engleski

Testing a link between cosmic rays, solar irradiance and cloudiness over short timescales

A hypothesized link between the solar-modulated cosmic ray (CR) flux and the Earth’s cloud properties is still heavily debated in the scientific community. Recently, it was shown that numerous long-term studies of satellite-based cloud observations are limited by non-trivial disadvantages (e.g. satellite data errors, ENSO, volcanic eruptions) and have failed to present compelling evidence of a CR–cloud link. The satellite- data limitations can be resolved by focusing on short-term (daily) timescales using Forbush decrease events and epoch- superpositional (composite) methods. Unfortunately, these studies have also arrived at a range of conflicting conclusions. However, such studies have often failed to properly isolate the effects of various solar parameters like solar irradiance and consequently this may have interfered with their results. In our work using the composite analysis we clearly isolated the effects of both irradiance and the CR flux to test this hypothesis, and found no robust evidence of a widespread link to cloud cover. It may be the case, that for the short-timescale studies, a hypothesized CR induced signal in clouds may be drowned in the meteorological noise of the datasets. Using extensive Monte Carlo simulation techniques and the two most widely used satellite cloud datasets (ISCCP and MODIS), we quantitatively demonstrate how the high noise levels present in composites of small sample sizes, or for overly isolated sample areas, may predominately account for the inconsistent results obtained. Furthermore, we find that the assumptions made by classical statistical tests (like the Student’s T-test) are frequently violated by both the restricted samples and methods employed in the literature (such as normalization to an averaging period). We conclude that such tests should be avoided, in favor of MC simulations, which offer a far more robust method of assessing significance and enabled us to correctly assess the significance of some recent short-term studies purporting to identify evidence of a CR–cloud link.

cloud cover; cosmic rays; Forbush decreases; Monte Carlo; statistics

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