CERN’s CLOUD experiment

Physics Today : CLOUD ( cosmics leaving outdoor droplets ) is a novel experiment at CERN to investigate the possible influence of galactic cosmic rays on Earth’s clouds and climate by studying the microphysical interactions.Using the 50-year-old proton synchrotron , researchers simulate cosmic rays passing through Earth’s atmosphere, in hopes of revealing the extent to which the constant background drizzle of charged particles affects cloud formation.Earlier experiments have suggested that ionization causes clouds to “seed” and that ionization is influenced by the type and quantity of cosmic rays that reaching Earth.CLOUD has been running since 2006 and proved that cosmic rays bombarding Earth’s atmosphere may have an influence on the amount of cloud cover through the formation of new aerosols (tiny particles suspended in the air that seed cloud droplets).This result is supported by satellite measurements, which show a possible correlation between cosmic-ray intensity and the amount of low cloud cover. Clouds exert a strong influence on Earth’s energy balance; changes of only a few percent have an important effect on the climate. Understanding the microphysics in controlled laboratory conditions is a key to unravelling the connection between cosmic rays and clouds. The experimentThe initial stage of the experiment uses a prototype detector in a particle beam. CLOUD uses CERN’s Proton Synchrotron to send a beam of particles - the ‘cosmic rays’ - into a reaction chamber. The effect of the beam on aerosol production will be recorded and analyzed.The roots of the experiment can be traced as far back as two centuries, to the time when the astronomer royal, William Herschel, noticed a correlation between sunspots and the price of wheat in England. Herschel’s observation was the first to suggest that solar variation may affect Earth’s climate. Solar-climate variability has remained a great puzzle since that time, despite an intensive scientific efforts.During the ‘Little Ice Age’ around the 17th and 18th centuries, when sunspots all but disappeared for 70 years, the cosmic ray intensity increased and the climate cooled. Apparently that was merely the latest of around a dozen similar events over the past 10 000 years. At present, there is no established reason for the Sun’s brightness to fluctuate on these time scales. The possibility that galactic cosmic rays, which are modulated by changes of the solar wind, may directly influence the climate is therefore attracting the interest of scientists.The CLOUD collaboration brings together atmospheric physicists, solar physicists, and cosmic-ray and particle physicists to address a key question in the understanding of clouds and climate change. “CERN is a unique environment for this experiment,” says CLOUD spokesperson Jasper Kirkby of CERN.The first beam data from the full CLOUD experiment is expected in 2010.Clouds are one of the primary factors in determining global surface temperature, but the United Nation’s Intergovernment Panel on Climate Change admits that current level of scientific understanding of them is limited.