Population growth is by far the biggest threat to mankind and earth for that matter, assuming no nuclear holocaust will happen in the near future.
It's a big problem, far bigger than most will admit, as technology advances, less and less people are needed for mankind to function normally, more automation and less jobs to go around. 
On the flip side, inhabitable corners of the earth are shrinking daily, we have drought and femine in places people used to live happy ten or twenty years back, poverty is increasing all around the globe.
You see where this is going?
In the following years we will start to see mass migrations due to uninhabitable lands and soon earth will not be able to sustains its population for multiple reasons.
Unfortunately, there is not enough for everyone to go around at the rate the populations is growing, you can check the predicted population growth - its doesn't look good, its actually scary to say the least.
By the time we hit 2050-2060 there's gonna be huge problems, and there's not much options that can solve this.
Either we start controlling population growth or there's gonna be cleansing, either by viruses or world wars.....
No doubt there's organizations around the world already planning how to get rid of mass amounts of people