Reuters: The top UN official on climate change said on Thursday that he sees a U.S. plan to cap rising emissions by 2025 as only a "first offer", adding that all three presidential candidates had promised a tougher stand. Yvo de Boer, head of the UN Climate Change Secretariat, also said that as the fight against climate change gets more urgent, the world will have to embrace nuclear power and provide more support to developing nations. "I see this very much as a first …