Originally Posted by ocellaris
It is burning more fuel to heat up the catalytic converter faster to reach the point of cleaner emissions. Also the engine operates more efficiently and burns fuel better when it reaches operating temp. With a cold cat and a cold engine block, there is a lot of pollution coming out the tailpipe.
I don't think it burns more fuel at startup. I think it runs leaner (less fuel) which heats up the catalytic converters quicker.