Total Solar Irradiance
Scientists have speculated that long-term solar irradiance variations might contribute to global warming over decades or hundreds of years. More recently, there has been speculation that changes in total solar irradiation have amplified the greenhouse effect, i.e., the retention of solar radiation and gradual warming of the earth's atmosphere. Some of these changes, particularly small shifts in the length of the activity cycle, seem to correlate rather closely with climatic conditions in pre- and post industrial times. Whether variations in solar irradiance can account for a substantial fraction of global warming over the past 150 years, however, remains a highly controversial point of scientific discussion.
Some researchers are convinced solar irradiance has increased between 1986-1996 (the years of the twentieth century's last two solar minima) and this increase is consistent with the conclusion that long term solar irradiance changes are occurring. But other scientists disagree, citing data inconsistent with such a conclusion. In particular, they have reported that solar irradiance was at similar levels in the years 1986 and 1996, but the global surface temperature of Earth had increased by about 0.2°C during the same decade. Although researchers disagree about whether recent changes in the total solar irradiance can account for global warming between 1986-1996, most agree that long-term solar irradiance measurements will help elucidate the role the Sun actually plays in driving global climate changes.