Can the observed changes be explained by natural variability, including changes in solar output?
Since our entire climate system is fundamentally driven by energy from the sun, it stands to reason that if the sun’s energy output were to change, then so would the climate. Since the advent of space-borne measurements in the late 1970s, solar output has indeed been shown to vary. With now 28 years of reliable satellite observations there is confirmation of earlier suggestions of an 11 (and 22) year cycle of irradiance related to sunspots but no longer term trend in these data. Based on paleoclimatic (proxy) reconstructions of solar irradiance there is suggestion of a trend of about +0.12 W/m2 since 1750 which is about half of the estimate given in the last IPCC report in 2001. There is though, a great deal of uncertainty in estimates of solar irradiance beyond what can be measured by satellites, and still the contribution of direct solar irradiance forcing is small compared to the greenhouse gas component. However, our understanding of the indirect effects of changes in solar output
Since our entire climate system is fundamentally driven by energy from the sun, it stands to reason that if the sun’s energy output were to change, then so would the climate. Since the advent of space-borne measurements in the late 1970s, solar output has indeed been shown to vary. There appears to be confirmation of earlier suggestions of an 11 (and 22) year cycle of irradiance. With only 20 years of reliable measurements however, it is difficult to deduce a trend. But, from the short record we have so far, the trend in solar irradiance is estimated at ~0.09 W/m2 compared to 0.4 W/m2 from well-mixed greenhouse gases. There are many indications that the sun also has a longer-term variation which has potentially contributed to the century-scale forcing to a greater degree. There is though, a great deal of uncertainty in estimates of solar irradiance beyond what can be measured by satellites, and still the contribution of direct solar irradiance forcing is small compared to the greenhous
