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If definitive proof emerged that GMO's were safe for human consumption, I don't think many environmentalists would be okay with that, because as far as I can tell, the safety aspect is a small part of opposition to GMO's. Many environmentalists (myself included) have strong preferences for 'the way things were' without human impact. Just think of the root of the word 'conservation'. I personally think much of the Anti-GMO furor is a bit too 1. Luddite and 2. tinged with anti-capitalist sentiment for my tastes, but scientific consensus declaring GMO's safe isn't going to have any impact on those two forces and preferences.
So comparing environmentalists reaction to consensus on Global Warming and consensus on GMO safety seems more than a little (genetically-modified) apples and oranges.