How come the Earth is negatively charged?
Where did you hear or read that?
Do you mean the Earth including the atmosphere or just the earth's surface?
Electrons are much more easily removed from molecules in the air and clouds. Rain can carry the electrons to the earth where the negative charge is distributed. At some point, a lighting discharge occurs and some of the electrons travel up the arc back to the positive ions.
from http://nofc.cfs.nrcan.gc.ca/science/research/lightning_e.html [Broken]
As far as I know, the Earth (including atmosphere is electrically neutral).
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