Conclusion: In this paper we have reviewed the physical mechanisms behind solar irradiance variation, and we have reviewed how on the timescale of solar evolution, the Sun cannot have been any dimmer than it is at the most recent activity minima. We have also shown how concurrent changes in the Earth’s reflectance can produce a much larger climate impact over relatively short time scales. Thus, a possible Sun–albedo link, would have the potential to produce large climate effects without the need for significant excursions in solar irradiance . These could provide an explanation for the apparently large climate response to apparently small solar changes, as well as how the 11/22 year solar cycle is imprinted on Earth. Regardless of its possible solar ties, we have seen how the Earth’s large scale reflectance—and the short wavelength part of the Earth’s radiation budget—is a much more variable climate parameter than previously thought and, thus, deserves to be studied in as much detail as changes in the Sun’s output or changes in the Earth’s atmospheric infrared emission produced by anthropogenic greenhouse gases.
Pinker et al., 2005
Long-term variations in solar radiation at Earth’s surface (S) can affect our climate, the hydrological cycle, plant photosynthesis, and solar power. Sustained decreases in S have been widely reported from about the year 1960 to 1990. Here we present an estimate of global temporal variations in S by using the longest available satellite record. We observed an overall increase in S [solar radiation] from 1983 to 2001 at a rate of watts per square meter (%) per year ; this change is a combination of a decrease until about 1990, followed by a sustained increase. The global-scale findings are consistent with recent independent satellite observations but differ in sign and magnitude from previously reported ground observations. Unlike ground stations, satellites can uniformly sample the entire globe.
Wild et al., 2005
A similar reversal to brightening in the 1990s has been found on a global scale in a recent study that estimates surface solar radiation from satellite data. This indicates that the surface measurements may indeed pick up a largescale signal. The changes in both satellite derived and measured surface insolation data are also in line with changes in global cloudiness provided by the International Satellite Cloud Climatology Project (ISCCP), which show an increase until the late 1980s and a decrease thereafter, on the order of 5% from the late 1980s to 2002 . A recent reconstruction of planetary albedo based on the earthshine method, which also depends on ISCCP cloud data, reports a similar decrease during the 1990s. Over the period covered so far by BSRN (1992 to 2001), the decrease in earth reflectance corresponds to an increase of 6 W m-2 in absorbed solar radiation by the globe. The overall change observed at the BSRN sites, estimated as an average of the slopes at the sites in Fig. 2A, is W m-2 per year ( W m-2 over the entire BSRN period) .
[T]here is general consistency of a reduction in total cloud cover as temperature anomaly increases , with cloud cover decreasing from about 1984 until year 2000 followed by a flattening out to 2009, which is the end of the available cloud cover data. The decrease in total cloud cover anomaly is approximately percent of sky, against the long-term average (all months 1984-2009 inclusive) of percent of sky, which means a reduction of % of the cover.