The Cloud Project: Climate Research with Accelerators

The current understanding of climate change in the in- dustrial age is that it is predominantly caused by anthro- pogenic greenhouse gases, with relatively small natural contributions due to solar irradiance and volcanoes. How- ever, palaeoclimatic reconstructions show that the climate has frequently varied on 100-year time scales during the Holocene (last 10 kyr) by amounts comparable to the present warming—and yet the mechanism is not under- stood. Estimated changes of solar irradiance on these time scales are too small to account for the climate observations. This raises the question of whether cosmic rays, which are modulated by the solar wind, may be directly affect- ing the climate, providing an effective indirect solar forcing mechanism. Indeed recent satellite observations—although disputed—suggest that cosmic rays may affect clouds un- der certain conditions. However, given the many sources of variability in the atmosphere and the lack of control of the cosmic ray flux, demonstrating overall cause and effect be- ginning with changes in cosmic rays and ending with obser- vations of perturbed clouds is challenging. For these rea- sons a novel experiment known as CLOUD has recently be- gun operation at the CERN Proton Synchrotron which aims to study, under controlled conditions, the effects of cosmic rays on aerosol nucleation and growth, cloud droplets and ice particles. The CLOUD project represents an unprece- dented opportunity for the particle accelerator community to combine their facilities and expertise with those of the atmospheric and climate communities to address a subject of major importance for science and society