Overpopulation. I think we will run out of resources before global warming or nuclear disaster will hit us.

Overpopulation is and will continue to be the driving force behind all our problems. No species can continue to grow without check and if we can't do it ourselves nature will (and is) do it for us.

We need to get beyond the 2 basic drives, survival and procreation, and see what continued growth is bringing on this finite planet. The only solution, at this time, is to hit bottom, as most addicts must do before they become clean and sober. We cannot tackle "Gloom and Doom" with "Smile and Denial"; we must use "Know and Grow".

Lack of Understanding. Too many people know and too few understand.

We cannot change IT until we understand IT. Condemnation will not liberate, merely oppress and fuel IT. Understand that IT is what IT is, you cannot force change upon IT, but you can change the world by understanding IT. The change due understanding the IT will be as simple as peace of mind at home and as complex as peace on earth. Horrible ITs will always exist so long as no one cares to understand the ITs behind them.

We look upon IT with disdain and desire IT gone, but we cannot force IT gone. IT is a natural occurrence, brought about due to a lack of understanding, on all sides. Genocide is an IT that a majority disdain, but few understand. Adolph Hitler, a prominent poster child of genocide, had the same consideration towards the holocaust victims as many do to ants, mosquitoes, and spiders. There are some who consider ants, mosquitoes, and spiders the way many others do each other, as equal. No one would claim that the latterly mentioned person is of any harm or threat to anyone, but that person would view an exterminator such as the decedents of holocaust survivors do Hitler.

Neither understands that the one they hold disdain for have their own bias that allow for such different views. Knowing your own IT is the best method for dealing with the ITs of others. Through understanding IT as IT is, without force others too will be willing to understand ITs of their own, and soon the ITs like genocide will disappear through the understanding of all the ITs behind genocide. Hitler's fault was not in his disdain, but in not understanding the IT of others that caused his disdain. He tried forcing IT away the IT he held in disdain was a cultural difference of opinion.

That lack of understanding lead to violent hasty actions, when a simple "IT is what IT is'' would have saved lives. To simplify, I am saying live and let live no matter what you do you can't satisfy everyone and everyone will not satisfy you. You do not need to like or integrate others' ways into your way but understand IT through their terms. So long as there are no unwilling physical or psychological harm in peoples' acts, directed to others, there is no loss. Everyone has different views, opinions and act different from each other. Understanding is all we need to get along, work together, and solve our problems as a species.

In Case it was not obvious or added more confusion, IT is my way of generalizing the many problems humanity faces.

our greatest problem is way too many human beings taking what they want without a second thought. the easy solution would be to all do what China did for a couple of generations till there are only 2 billion people but we all know that will never happen. the hard brutal way it is it seems. mother nature will address it if we don't.

I think HATE is our biggest problem. What trump and the Pubs are doing is using hate of black, brown, Muslim, LGBTQ community and liberals to mold their electorate. They will develop social and economic policies that will affect the respect of the States throughout the world. The lack of respect will lead to miscommunication among our allies which will lead to conflict and war. The solution is to vote Pubs out in November so we can impeach trump and pence.

Capitalism!!! It has always been a plague on humanity. The poor do all the work in sub-par conditions while the privlaged reap all the benifits with little effort.

Humans. Solution, genocide.

Overpopulation, I did the responsible thing and got vasectomy after 2 kids, ie replacement scheme.

I cannot think of a better answer or way to state it than she does in this video. The title doesn't really reflect the whole message. This is a very western-centric answer, but I think western civilization is the biggest threat to the human race. It's about 23 minutes long, but very well put together.

Emprison Centro bankers and bring debt back to zero. One way or another, it must happen .

I tried to think of an answer but it felt like my head was going to explode.

Hard to pinpoint correctly the greatest. Individual and location will vary. War will always comes to mind. Also those that try to sell us "overpopulation", there is plenty of land on earth for everyone to live on and food to feed everyone. We have the technology available to produce more. Money also comes to mind. War and Money. Money for War and War for Money. Always found funny that there is not enough money made on earth to pay all the debts on earth yet they keep lending more. My final answer.

Voluntary stupidity. Need I say more?

We keep killing each other. So one country had an asshole president so another country went to that country to make peace, but at the same time killed a whole of lot of innocent people, so those innocent people families get angry and want to kill the other people and so on.
Also over land, fighting and killing over land of two people who happened to be different religions, They want the land so they kill some people then the family and brothers of those dead people get angry and go and kill the first people and so it goes on and on and on and on.... So much hate