Famine. When the environment is exhausted, the waters are contaminated and no one is able to produce or distribute food, mass starvation will be inevitable.
Note: perhaps for the first time in recent history, the world population has DECREASED. That, along with vastly improving agriculture, could save the planet.
While that sounds encouraging, we should look closely at why that is. How much has to do with —shall we say—mass misfortune, than a function of any coherent plan (I cite the grossly mismanaged ‘One Child’ policy in China). We will need all of the forces and techniques at our disposal, a greater common agreement on our circumstance and some damn good fortune to equal the demands our billions place on the earth to sustain us.
The oncoming E-plague. The plague of autonomous production, electronic gadgetry, the obsoletion of humans, and the resulting contamination of heavy metals from it.
I knew a nurse in a HIV wing of a prison. She said all the patients bandages are destroyed but the contiminated blood from the HIV patients just goes down the drain and into the sewer system just like it was nothing. Who knows what really lurks in our water and when the continimation will reach levels that cannot be washed away. The Hormones expelled in waste from those who take them has already shown up. This is the reason why I filter all my drinking water, especially for lead and other metals, I have instant hot water for tea.