This work sits at the fringe of astrophysical studies: nearby are analyses of black hole flares, planetary deaths, and cosmic dust. But here the telescope is used not to probe distant galaxies but as a sensitive sensor of Earth's atmosphere. Long records of atmospheric extinction from observatories are rarely analyzed for dust climatology, even though they offer a continuous multi-decadal picture. This sets the work apart from its neighbors: it turns a byproduct of astronomical observations into a standalone climate archive.
On every photometric night, the Carlsberg Meridian Telescope measured how much the light of standard stars was attenuated in the optical range. Until 1998, a V filter (551 nm) and a scanning slit micrometer were used; after 1999, an r' filter (625 nm) and a CCD camera. To isolate dust events, a 100-day moving median was subtracted from nightly values, removing slow variations such as volcanic aerosol after the Pinatubo eruption. The threshold for a dust event was defined separately for each period: values above the 99.9th percentile of the normal distribution were considered dust. Frequency was calculated as the fraction of nights exceeding the threshold among all clear nights. To search for links with climate indices, correlation analysis was applied with lags from -12 to +12 three-month intervals, with significance assessed via Monte Carlo methods.
During the summer months from July to September, the frequency of Saharan dust events over La Palma is 28±6%, whereas during the rest of the year it is only 10±4%. From 1984 to 1997, the frequency declined from 0.47±0.11 to 0.10±0.07, then partially recovered and remained around 0.26±0.09 until 2012. The intensity of dust episodes, estimated from the mean light attenuation, followed the frequency, but because of the instrument change in 1999, absolute values cannot be compared. Correlation analysis revealed that 55±4% of the interannual variance in dust frequency during July–September is explained by the North Atlantic Oscillation (NAO) index for the preceding October–December, and 45±4% by a negative correlation with the Sahel Rainfall Index (SRI) for February–April. Links with El Niño (MEI index) proved statistically unstable.
Unlike neighboring studies that examine individual astrophysical objects or one-off events, this work analyzes a thirty-year series of atmospheric measurements as a climate indicator. It is the first attempt to use Carlsberg Meridian Telescope data for long-term statistics of Saharan dust specifically in the summer season. The study shows that astronomical archives can serve as a source of climate information where dedicated observation networks appeared later or have gaps. The lagged correlations found suggest potential predictability of dusty seasons several months ahead.
The topic could evolve toward combining data from different Canary Islands observatories and comparing with modern satellite and ground-based lidar observations. Statistical models for forecasting dusty seasons based on winter NAO and SRI indices could be built. Of particular interest is using longer records of atmospheric extinction to reconstruct dust activity before the satellite era.
The results may be useful for planning astronomical observations in the Canary Islands and for validating models of mineral dust transport in the atmosphere.
A next step could be calibrating data from different instruments to obtain a continuous series of dust intensity over the whole period. It would also be worth checking the found correlations against independent data from other stations.
The work addresses a long-standing problem in astronomical climatology: how to separate the influence of local atmospheric conditions from global climate signals in long observational records. Uncertainty in dust event thresholds and instrument changes remain the main limitations for climate conclusions.
The telescope, designed to measure stellar positions, turned out to be a sensitive detector of Saharan dust: on the dustiest nights, light attenuation reached 0.3 magnitudes, comparable to losing a third of a star's brightness.