The two-week "Conference of the Parties", or COP, as it is known, comes as extreme weather events around are being linked to manmade global warming, putting pressure on the summit to strengthen the implementation of the 2015 Paris Agreement on limiting the rise in temperature.
"For future generations.
This is a mission, this is a passion, this is a scientifically-based approach, to all.
We are here to say to you, on behalf of the House of Representatives to the Congress of the United States, we are still in it.
We are still in it," said Pelosi.
Pelosi's appearance the COP25 comes as President Donald Trump has started withdrawing the United States from the Paris Agreement.
The summit runs until Dec.13.