Deforestation is 3rd largest source of emissions

Vegetarian/vegan food is much less environmentally damaging than meat.

From Elena Sgarbossa

A vegan burger uses less than 10% land in its production than production of a beef burger does.

Which means that less forest is cleared.  And if tropical deforestation was a country, its emissions would be behind only China and the USA.  

Moreover, to the emissions produced by land clearing, you have to add in the methane directly produced by beef and mutton.  The rise in atmospheric methane is responsible for 1/3rd of the rise in global temperatures since 1850-1900.  The biggest step you can take to personally help cut emissions and global temperatures is to give up meat and milk.