Why is human population increasing? Better kamma?

Human population is increasing whether we like it or not and might even hit 10bn in the next few decades. But why is this happening according to a Buddhist's perspective? Some that I know that don't believe in kamma use this as a counter example saying that if people are becoming more sinful they ask how come the population is increasing. While I have my own understanding (devas being born back among humans, kamma alone doesn't determine the next birth "cuti sitha" does) I would like your expert opinion this.