Bright Clouds with Added Pollution
University of Manchester scientists, writing in the journal Nature Geoscience, have shown that some natural emissions and man made pollutants can have an unexpected cooling effect on the world’s climate by making clouds brighter. Clouds are made of water droplets, condensed on to tiny particles suspended in the air. When the air is humid enough, the particles swell into larger cloud droplets. It has been known for some decades that the number of these particles and their size control how bright the clouds appear from the top, which affects the the efficiency with which clouds scatter sunlight back into space. A major challenge for climate science is to understand and quantify these effects which have a major impact in polluted regions of the world.
The tiny cloud seed particles can either be natural (for example, sea spray or dust) or man made pollutants (from vehicle exhausts or industrial activity). These particles often contain a large amount of organic material and these compounds may be volatile, so in warm conditions exist as a vapor.
The researchers found that the effect acts in reverse in the atmosphere as volatile organic compounds from pollution or from the biosphere evaporate, but under moist cooler conditions where clouds form, the molecules prefer to be liquid and make larger particles that are more effective seeds for cloud droplets.
"We discovered that organic compounds such as those formed from forest emissions or from vehicle exhaust, affect the number of droplets in a cloud and hence its brightness, so affecting climate," said study author Professor Gordon McFiggans, from the University of Manchester’s School of Earth, Atmospheric and Environmental Sciences.
"We developed a model and made predictions of a substantially enhanced number of cloud droplets from an atmospherically reasonable amount of organic gases."
"More cloud droplets lead to brighter cloud when viewed from above, reflecting more incoming sunlight. We did some calculations of the effects on climate and found that the cooling effect on global climate of the increase in cloud seed effectiveness is at least as great as the previously found entire uncertainty in the effect of pollution on clouds."
This phenomena has been noticed before. Some researchers as early as 1990 found that slight dimming continued over land while brightening occurred over the ocean. A 2007 NASA sponsored satellite-based study shed some light on the puzzling observations by other scientists that the amount of sunlight reaching Earth's surface had been steadily declining in recent decades, began to reverse around 1990. This switch from a global dimming trend to a brightening trend happened just as global aerosol levels started to decline.
For further information see Bright Clouds.
Cloud Formation image via Wikipedia.