Approximately 1360 watts per square meter of irradiance arrive at the earth from the sun. Averaging over the earth’s spherical surface (dayside, nightside, poles), this irradiance is 340 watts per square meter of earth, but of this only 200 watts per square meter makes it through the atmosphere and heats the surface of the earth. Water vapor is by far the major absorber of incoming sunlight, and together with dust and ozone it blocks 52% of the sun’s incoming energy.
Source: https://commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg
The sun’s irradiance typically becomes stronger as the number of sunspots increase and diminishes as the number declines. Sunspots usually occur in an 11-year cycle, but there have been long periods when the sun’s radience has been more or less than normal. When irradiance decreases, not only is the earth’s temperature directly reduced, but also more galactic cosmic rays enter the atmosphere. This increases cloudiness, which then reflects more of the sun’s radiation back to space. However, this effect is minor.
Studies disagree on how much the sun’s irradiance has varied over history; the estimates range from 3 to 7 watts per square meter1 — roughly a 0.2% to 0.5% change. If solar radiation diminished for a long period by 0.26% — 3.6 watts per square meter, the earth’s temperature would fall by approximately 3/4ths of a degree C and vice-versa. Hence, past changes in climate, such as in the Little Ice Age and the Minoan Warm Period, may be explained by solar changes.
In the three most recent decades satellite measurements indicate that irradiance changes have been minimal and have not departed from norms, causing at most a temperature change of 0.01ºC2. Therefore, the sun is not responsible for recent climate warming.