I am pretty sure that is not the way it works. Everything I have read reffers to the upper layer of the oceans waters top 200 +/- feet. The life sustained below that level is not nearly as dependant on plankton, alge and the other microrganisms. IIRC there are currents at deeper levels where the temp change is dramatic (I think subs use it for cloaking their signatures). The minor temp fluctuations on the surface of the ocean can have dramatic affects on the alge blooms which are basis for huge feeding cycles for the food chain. There was a chapter in the Discovery Earth series that described/showed it quite well.
I have been looking into starting up a marine aquarium and everything I have read indicates that temperature fluctuations are one of the primary concerns.
I am not so certain than you can just look at the output of a site as your sole determination. What about the forests that have been clear cut. What about all the concrete laid out for roads? What about the pollutants put in the waters and the grounds? What about all the animals that have been eliminated due to loss of habitat or hunting? There are so many other factors that have an effect on our environment.
When you look at nearly any animal (human included) there seem to have evolved to function with in a very narrow spectrum. Humans can only see a narrow band of light, survive with in a narrow temp range (with out needing additional protection)….etc. The same can be said of most animals on the planet. I think an argument could be made that the planet could be viewed the same way. The sea-saw we call the eco-system may have evolved to operate under a somewhat narrow scope. If that equilibrium is disturbed beyond its ability to compensate, it could knock the entire cycle off and send it into a tail spin. The planet will survive. And in all likely hood life will survive as well albeit in a quite limited state and only the most hardy would make it. Eventually in a few thousand years or so the planet might repair its self or other life forms may evolve to the new conditions. Either way, we may or may not make it.
Like I said before, are we so sure that the GW folks are wrong? Are we willing to stake our survival as a species on it?