Tip number 153: Unplug your charger

It takes a forest with an area equivalent to 500 football pitches to
absorb all the carbon dioxide produced by mobile phone chargers that
are left plugged into electrical outlets. In the UK alone, 95% of the
energy used by mobile phone chargers is wasted - only 5% is actually
used to charge the phone and the rest goes to waste when the charger
is left plugged in.

Taken from 1001 little ways to save the planet.