Since we regularly get questions about this type of thing, I wanted to repost this excellent post from Erik Klemetti's blog 'Eruptions.'

"I recently read a post about how humans couldn't be to blame for the warming of the oceans.    
The "logic" of their post was this:

• We see water getting warmed by magma at volcanoes worldwide, like Taal andRuapehu.
• The bottom of the ocean is covered in volcanoes, whether it be seamounts or the mid-ocean ridge system.
• Volcanoes at the bottom of the ocean must heat up the ocean water (like in crater lakes), so humans aren't to blame.

There are a number of ways to attack this poor reasoning, but I'll try to go with the ones I think are most convincing.

• This argument suggests that volcanic activity (not just volcanoes) on the ocean floor must be increasing over the last few hundred years. We see no evidence of this - and the author of the post only uses evidence that we have discovered more volcanoes (up to 3.4 million) on the seafloor.
• The author claims "that 3,477,403 number, coming from two well-respected oceanographers, does reinforce my point rather nicely, namely, that underwater volcanoes are heating the seas." Sure, there might be a lot of volcanoes but he makes no mention of how many might be active enough to heat the ocean water. We can see at surface volcanoes that not all of them are erupting or puffing away simultaneously - why would undersea volcanoes be different? Additionally, of those 3.4 million 'volcanoes' he cites from a 2007 study, how many were considered active?
• Finally, you would expect in this situation that ocean water would warm from the bottom (near the volcanoes) upwards, but it is actually the surface waters that are warming, not the deep ocean. This suggests that heating is coming from exchange with the atmosphere, not from some deep source on the ocean floor.

It is very easy to try to throw around some partially baked ideas about volcanism to try to explain the changing in the ocean temperature worldwide, but they would require extraordinary circumstances where ocean volcanic activity was increasing exactly when human carbon dioxide production was also increasing. Sorry, the subsurface volcanoes are no source for your ocean heating (and if I had the time, I'd calculate how much energy would take to heat all the oceans by 0.5C - it is bound to be more than you can get out of a few hundred thousand Ruapehus)."

For more information on Submarine Volcanoes, check out our lesson here!