The only other possible sources are 1) a change in earth's orbit leading to more insolation, or, 2) a change in the energy output of the sun, or, 3) an increase in the energy released by our industrial activities.

However,

1) There has been no change in the earth's orbit for the centuries that we have been measuring it.

2) There has been no measurable change in the solar constant since we have been monitoring it.

3) All the energy added by all of Man's industrial activities is not sufficient to raise the global temperature appreciably, there isn't anywhere near enough and because the energy would leak back out into space as excess heat (infrared radiation) anyway.

However, the concentration of CO2 greenhouse gas has increased by about a third in the sixty years that we have been monitoring it. Adding greenhouse gases to the atmosphere makes it increasingly opaque to infrared. THAT is why the world is heating up. Period. Full Stop. The global temperature varies in step with the level of greenhouse gases in the atmosphere. And we know that to be true because we can prove it happened in the past, before there were people or industries or nuclear power. As the amount of atmospheric CO2 goes up and down, for whatever the reason (natural or man-made), so does the temperature. We know this because fossil evidence tells us what the temperature AND the CO2 concentration was in past eras. We can compare them and they are correlated. Natural CO2 levels are driven by events in nature, they have fluctuated wildly long before there were people or nuclear reactors on earth.

But when people started burning fossil fuels in large amounts, we started artificially (and very rapidly) adding CO2 to the atmosphere. All fossil fuels contain carbon. What our industrial activity over the last 200 odd years (when we started burning fossil fuels) has done is greatly increase the concentration of atmospheric CO2. And this increase has become particularly extreme in the last half-century.

That's all you need. We have not added heat to the earth, what we have done is to alter the earth's ability to regulate its temperature. The atmosphere is transparent to most light from the sun, it heats up the planet. At night, the warm Earth emits the heat as infrared back into space. THAT is what we have interfered with. We are to puny a species to add significant heat to the earth's energy budget, but altering the atmosphere's infrared opacity turns out to be quite easy, and we are doing it in spades.

Think of it this way. If you roll up the windows in your car on a hot sunny day it will soon become very hot inside the car. Whether or not you smoke a cigarette will make absolutely no difference one way or the other. It just doesn't matter.