WASHINGTON (AP) - President Donald Trump is aiming to make the annual United Nations-sponsored climate talks underway in Madrid the last ones for full participation by the United States, which is the world's No. 1 economy and the second-biggest carbon emitter.

Mr Trump dismisses climate change and he thumbed his nose at previous climate talks by twice sending White House delegations to promote climate-degrading coal.

He is due to complete the US withdrawal from the landmark Paris global climate accord on Nov 4, 2020, the day after next year's US presidential election.

If Mr Trump loses that election, the next president could put the brakes on the withdrawal.

House Speaker Nancy Pelosi, a California Democrat, and other political and business leaders, scientists and activists are travelling to Spain this week and next to drive home a counter message: US cities, states and businesses representing a sizeable chunk of the US population and economy are committed to a global effort to slash emissions.

"We're still in it," Ms Pelosi told reporters at the talks, where she appeared with 14 other congressional Democrats on Monday (Dec 2) to call climate change a growing threat to public health, economy and national security in the US.

Her comments were echoed by Mr Mandela Barnes, Wisconsin's lieutenant-governor.

"Regardless of whether or not we have the support of the nation's highest office... this work is going to get done," Mr Barnes said.

This year's conference is expected to focus on fine-tuning the rules for reducing fossil fuel emissions by the roughly 200 signatories of the Paris Agreement.

It comes ahead of a big push at next year's climate summit for more ambitious emissions-cutting targets.

Experts say the US' repeated about-faces on the threat of climate change likely have done lasting damage.

Even before Mr Trump repudiated the deal backed by President Barack Obama, Mr George W. Bush's administration renounced the landmark Kyoto emissions protocol, negotiated in the late 1990s during Mr Bill Clinton's presidency, said Mr Nigel Purvis, a State Department climate negotiator under Mr Clinton and Mr Bush.

"The international community has concluded the United States is an unreliable partner," Mr Purvis said.

Although the US served formal notice last month that it intends to become the first country to withdraw from the Paris accord, it technically remains a participant until next Nov 4.

Ms Marcia Bernicat, a senior State Department official, is leading the official US delegation.

"The administration is taking part to ensure a level playing field that protects US interests," the State Department said in a statement.

Advocates of the Paris accord say the US withdrawal will leave American businesses to compete internationally under carbon-cutting rules set by other countries.

Behind the scenes, US diplomats have played a helpful role despite the planned US withdrawal, pushing for transparency and solid rules as countries commit to specific targets for cutting emissions, delegates from other nations say privately.

Publicly, Mr Trump has catered to his base at the yearly talks. That includes dispatching a team to the 2017 and 2018 climate meetings to stage side events promoting coal-fired power production, one of the main sources of climate-wrecking emissions.

His administration stood by fossil fuels "unapologetically", White House energy envoy Wells Griffith said at the US pro-fossil fuel event at last year's talks in Poland.

That drew chants from the audience of "Shame on you!"

Mr Griffith, who helped broker a coal deal in Ukraine, apparently refused a request by House impeachment investigators to discuss administration actions there.

A woman who answered the phone at Mr Griffith's office on Tuesday said no one there would say whether he planned to appear at this year's climate negotiations.

Secretary of State Mike Pompeo last month repeated Mr Trump's argument that the Paris accord was an economic burden for the United States.

Mr Pompeo said technological innovation and the free market have made for continued US declines in climate-changing emissions.

It is true that US carbon emissions are still falling under Mr Trump, according to a study by the Global Carbon Project, a group of international scientists who track emissions.

The country saw emissions drop 1.7 per cent from 2018 to 2019, the same decline as in the European Union, even as China led in a 0.6 per cent rise in emissions globally over the last year, the study said.

US experts say the drop in US fossil fuel emissions is due in part to the decline of coal-fired power plants, losers in marketplace competition against cheaper natural gas and renewable sources despite Mr Trump's 2016 campaign pledges to save coal.

The 2018 midterm elections, which gave Democrats control of the House, showed that embracing top-down government action to cut fossil fuel emissions can be part of a winning platform, at least in some parts of the country.

In June 2017, 46 per cent of Americans opposed US withdrawal from the international agreement, while 29 per cent supported it, according to a poll by The Associated Press-NORC Centre for Public Affairs Research.

This August, another AP-NORC poll found nearly two-thirds of Americans said the federal government should bear a lot of responsibility for combating climate change.

"We hope... this is only a temporary farewell" for the US, German Foreign Minister Heiko Maas said last month at a diplomatic conference.

Regardless, he said, other governments can't count on Americans sorting out a lasting climate policy anytime soon.

Ultimately, said Ms Carla Frisch, a former energy policy expert at the Department of Energy under three US administrations, US climate action demands US climate regulation, making cutting emissions the law and policy of the land.

"We have to be all in," she said. "We also need the federal government, to get where we need to go."