The total energy received by the Earth from the Sun is equivalent to 131,400 billion tons of oil per year.
The total heat output of all of man's works is less than .00008 of the solar input. There is no way that we can measure the temperature that closely. The variability of the clouds on a single day may be more than that. It is less than 1/10,000 of the heat input to the planet.
I believe that to be insignificant in the overall heat budget. Here is where I found the numbers.
http://www.mpoweruk.com/solar_power.htm
This is not to indicate that there is no “heat island” effect in urban centers. That effect is real, but is different from the heat input into the total Earth heat budget, and includes a lot of energy streams going in and out of urban heat islands due to the large presence of so much asphalt and concrete, and so few trees, more than anything else.
The Earth is giving up solar derived energy and only retains a specific amount. Is that net energy retained ? Minus what is given back out to space at night ? Without radiating solar energy back into space in the short term, the Earth would over heat. Without retaining some of the solar energy long term, the Earth would freeze.
I agree however that what we are looking at are the causes of the Urban Heat Island effect and not the causes of Global Climate Change. My guess would be excess heat retention from concrete and excess heat release from burning fossil fuels within the urban areas.