Reuters: After two years of work, and 12 years after their last attempt, 190 nations gather in Copenhagen from Monday to try to avert dramatic climate change — what one minister called "the most difficult talks ever embarked upon by humanity." Already the sheer size of the measures needed, and splits between rich and poor about who should pay, mean that a historic U.N. pact to fight global warming and ease dependence on fossil fuels may be put off in favor of a less binding …