Radiation emitted by the sun.
Wallace Broecker on irradiance:
"Skeptic claim: The warming of the last 30 years has been driven by an increase in the Sun's energy output.
Rebuttal: Satellite-based measurements carried out over the last quarter century (a time during which the Earth has steadily warmed) reveal no upward trend in the Sun's irradiance. Rather, they show only a tiny (less than 0.1 percent) cyclic change which correlates with the 11-year sunspot cycle."