The argument was never discredited, only the application of the argument. We could fire all of our nukes into the sun, with no noticeable, not to mention ill, effects. It's the same argument, still being applied correctly.
Heavy or organic pollutants which radically alter metabolism, and heat, are two completely different things. One can kill a lot of stuff in small quantities, the other is pumped in by the sun, all the time, and is considered part of the status quo.
You can kill all of the bacteria in a pot of water by adding a couple drops of bleach, but you have to add a
huge amount of energy to achieve the same effect.
(I worry that you barely read my post, because I did prefix that statement with "when it comes to energy" specifically to keep you from wasting a line on mentioning something like interplanetary travel =/ )
When I said "human scale" I meant, "within human capacity to change." Right now, we could (and have) overheat medium-sized lakes with our coal, nuclear, "hot" power plants. We overcool other bodies of water with the reservoirs of hydroelectric power plants. Chemically, we can take out small to medium-sized ecosystems, physically, we can chop down, drain, or otherwise displace bigger ones. But heat? Even if we dedicated the whole of human activity to (directly*) heating the oceans, we'd be completely incapable of making a dent.
This post makes the basic point rather vividly:
http://news.slashdot.org/comments.pl?sid=1376893&cid=29497453
5.6*10^24J is
not the kind of energy humans, as a species, can hope to move from the land to the sea. And that's for 1 degree.
As it stands, our current means of cooling uses the atmosphere, in a less-efficient process. As the same post mentions, the atmosphere is less massive and has a much lower specific heat. If you're worried about screwing up the environment, using ocean-based cooling is
exactly the kind of thing you'd be into, as it is more efficient, and is placed into a system that is very capable of dealing with it.
*Directly, as in the thermal result of combustion or electrical resistance (or giant orbital doom-mirrors), as, indirectly, chemically, it appears to be very much within our capacity (read: atmospheric hydrocarbons.)