Global warming, garbage and recycling problems, unsafe water and food. And here in the US I'd add the current Republican agenda to this list as we will soon be a country with few human rights.

We are the biggest threat. I don't expect the world to remain comfortable.

If you truly want to control the population, you don't have to look very far, as population control is possible and I have a feeling that religion does not want to happen. So how do we control the growth of this earth? Empower women.

It’s official: Japan’s population is dramatically shrinking
[washingtonpost.com]

World Population Out of Balance
[manataka.org]

First Nations women have always been leaders
[winnipegfreepress.com]

When women are in control, our population is controlled. In Japan, by the year 3011, there will not be a full-blooded Japanese (like me) ever. Why is that? Many women want to work than raise kids. If we empower women, and let them control their own bodies, and stop the pro-life B.S. of making women baby factories, then we can truly control the size of this planet. Conservatives hate abortion, so if this were to happen, watch Planned Parenthood go out of business.

The dishonest propaganda campaign by those in power to keep themselves rich and in charge at the cost of anything else. It makes the uneducated masses their own worst enemy.

Climate change. Water and food shortages will cause major panic everywhere. Look at Cape Town for example: when they were told they would run out of water in 78 days gangs started stealing and stock piling water. Resources will become vital as crops begin to fail from extreme temperatures. No way a changing climate can sustain the amount of people currently on this earth.

Over population. We need a good plague.

But seriously, we need world wide population control.
Immediately....although the tipping point toward destruction may have already zoomed past us...

Climate change.

In America our food is shit. Have a nice healthy salad, grown in contaminated crap, sprayed with carcinogenic shit, what doesn't give you cancer will screw up your metabolisism and immune system and make you diabetic. Everyone lies. There is a vicious cycle of make 'em sick then make them pay to get well, which won't happen so then pay to bury them. In the mean time work them to death and keep them as ignorant as possible. In otherwords, other humans.

Overpopulation. the more people the more problems the more problems the more violence. Our current political climate I think is an example that we are beginning to feel the massive unrelenting overpopulation issue. Humans are social animals and when social animals are overpopulated extreme aggressive behavior and killing becomes extremely Common Place.

Climate change.

ourselves

Greed! From public parks and beaches to water rights and the very air we breath, these things that we all need to sustain us should not be owned by Corporations and the uber rich.

Get rid of religion,use birth control to reduce poverty and need for large families,abortion for all women who want it,limit number of children to a couple exclude multiple births.These families having 8 and 12 children need to be penalized someway.

Religion.

I was thinking this myself because I grew up in the smallest London borough in 1948- post war we were decimated but all the survivors were amazing =so glad to be alive so happy to be able to buy 'things' and we all seemed to know each other and people were cheerful and very happy with the Irish and Hispanic people who came to live with us and delighted with the huguenot silk weavers in the East end - I don't remember people being unhappy at all - People were kind and helpful because we were not overcrowded. if children were out late men would stand under the street lamps to make sure we got home safely without being intimidated . we biked, roller skated,and played on the bomb sitesas well as playing in the road as there were very few cars and the ones that came were always very slow - It seems that everyhting has speeded up since then - and thats just my lifetime .

Oligarchy, the end of a Democracy.

the rapid increase in contaminated food & water for a staggeringly increasing population, angry & desperate: chemicals, hormones, gmos, etc.

The corporations that are ruining the world

Too many children is the biggest threat to most nature including us.

We can't until we start right now educating people of the damage we do to our planet. Get a way of quieting the anti-global warming sect. Stop using plastic for all liquids; go back to real glass bottles and require a deposit on them like there was at one time, use metal containers for liquid non-food items, go back to powdered products and use cardboard paper boxes, and a lot more. OK, I'm done with my rant.

A penny doubled then those pennies doubled then those pennies doubled...

I think the last war will be fought over water

(1) Nuclear war.
(2) Islam.

Trump and republicans. Especially the funnymentalist ones. They think their sky daddy is going to save all of them while killing off all of the folks who don't buy their bullshit.