Look at the statistics.
During the 1990s, the number of vegetarians in the UK almost doubled. Many people became vegetarian after various meat scares like BSE and Foot and Mouth, which led large numbers of people to rethink their diets, with 27 % of the population saying that they would consider giving up meat, and 12 % saying that they were vegetarian or meat-reducing.
Now that those scares, exaggerated and distorted by media and governments alike, have disappeared from the horizon, the number of UK vegetarians has decreased from 3 million in 2001 to 1.9 million in 2011.
The global warming theory is not a valid scientific theory. The predictions from computer models based on it have been refuted by the evidence of global temperatures going down when the predictions had said they would go up. And the behaviour of its supporters has contributed to discredit the theory repeatedly, including the latest global warmists scandal: "even the United Nations scientists who still support it formally asked for immunity from criminal prosecution before Rio de Janeiro's international climate talks in June 2012".
When this hypothesis will be well and truly considered false, as all signs indicate it will, the number of vegetarians will go down again. Not only that: the vegetarian movement will have lost credibility for supporting a theory which, under even a superficial scrutiny, was clearly unfounded and fallacious.
We shouldn't bet on a losing team.
Even more importantly, we should only recommend a vegetarian diet on the strength of arguments that have a solid empirical and logical foundation. In Italy we say "Le bugie hanno le gambe corte" (Lies have short legs). Allegations which are not evidence based don't last in the long term.