Usually we use 'the' with world or earth because there is only one of these things. The phrase I would use would 'The world's population' but you could also use 'Global population' (without 'the') Let me know if you need more help with articles. :)

I would also capitalise "Earth" as it is the name of the planet. In terms of "global population" it would take "the" if you were referring to something specific, e.g. "The global population estimate from the last UN report is seven billion; or talking about the population of Earth, e.g "The global population will rise exponentially over the next 10 years". There would be no absolute need for an article if you were referring to a plural noun, e.g. "Global population estimates from the last century were inaccurate"; but "The global population estimates from the last century were inaccurate" is also correct.