eat less meat

According to a report by the Food and Agriculture Organization of the United Nations, entitled "Livestock's Long Shadow":

The livestock sector generates more greenhouse gas emissions as measured in CO2 equivalent -- 18 percent -- than transport.
...
Livestock now use 30 percent of the earth's entire land surface, mostly permanent pasture but also including 33 percent of the global arable land, [which is] used to produce feed for livestock
...
The livestock business is among the most damaging sectors to the earth's increasingly scarce water resources, contributing among other things to water pollution, euthropication and the degeneration of coral reefs.
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Meat and dairy animals now account for about 20 percent of all terrestrial animal biomass.

I suppose we can add to the list of "simple things you can do to save the Earth" this bon mot:

Eat Less Meat

Not to mention the fact that it also has the potential to extend your life.