similar changes in cloud cover as increase of greenhouse gases. There are, however, a little complexity in case of absorbing aerosols – the relative position of clouds and absorbing aerosols matters. Undermost circumstance, absorbing aerosols will warm up the troposphere, increasing its stability and reducing upward moisture fluxes. As a result, the cloud cover will be reduced. When absorbing aerosols are embedded in clouds, the absorbed radiation will further heat up the cloud droplets and speed up the cloud dissipation [Ghan et al., 2012]. However, if absorbing aerosols are right above the stratocumulus cloud deck, the capping inversion above the cloud layer can be enhanced. As a result, the stratocumulus cloud deck can be thickened, which has been evidenced both by satellite observation [Wilcox, 2010] and modeling results [Johnson et al., 2004].

 

3. Aerosol Feedbacks to Climate Change

 

    Climate change can be physical (in temperature, humidity, winds, precipitation, solar radiation, etc.), chemical (in oxidation state, etc.), or biological (in vegetation cover, primary production, etc.). Therefore, the influence of climate on aerosols is also multi-facets.

 

    Some modeled feedbacks of aerosols on climate change are:

• Dust loading might increase (due to increase in bare soils) or decrease (due to increase in vegetation fertilized by increased CO2) (low confidence);

• DMS-sulfate-cloud-climate feedback is a weak one due to a weak sensitivity of CCN population to variations of DMS emission (medium confidence);

• Formation of nitrate aerosol might be impacted by changing temperature. In the meantime, formation of ammonium aerosols might be affected as a consequence of changes in sulfate and nitrate aerosols. However, it is hard to project their future loading due to great uncertainties in the emission of their precursors.

• Changes in carbonaceous aerosols (mainly biogenic) due to climate change can lead to counteracting radiative effects. Therefore, feedbacks associated with this type of aerosols are also very uncertain.

 

    Overall, AR5 estimates that changes in emission of aerosols due to climate change corresponds to relatively small feedback parameters (within ±0.2 W m^–2 °C^–1, with low confidence), compared with uncertainties in other radiative forcing.