Yeah, they cut most of it out of the first movie but it's in the deleted scenes.
This is from the Avatar Wiki:
"Due to pollution, Earth's atmosphere is covered in a haze of greenhouse gasses and had many other problems such as overpopulation, nuclear warfare, environmental terrorism, significant deforestation, world hunger,
ozone depletion, resources depletion, water shortages, and overhunting of what is left of Earth's very few still living animal species are the main things that are slowly consuming what is left of the once beautiful planet. Excessive mineral mining in Africa and the deep sea also expedited the planet's natural resources.Much of the western coast of America was damaged in a tsunami event in the
2140s."
I think in the deleted scenes from the first movie, it's revealed that the only natural food source left is algae.