The irony is that Romney was mocking Obama exactly for the unscientific fantasy he had promised in 2008.
Check out this treatment by climate scientist Tom Wigley. Wigley, for the sake of argument, lays out an absurd scenario: what if manmade greenhouse gas emissions were slashed severely, immediately, and then brought down to zero by 2050. Keep in mind, nobody — certainly not Barack Obama — is seriously proposing this. This would require our eliminating all use of natural gas, oil, and coal.
Even under this impossibly severe regime, which would probably require totalitarian enforcement of global poverty, CO2 concentrations would continue to increase until 2040. As long as CO2 emissions are growing, the rise of the ocean isn’t going to be slowing.