The Latest: Brazil's president addresses domestic turmoil

Brazil's president is using the signing ceremony to briefly address the political crisis at home, calling the effort to oust her a "grave moment" for the country and thanking leaders who have expressed solidarity with her.

On climate, Rousseff says her country will restore and reforest 12 million hectares (30 million acres) of forests and 15 million hectares of degraded pastures and increase the nation's reliance on renewable sources to 45 percent of the energy matrix in order to reduce emissions, without offering any timetable.

Hollande, the U.N. chief and French Environment Minister Segolene Royal, who is in charge of global climate negotiations, invited leaders from all 193 U.N. member states to Friday's event.

U.N. Secretary-General Ban Ki-moon says at the start of a high-level U.N. ceremony where a record 171 countries are expected to sign the landmark climate change agreement that "history is in the making."

The U.N. chief told global leaders and ministers on Friday that the world is in "a race against time," citing record global temperatures, record ice loss and record carbon levels in the atmosphere.

Ban, who recalled that climate change has been his top priority since he became secretary-general over nine years ago, urged all countries to ratify the agreement so it can come into force as early as possible.