The IPCC is the umbrella organization that articulates not only the consensus among climate scientists, but also the breadth of opinion among them. It assembles and synthesizes the work of some 2,000 scientists in scores of disciplines from about 150 countries. Wikipedia.  Press Contact. 
The UNFCCC is a treaty that was negotiated at the 1992 Earth Summit in Rio de Janiero, was ratified by some 189 countries, and went into effect in March 1994. It initially had no mandatory controls or deadlines. The 1997 Kyoto Protocol, which did set mandatory limits, was an add-on to the UNFCCC. Wikipedia.  Press Contact. 
The WMO is the U.N.-based international organization for cooperation among national weather agencies on all sorts of weather-related projects, including the instrument observations at a vast network of weather stations that provide basic data on climate. Wikipedia.  Press Contact. 
UNEP is the administrative umbrella for most major UN environmental efforts, including those on climate change. Their site is a good place to look for current and upcoming events, reports, and documents.