Well, there would be food and water shortage.Then, the more Oxygen intake, the more Carbon Dioxide would be released. Carbon Dioxide is part of the plants food which would be good for the plants but what if they'll be cut down and since your question is what challenges to world would face is right? So, the more people that will live in the cities, the more plants and trees would be gone. Why would they be gone? They would be gone because it needed to be removed to occupy the people that continues to be more and more in number. What would happen, then, if the plants and trees are gone? Surely, to make it all short, we will all be extinct. That is all.