How much does increased world population contribute to global warming? In 1974 there where 4 billion people on earth. Now in 2013 we passed 7 billion people. So the world population is nearly doubled in 40 years. Every living human being also haves a body temperature of $37.5^\circ \: \mathrm{C}$ not to mention all the animals it need to feed on. Not taking into consideration the heat generated by machines and animals required to keep the population alive; by how much will 3 billion people at $37.5^\circ \: \mathrm{C}$ potentially affect the world temperature?
 A: Your question seems to be about human body heat rather than other human activities that contribute to global warming.  Humans body heat doesn't actually add any energy to the whole-Earth system (see below) but for a moment, I will assume that it does.
Instead of looking at the mean temperature of humans, it's easier to look at the amount of energy our bodies consume in food as a stand-in for the energy we dump into the environment.
We measure food energy in (kilo)calories which are exactly $4.184\: \mathrm{kJ}$.  Total food energy intake varies from country to country however at the top, people consume about $3800\: \mathrm{kCal}$ per day.
Assuming all 7 billion people ate $4000\: \mathrm{kCal}$ per day, the energy output would average:
$$7 \times 10^9 \mathrm{people} \times \frac{4000 \cdot 4.184\: \mathrm{kJ}}{\mathrm{people} \cdot \mathrm{day}} \times \frac{\mathrm{day}}{24 \cdot 60 \cdot 60\: \mathrm{s}} \approx 1.3 \times 10^{12}\: \mathrm{W}$$
Or, in English, all humans combined output about 1.3 terawatts of energy.
Now, to compare that to something else like sunlight, you'll see how small it really is.  According to Wikipedia:

The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses.

$$\frac{1.3 \times 10^{12}\: \mathrm{W}}{174 \times 10^{15}\: \mathrm{W}} \approx \frac{1}{128000}$$
As you can see, human body heat is negligible.

Also, if you trace back the source of the energy for our food, you find that all of the energy that went into making the food was derived from solar radiation.  More humans means more humans eating which means more food produced which means more solar energy absorbed to produce the food.  In the end, human body heat has no net impact on the warming of the Earth.
