Asked about comments by Spain's interim prime minister, Pedro Sánchez, that "only a handful of fanatics deny the evidence" of man-made global warming, Pelosi responded that she had "three words that describe how we address this: science, science and science.
"This must be the start of a decisive year for climate ambition".
"The global contributions to climate change fall far short", said Berthold Kuhn, an expert on sustainable development with the Free University of Berlin and senior advisor to the Dialogue of Civilizations Research Institute. Much of the focus this year will be on China, which burns half of the coal used globally each year, making it the world's biggest emitter of greenhouse gases. The 197 countries which have ratified the United Nations convention on climate change are called the Parties to the Convention.
It comes amid of a wave of global youth-led fury, but also the USA withdrawal, and dire scientific reports on the state of the planet. This is putting the Earth on course to experience a temperature rise of 3.2 degrees Celsius by the end of this century, rather than the 1.5 degree goal set in the Paris Agreement.
Leading the 15-strong Congressional delegation, Pelosi came to Madrid even as her colleagues in the House consider articles of impeachment against Trump.
Madrid then volunteered to host the two-week summit.
"There is no sign of GHG emissions peaking in the next few years; every year of postponed peaking means that deeper and faster cuts will be required", the emission gap report said.
House Speaker Nancy Pelosi (D-CA) declared Monday that the United States will join other nations in their fight against climate change at a United Nations-led press conference in Madrid, Spain.
The congressional group is separate from the official USA delegation at the "COP25" conference, which is led by Ambassador Marcia Bernicat, a senior State Department official.
Around the world, extreme weather ranging from wildfires to floods is being linked to man-made global warming, putting pressure on the summit to strengthen the implementation of the 2015 Paris Agreement on limiting the rise in temperature.
The conference is important because it's where countries will discuss how they can deal with climate change and get their country to net zero emissions.
United Nations secretary general Antonio Guterres has opened the COP 25 climate summit in Madrid warning that Governments risked sleepwalking "past a point of no return" if they remained idle.
"Political will to stop building coal power plants from 2020 onwards".
That involves creating a functioning global emissions-trading system and compensating poor countries for losses they suffer from rising sea levels and other consequences of climate change.