An international team of scientists has come up with either a surprising or unsurprising finding about whether air pollution increases or decreases rainfall. Their conclusion: a rather frustrating both, depending on local environmental conditions.

In an era of climate change, understanding how rain is impacted by pollution has significant consequences and in an article appearing in Science, the scientific team has published the results of its research untangling the contradictions surrounding the conundrum. They do this by following the energy flow through the atmosphere and the ways it is influenced by aerosol (airborne) particles. This allows the development of more exact predictions of how air pollution affects weather, water resources and future climates.

Mankind releases huge amounts of particles into the air that are so tiny that they float. Before being influenced by man, air above land contained up to twice as many of these so called aerosol particles as air above oceans. Nowadays, this ratio has increased to as much as a hundredfold.

Natural and manmade aerosols influence our climate – that much is agreed. But which way do they push it? They produce more clouds and more rain, some say. They produce fewer clouds and less rain, say others. This disputed role of aerosols has been the greatest source of uncertainties in our understanding of the climate system, including the question of global warming.

“Both camps are right”, says Prof. Meinrat O. Andreae, director of the Max Planck Institute for Chemistry in Germany, a coauthor of the publication. “But you have to consider how many aerosol particles there are.” The lead author, Prof. Rosenfeld of the Hebrew University, adds: “The amount of aerosols is the critical factor controlling how the energy is distributed in the atmosphere.” Clouds, and therefore precipitation, come about when moist, warm air rises from ground level and water condenses or freezes on the aerosols aloft. The energy responsible for evaporating the water from the earth’s surface and lifting the air is provided by the sun.

Aerosols act twofold: 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. On these tiny particles with diameters of less than a thousandth of a millimeter the water condenses – similar to dew on cold ground – 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.
But if there is a surplus of these gathering points, the droplets never reach the critical mass needed to fall to earth as rain – 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.

In a nutshell, then, the study results show the following: 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 manmade conditions, the continuation or even aggravation of those conditions can lead to lower than normal rainfall or even drought.

Prof. Rosenfeld states: “These results have great significance for countries like Israel where rainfall is scarce and can be easily affected by over-production of aerosols. Our study should act as a red light to all of those responsible for controlling the amounts of pollution we release into the atmosphere.”

“With these results we can finally improve our understanding of aerosol effects on precipitation and climate,” summarizes Andreae, “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.”

Article: Science, 5 September 2008, 321