Since hosting the COP22 meeting in Marrakech in 2016, Morocco has emerged as one of the few countries making "commendable" steps in adopting environmentally friendly policies.
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.
UN Secretary-General Antonio Guterres has warned that pledges to reduce emissions of gases responsible for rising temperatures so far are insufficient to overcome the "point of no return" in climate change.
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.More news: Trump to tariff steel and aluminum from Argentina and Brazil
Mr Guterres noted some 70 countries - many of them among the most vulnerable to climate change - have pledged to stop emitting more greenhouse gases by 2050.
Security is tight at the IFEMA centre, with more than 5,000 Spanish police on duty outside the buildings, as well as 450 private agents backing up the United Nations own blue-clad operatives. "Today, we need to reduce emissions by 7.6% each year".
Trump says complying with the accord would hurt the US economy, restricting domestic producers while other big polluters such as China increase emissions. "The only solution is rapid, ambitious, transformative action by all - governments, regions, cities, businesses and civil society, all working toward a common goal".
50 Heads of State and government will attend the event and also representatives of the main worldwide organisations.
In contrast to the European Union, which sent its newly sworn-in executive leadership team to the summit, the rest of the world's largest carbon emitters - the United States, China and India - have sent lower-level ministers and officials.More news: New Facebook tool lets you move photos to Google
Many Governments and environmental groups are aghast at Mr Trump's move last month to begin formal proceedings to yank the United States, one of the world's biggest polluters, from the 2015 Paris Agreement to avert catastrophic temperature rises.
"We're still in it", Pelosi said.
Spain's acting Prime Minister Pedro Sanchez, whose government stepped in to host the summit after unrest erupted in Chile, the original venue, urged delegates to take ambitious action to preserve the "fragile balance" of life on Earth. And maybe four: "science, again".
"By coming here we want to say to everyone, we're still in", Pelosi said of the U.S.'s commitment to tackling global warming.
The major task for the delegates is to discuss Article 6 of the Paris Agreement, the measures to promote voluntary worldwide cooperation by paying a price on carbon, emission trading schemes and other market mechanisms.More news: Wintry weather wreaks holiday weekend havoc across much of the U.S.