Assuming that the simplifications you've made are OK -- which is a question for which you'd need an actual climate scientist or oceanographer -- there's nothing wrong with the calculation. I just don't see why it's a calculation whose answer we should be very interested in. Are you doing it just out of curiosity or is it intended to tell us something important about the impact of AGW?
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.