Solar output in fact has decreased since the early 60s.
Also according to the Milankovitch cycles we should be in the middle of a cooling period, although the actual effect is quite complex (e.g. it makes a difference whether perihelion occurs in the austral or boreal summer). So it is also possible that we might be in for slight warming over the next twenty thousand years. But even if we were in for dramatic warming due to orbital resonance, that would be on the order of 0.1C/century, much lower than the changes we've observed.
You left out volcanoes, which are a natural source of CO2 (as well as cooling particulates).
If you add up all the known sources of natural climate variation you end up with no warming trend since 1900 (source).