Answer: I would say water supply.
Explanation: Due to the ever changing temperature of the earth, many glaciers which contain freshwater are melting into oceans, and turning that water into undrinkable salt water. The world as it stands now is already low in drinkable water already, with only about .6% of it being drinkable, which in itself is being generous. Due to climate change the supply is only becoming shorter, melting glaciers and the drying up of lakes and rivers; which are our primary sources of water, are drying up. And with an ever so diminishing economy, one of the key basic principles for human and animal survival, will be turned into pure luxury.
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