How much do we humans contribute to the heating of the world? The heat we output from our bodies and population increases must mean more heat.
I say this because a friend has a small barn and in it two horses, two pigs and some chickens. He said the barn doesn't use any heat source because the heat from the two horses is enough to warm the barn. Even a bucket of water in the barn won't freeze.
A human uses about 100W. I don't know about horses, but depending on how well-insulated the barn is, a few hundred watts would make a significant difference.
About those humans heating the Earth -- the effect of the GHG's we emit is tapping into solar insolation, which is huge, so it probably dwarfs the above figures (see next paragraph). Also, the energy that powers us (food) would be turned into heat by other processes anyway.
Order of magnitude: population of 7 billion * 100W/person = 9.7TW. Solar insolation 1361 W/m^2, over the projected area of the earth, which totals 175,000,000 TW.
I just throw that figure (100W) as a useful order of magnitude estimate.
I say this because a friend has a small barn and in it two horses, two pigs and some chickens. He said the barn doesn't use any heat source because the heat from the two horses is enough to warm the barn. Even a bucket of water in the barn won't freeze.