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Incoming global solar irradiance measured at the surface at the South Pole unexpectedly decreased steadily by 15% from 1976 through 1987 during the late austral summer season, whereas no trend is apparent for September through December. February's irradiance trend, − 1.24% yr−1 on the average, is statistically significant at greater than the 99.9% confidence level. The irradiance observations were made continuously with the same calibrated sensor and are confirmed by a second simultaneous solar irradiance measurement series. Associated changes in seasonal sky cover (clouds) and surface air temperature were also observed. Seasonally increasing cloud cover is directly associated with the decreasing irradiance trends, whereas temperatures show a warming trend significant only in March, followed by a cooling trend significant only in May. Cloudiness and temperature records for 32 years suggest that the observed cloudiness trend began in the late 1970s, while the temperature trends become apparent only in the early 1980s. The observed sensitivity of total global solar irradiance to the change in sky cover is roughly six percent per one-tenth and is shown to vary spectrally. Although the annual averages of solar irradiance at the South Pole display an overall decrease between 1976 and 1989, the most recent years in this period show some recovery from earlier declines. Likewise, the downward trends in January and February irradiance diminished in 1988 and 1989.
Dutton et al. (Thu,) studied this question.