Pelosi was speaking on the sidelines of COP25, a two week conference in Madrid aimed at bolstering the 2015 Paris Agreement to curb global warming against a backdrop of unusually severe weather disasters this year, from fires in the Arctic, Amazon and Australia to intense tropical hurricanes.
Announcing that the commitment to the climate crisis remains "ironclad", the speaker dismissed questions on U.S. President Donald Trump.
The Trump administration filed paperwork to withdraw the U.S from the Paris Agreement in November, the first formal step in a one-year process to exit the global pact to fight climate change.
Once it exits, the U.S. - the top historic greenhouse gas emitter and leading oil and gas producer - will become the only country outside the accord.
Delegates are hoping to use the summit to inject fresh momentum into the process, which faces a moment of truth next year when governments are due to unveil more ambitious proposals to cut the production of planet-warming gases.