The findings provide a hint as to the role of humans in the formation of clouds and precipitation. It is hoped that this knowledge could form the basis for more accurate climate models in the future.
All cloud droplets are made up of minute particles, known as aerosols, to which the water molecules can attach themselves. The clouds over Europe and other industrialised regions usually contain equal amounts of organic particles, such as fungal spores and plant fragments, and non-organic particles such as salts and minerals.
“Our results show that the clouds in the Amazon comprise up to 90 per cent organic material! The rest is primarily silicon, which is transported all the way from the Sahara, and salts that blow in from the sea”, explains Pontus Roldin, doctoral student in nuclear physics at the Faculty of Engineering and a co-author of the study.
The researchers also noted that the particle concentration in the rainforest is ten times lower than in Europe. In the Amazon the air contains 200 particles per cubic centimetre. The corresponding figure in industrialised countries is around 2 000 particles.
It is hoped that the results will make it easier for climate researchers to find out how great the cooling effect really is; today there is great uncertainty around this.
“The situation puts us in a tricky position. As regards global warming, cooling clouds are actually good. Meanwhile, every year hundreds of thousands of people die prematurely in Europe alone as a result of harmful particles in the atmosphere”, points out Pontus Roldin.
However, the aspect of the results that perhaps most fascinates the researchers themselves is how the plants and clouds appear to work together in a sophisticated manner. Could it be the case that evolution has favoured the vegetation that releases the exact type of particle that causes precipitation to form where it is needed?
“During the rainy season in the Amazon it rains almost every afternoon from high, heavy and cold Cumulus clouds. In order for rain to form easily, a certain type of large plant particle is needed, around which ice crystals can form. The number of particles of this type was very low compared with the other, smaller particles, but they still proved extremely important”, says Pontus Roldin.
How the study was carried out:
The measurements were made over an intensive month in February 2008. In the middle of the Brazilian rainforest the researchers managed to find an environment that, during the rainy season, had completely clean air (during the dry season large areas of rainforest are burnt to clear the land for agriculture). The aerosols were collected above the treetops with the help of a device placed on a 40-metre-high mast. The device was very complex and required round-the-clock monitoring by several researchers. Since then, the measurements have been analysed.
Read the article ‘Rainforest Aerosols as Biogenic Nuclei of Clouds and Precipitation in the Amazon’ at www.sciencemag.org
The main authors of the article are researchers from the Max Planck Institute for Chemistry in Mainz, Germany, and researchers from Harvard University in the USA. Researchers from the following universities also participated: the University of Colorado, Boulder; the National Institute of Amazonian Research; St Petersburg State University; Colorado State University; North Carolina State University; and Lund University.