Cosmic Rays and Atmospheric Ions: Their Importance for Clouds and Climate

Palaeoclimatic data provide extensive evidence for solar forcing of the climate during the Holocene and the last ice age, but the underlying mechanism remains a mystery. However recent satellite observations suggest that clouds may be influenced by cosmic rays, which are modulated by the solar wind. Physical mechanisms have been proposed to explain the cloud observations, but definitive experiments are lacking. It is clear, however, that there are several mechanisms by which cosmic ray ionisation may influence clouds and the climate system. In order to test whether cosmic rays and clouds are causally linked, a novel experiment known as CLOUD1 has been proposed, using a beam from a particle accelerator. INTRODUCTION It has been proposed that the Earth’s climate could be affected by changes in cloudiness in response to variations in the intensity of galactic cosmic rays (GCRs) in the atmosphere. This proposal stems from an observed correlation between cosmic ray intensity and the Earth’s average low cloud cover over the course of one solar cycle (Fig. 1) [1]. Some scientists question the reliability of the satellite observations, while others who accept them as reliable suggest that the correlation may be caused by other physical phenomena with decadel periods or by a response to volcanic or El Niño forcings. With the recent extension of the ISCCP-D2 cloud dataset for the period 1995-2001, there remains considerable uncertainty as to whether the cloud data show a significant long-term correlation with GCR intensity [2, 3]. Nevertheless, the observation has raised the intriguing possibility that a cosmic ray-cloud interaction may provide the key to the long-sought mechanism for Sun-Earth climate variability, and atmospheric electricity is a central topic under consideration. Changes in cloud cover are important because clouds exert a strong control over the Earth’s radiative energy balance. Increased GCRs are associated with more low clouds and cooler temperatures. The observed variation of low clouds by about 1.7% absolute corresponds to a change in the Earth’s radiation budget of about 1.2 Wm−2 between solar maximum and minimum. This change in energy input to the lower atmosphere is highly significant when compared, for example, with the estimated radiative forcing of 1.4 Wm−2 from anthropogenic CO2 emissions. The GCR intensity has varied considerably in the past on 100–1000 year timescales, and persuasive evidence for a significant solar/GCR influence on climate change has emerged from recent palaeoclimatic studies. For example, the occurrence of ice-rafted-debris cold events in the North Atlantic is found to be highly correlated with solar/GCR variability (Fig. 2) [4]. The correlation embraces the 17th–18th century Little Ice Age, which appears to be merely the most recent of around 10 such cold events during the Holocene, when North Atlantic Ocean surface temperatures fell by around 2◦C. More recently, during the 20th century, the GCR flux declined by about 15% due to an increase in the solar open magnetic flux by more than a factor of two. This 100-year change in intensity is about the same magnitude as the observed change over the last solar cycle. If the cosmic ray-cloud effect is physically-based then these long-term changes of GCR intensity could significantly influence climate, introducing a new solar indirect contribution to the current global warming and bringing additional uncertainties into climate change projections. Such possibilities make this a fiercely debated geophysical phenomenon and make it all the more important to understand the cause of the cloud variations. CLOUD is an acronym for Cosmics Leaving OUtdoor Droplets. C ha ng e in c lo ud fr ac tio n (% ) C ha ng e in c os m ic ra ys (% ) low clouds solar irradiance cosmic rays (>13GeV) C ha ng e in s ol ar ir ra di an ce (% )