There are natural explanations for global warming of which a change in the output of solar energy is a candidate. However, none of the published work has convinced me that this can explain much of the observed global warming over the last several decades. Volcanic emissions are another natural global forcing, and it is well known that they produce cooling, such as after the eruption of Mount Pintatubo, where in August of 1991 it was estimated as -4 Watts per meter squared. There have not been eruptions of that magnitude since, such that the absence of such major eruptions might permit greater absorbed solar radiation in the climate system than otherwise would occur. However, this absence of eruptions resulting in any positive radiative imbalance for a period of time well after a major volcanic emission has also not been shown to occur. This leaves anthropogenic emissions as a source for global warming.