[ksherlock]

The earth loses about 100,000 tons/year via hydrogen and helium escaping. The earth gains about 50,000 tons per year via space dust, etc.

Since temperature = energy, e=mc^2, therefore temperature = mass, the earth gains 160 tons/year due to global warming.

So Earth's mass and gravity is therefore decreasing over time. If you assume global warming is directly proportional to population, we'll need ~2.5 trillion people before we can start increasing gravity.