Although there is no compelling evidence from satellite-based cloud measurements that cloud changes respond to the cosmic ray flux, it is possible that a small link exists and remains undetected due to signal-to-noise limitations and the overwhelming influence of long-term measurement artefacts. The diurnal temperature range provides another way of testing this hypothesis and has several important benefits over satellite-cloud data, including coverage over a longer time-span – an important feature for examining Forbush decrease and ground-level events.

As part of the COST Action ES1005 TOSCA (Towards a more complete assessment of the impact of Solar variability on the Earth’s climate) group, Benjamin Laken of the Instituto de Astrofísica de Canarias and Jaša Čalogović at the Hvar Observatory in Croatia investigated the claims of the Serbian research group. The researchers report their results in the ERL Focus on High Energy Particles and Atmospheric Processes.

Laken and Čalogović say that the earlier reports of unusual diurnal temperature variations were connected to the effects of sample restrictions. “Since there are only very few high-magnitude Forbush decrease and ground-level enhancement events that have occurred in the past 50 years, if you were to progressively isolate the most extreme of these events the sample size rapidly dwindles,” Laken points out. “If you analyse atmospheric properties corresponding to a small number of events, noise dominates the data. For these data, such noise is far larger than even the most favourable estimates of a cosmic ray related signal.”

Small samples make accurately identifying statistical significance values difficult but Laken and Čalogović employed robust Monte Carlo analysis methods to overcome this problem. The researchers conclude that temperature changes under moderate Forbush decrease and ground-level enhancement events are not different from those occurring under more intense conditions. “We clearly show that only regular variations in the diurnal temperature range occur during strong cosmic ray changes, and are indistinguishable from meteorological noise,” said Laken.

Speaking about the field in general, Laken concludes: “although we found that cosmic rays do not dominantly influence cloud properties, there is still much work to do. Working with the TOSCA group, we hope to understand better the role of the Sun and space weather on the Earth’s environment, and aim to provide useful resources that will bring together the research communities involved in this topic. In the future, we could help provide a more accurate estimate of natural solar-induced climate forcing than we currently possess, so enabling improved projections of anthropogenic climate change.”