The 50th World Economic Forum has kicked off in the Swiss city of Davos, with an agenda that focused heavily on climate change as world leaders struggle to tackle the crisis.

The four-day annual gathering of some of the world's top political and business leaders in the Swiss Alps is seeking to meet head-on the dangers to both the environment and the economy from global warming.

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US President Donald Trump, who has repeatedly expressed scepticism about climate change, lauded the US economy in his keynote address on Tuesday morning, hours before his impeachment trial opens at the Senate in Washington, DC.

Trump told business and political leaders in the mountain resort that US's economic turnaround had been "nothing short of spectacular".

He also lashed out at environmental campaigners as Swedish teenage activist Greta Thunberg watched on.

"We must reject the perennial prophets of doom and their predictions of the apocalypse," said Trump, hours after Thunberg told the World Economic Forum that governments had done "basically nothing" to reverse climate change.

Trump's opposition to renewable energy, his withdrawal from the 2015 Paris climate accord and the free hand extended to the fossil fuel industry puts him at odds with the entire thrust of the event.

His administration's trade spats with China and the recent deal to ease tensions between the rival giant economies weigh heavy on the minds of leaders in Davos after the IMF slightly downgraded global growth prospects for 2020 on Monday.

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Middle Eastern issues are also on Trump's agenda, as he is set to meet Iraqi President Barham Salih, the president of the Kurdish region of northern Iraq, Nechirvan Barzani, and Pakistani Prime Minister Imran Khan.

Al Jazeera's James Bays, reporting from Davos, said Trump decided to attend the summit on a day "when there is so much going on in Washington".

"There was lots of speculation that he would cancel at the last minute but he didn't," Bays said.

"His advisers [believe] that this is a good look on this day, a chance to have him on the world stage, looking presidential and showing his strong suit because he's going to be among the world's top business leaders and that really is the strong card of the Trump administration - the economy and stock market."

'Climate apocalypse'

Greta Thunberg, who inspired the global climate strike movement, slammed global inaction on climate change at a youth panel in Davos hours before Trump spoke on Tuesday.

"We are all fighting for the environment and climate. If you see it from a bigger perspective, basically nothing has been done. It will require much more than this. This is just the very beginning," she said.

The teenager will also address a session titled "Averting a Climate Apocalypse" later on Tuesday.

Hundreds of activists marched for three days from the Swiss city of Landquart towards Davos, calling on world leaders to do more to tackle climate change.

Sustainability is the buzzword at the forum, which began in 1971, with heel crampons handed out to participants to encourage them to walk on the icy streets rather than use cars, and the signage paint made out of seaweed.

The forum's own Global Risks report published last week warned that "climate change is striking harder and more rapidly than many expected" with global temperatures on track to increase by at least 3 degrees Celsius (5.4 degrees Fahrenheit) by the end of the century.

Business leaders attending the forum will be eager to tout their awareness of climate change but are likely to be concerned also by the state of the global economy whose prospects, according to the IMF, have improved but remain brittle.

The IMF cut its global growth estimate for 2020 to 3.3 percent, saying that a recent truce in the trade war between China and the US had brought some stability but that risks remained.

"We are already seeing some tentative signs of stabilisation but we have not reached a turning point yet," said IMF chief Kristalina Georgieva.