Under inflated tires create more resistance when your car is moving. That means your engine has to work harder, so more fuel is used and more CO2 emissions are produced. Simply check and adjust your tire pressures regularly and also before long journeys. This will also help to increase the life of your tires.
Under inflated tires increase CO2 but over inflated tires can be unsafe, so check your car manual for the correct tire pressure. Remember, a car with a heavier load may need different air pressure in the tires.