Climate change is not a fully understood phenomenon at this point. While the basics are understood, there are so many gaps in our understanding that undertaking a large project to reverse it could prove to be a large waste of time and money. And its entirely possible that advances in technology and production could drastically reduce CO2 emissions on their own. I do however believe that the UN could significantly impact global poverty rates. They could subsidize wages in sweat shops so the companies would stay in the nation and workers would have slightly better pay. They could help clamp down on corruption. They could fly in experts to get factories running more efficiently. They could supply scholarships to build up a base of skilled workers in third world nations. Actions like these are far more effective at improving world welfare than getting China to sign some more non-binding resolutions about their coal factories.
The UN is in a better position to effectively address the issue of climate change than the issue of poverty. Environmental issues are globally interconnected and require a global response, while issues of poverty are localized and require individual responses based on a variety of factors that an international organization may not easily be able to address. Since both are serious issues, the UN should put its effort toward that which it can solve, and focus on climate change and the environment.