CERN experiment looks at cosmic rays, clouds and climate
CLOUD (Cosmics Leaving OUtdoor Droplets) is a novel experiment at CERN conducted by an interdisciplinary team of leading scientists from 18 institutes in 9 countries. The goal is to investigate the possible influence of galactic cosmic rays on Earth's clouds and climate, by studying the microphysical interactions involved. This is the first time a high energy physics accelerator is being used to study atmospheric and climate science.
Cosmic rays are charged particles that bombard the Earth's atmosphere from outer space. Studies suggest they 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 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 the Earth's energy balance; changes of only a few per cent 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 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 analysed.
The roots of the experiment can be traced as far back as two centuries, when the Astronomer Royal, William Herschel, noticed a correlation between sunspots and the price of wheat in England. This marked the first observation that Earth's climate may be affected by variations of the Sun. Solar-climate variability has remained a great puzzle since that time, despite an intensive scientific effort. 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. This seems to be merely the latest of around a dozen similar events over the last ten thousand years. At present, there is no established reason for the brightness of the Sun to fluctuate on these time scales. The possibility of a direct influence on the climate of galactic cosmic rays (which are modulated by changes of the solar wind) 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. "The experiment has attracted the leading aerosol, cloud and solar-terrestrial physicists from Europe; Austria, Denmark, Finland, Germany, Switzerland and the United Kingdom are especially strong in this area," says the CLOUD spokesperson, Jasper Kirkby of CERN. "CERN is a unique environment for this experiment. As well as our accelerators, we bring the specialist technologies, experimental techniques and experience in the integration of large, complex detectors that are required for CLOUD." An example in the present CLOUD prototype is the gas system, designed by CERN engineers, which produces ultra-pure air from the evaporation of liquid oxygen and liquid nitrogen. "It's probably the cleanest air anywhere in the world", says Kirkby.
The first results from the CLOUD prototype are expected by the summer of 2007. The full CLOUD experiment includes an advanced cloud chamber and reactor chamber, equipped with a wide range of external instrumentation to monitor and analyse their contents. The temperature and pressure conditions anywhere in the atmosphere can be re-created within the chambers, and all experimental conditions can be controlled and measured, including the ‘cosmic ray’ intensity and the contents of the chambers. The first beam data with the full CLOUD experiment is expected in 2010.