A REVIEW OF LONG TERM TRENDS IN SOLAR RESOURCES

摘要

A sample of recent literature on the topic of long-term trends observed at solar-monitoring stations has shown that solar resources have decreased slightly, yet noticeably, at the majority of the sites surveyed around the world. At many sites, the trends are statistically significant, showing decreases in global horizontal solar radiation of up to 2-8% per decade. Only a few stations showed increases. In some studies, concurrent cloud cover observations were compared with the solar measurements. It was found that the decrease in insolation was greater under normal or cloudy conditions than with clear sky conditions, indicating that a change in cloud fraction over time is a primary source of the insolation trend at these locations. Although the studies generally indicate that changes in cloudiness had more influence on the trends than changes in atmospheric aerosol concentrations, there are exceptions to this. In some studies cloud fraction actually decreased concurrently with the insolation decrease. In these cases other factors, such as increases in long-range transport of pollutants from industrial areas, or increases in local pollution sources (such as automobile traffic), were attributed to the decreasing insolation trends. For all these studies, however, it is important to understand that maintaining long-term quality measurements is extremely difficult, and that features such as length of record, calibration drift, sensor degradation, instrument maintenance, and changes in instrumentation and local conditions must be considered when evaluating these long-term trends.