Carbon offsetting is effectively a person or a company doing something "green" every time they do something which damages the environment. The driver is claiming that every time they drive their gas-guzzling car (bad for the environment), a seedling is planted in Uganda (good for the environment). Each time they use a gallon of petrol (gas), a penguin is somehow saved. But this offsetting does not actually decrease greenhouse gas emissions. At a stretch, it might keep the emissions the same.
The driver is trying to sound as if they are saving the planet by being so environmentally responsible (even though they're not). The penguin sarcastically suggests that this person must be an "angel" because they are so "green"! If this angel is on earth, then he/she can't also be in heaven, so heaven is missing (ie without) one angel.
It's also the title of a successful pop song from 1976 by Tavares.
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