Aerosols and clouds are classified as maritime or continental. Before humankind began heavily polluting the air, aerosol concentrations over land were up to double those over the oceans. Now, however, aerosol concentrations in polluted air masses are one to two order of magnitude greater than in pristine air.

Anthropogenic aerosols alter Earth’s energy budget by scattering and absorbing the solar radiation that energizes the formation of clouds. Because all cloud droplets must form on preexisting aerosol particles that act as cloud condensation nuclei (CCN), increased aerosols also change the composition of clouds (i.e., the size distribution of cloud droplets). This, in turn, determines to a large extent the precipitation-forming processes.

Precipitation plays a key role in the climate system. About 37% of the energy input to the atmosphere occurs by release of latent heat from vapor that condenses into cloud drops and ice crystals. Reevaporation of clouds consumes back the released heat. When water is precipitated to the surface, this heat is left in the atmosphere and becomes available to energize convection and larger-scale atmospheric circulation systems.

The dominance of anthropogenic aerosols over much of the land area means that cloud composition, precipitation, the hydrological cycle, and the atmospheric circulation systems are all affected by both radiative and microphysical impacts of aerosols, and are likely to be in a different state relative to the pre-industrial era.

—Rosenfeld et al. (2008)

Aerosols act in two ways. On the one hand, they act like a sunscreen reducing the amount of sun energy reaching the ground. Accordingly, less water evaporates and the air at ground level stays cooler and drier, with less of a tendency to rise and form clouds.

On the other hand, there would be no cloud droplets without aerosols. Some of them act as gathering points for air humidity, so called condensation nuclei. Water condenses on these tiny particles, releasing energy in the process. This is the same energy that was earlier used to evaporate the water from the earth’s surface. The released heat warms the air parcel so that it can rise further, taking the cloud droplets with it.

If there is a surplus of these gathering points, the droplets never reach the critical mass needed to fall to earth as rain&madsh;there just is not enough water to share between all the aerosol particles. Also, with a rising number of droplets their overall surface increases, which increases the amount of sunlight reflected back to space and thus cooling and drying the earth.

The study shows that with rising pollution, the amount of precipitation at first rises, than maxes out and finally falls off sharply at very high aerosol concentrations. The practical result is that in relatively clean air, adding aerosols up to the amount that releases the maximum of available energy increases precipitation. Beyond that point, increasing the aerosol load even further lessens precipitation. Therefore, in areas with high atmospheric aerosol content, due to natural or human-made conditions, the continuation or even aggravation of those conditions can lead to lower than normal rainfall or even drought.

With these results we can finally improve our understanding of aerosol effects on precipitation and climate, since the direct contradiction of the different aerosol effects has seriously hindered us from giving more accurate predictions for the future of our climate, and especially for the availability of water.

— Prof. Meinrat O. Andreae, director of the Max Planck Institute for Chemistry in Germany, co-author

Resources

• Daniel Rosenfeld, Ulrike Lohmann, Graciela B. Raga, Colin D. O’Dowd, Markku Kulmala, Sandro Fuzzi, Anni Reissell, and Meinrat O. Andreae (2008) Flood or Drought: How Do Aerosols Affect Precipitation? Science Vol. 321. no. 5894, pp. 1309 - 1313 DOI: 10.1126/science.1160606