If we don't care for the environment now, our great-grandchildren could start using dictionaries with entries like these. The dictionary is fully digitized, but only because trees no longer exist.
A clean bill of health (noun): The world record of being fully healthy without any diseases at all. Once commonplace before 2050, there has only been one such person since 2052.
Air (noun): The mixture of gasses, mainly nitrogen, sulphur dioxide, carbon monoxide, carbon dioxide, that constitute the atmosphere, and are harmful to living things. Oxygen, once abundant, in air is now carried in special tanks (see 'gas tank')
Air corridor (noun): Special, cleared routes for aircraft to fly in, which, unlike the air surrounding them, is clear of smog (see 'smog')
Air plants (noun): A primitive method of growing plants in the air, discontinued since 2050, when the air ended up killing plants grown in this method. (see 'air pollution')
Air pollution index (noun): A numerical scale used for measuring how polluted the air is. In 2050, the maximum point of this scale has been increased numerous times, from 300, to 500, to 750, etc. As of 1st Jan 2120, it currently stands at 2050. (see 'air pollution')
For now, these entries are unthinkable, but they could become reality if we persist with huge petrol guzzling SUVs, needless air conditioning, and if we refuse to recycle. Start a change. Save the world.