"People are underestimating the force of angry kids," the Swede told a crowd of reporters and supporters as she disembarked in Portugal on Tuesday from the catamaran owned by an Australian couple.

"They're angry and frustrated."

The prospect of another fiery intervention by the 16-year-old, whose ability to stare down politicians has inspired a global protest movement, electrified younger delegates at the international climate talks underway in neighbouring Spain.

Since staging a solitary protest outside the Swedish parliament more than a year ago, Thunberg has channelled the anger felt by millions of teenagers saddled with the prospect of an escalating climate crisis their parents failed to avert.

In September, she carried her message to a one-day climate summit at the UN in New York, furiously telling leaders "you have stolen my dream", before sailing back to Europe for the latest round of talks.

Thunberg was due to spend the rest of Tuesday meeting activists in Portugal before departing for Madrid, where the negotiations are being held in a series of hangar-like halls.

Carolina Schmidt, environment minister of Chile, which is chairing the negotiations, said she hoped Thunberg's presence would galvanise more ambitious commitments by governments at talks aimed at bolstering the 2015 Paris Agreement to avert catastrophic temperature increases.

"We need that tremendous force in order to increase climate action," Schmidt told Reuters television.

"We need Greta in here with all that force."

Increasingly erratic weather patterns, from bushfires in Australia and California to floods in Europe, have added to the sense of urgency around the two-week summit in Spain.

Underscoring the pace of change, the Geneva-based World Meteorological Organisation launched a report at the talks that found the past decade was almost certain to have been the hottest on record.

In a stark reminder that burning fossil fuels has fundamentally changed the composition of the earth's atmosphere, the report said the concentration of carbon dioxide hit a record 407.8 parts per million in 2018 and rose again in 2019, hovering at the highest levels seen in millions of years.

Opening the climate summit on Monday, UN Secretary-General Antonio Guterres had warned that 400 parts per million had once been considered an "unthinkable" tipping point.

Despite the grim statistics, young delegates said they were inspired by Thunberg's determination to minimise her own carbon footprint by twice braving the Atlantic instead of flying, which they saw as a symbol of resourcefulness and defiance.

"She is making a statement that you don't always have to take the easy way," said Lander Wanters, 20, a Belgian climate activist. "We have to act now to do something for the climate."

Delegates at the talks aim to finalise groundwork to support the Paris pact to curb the rise in global temperatures, which enters a crucial implementation phase in 2020. Last year, greenhouse gas emissions hit a record high.

"At the negotiators level, they are working to try and close any loopholes to make sure that you have a strong agreement that works," said Stephen Cornelius, chief adviser on climate change at the World Wildlife Fund UK.

"We are hoping it will all be completed here in Madrid."

Existing commitments fall far short of the kind of radical action to transform energy, transport, heating and agricultural systems that scientists say is needed to steer the world off its current course towards disastrous levels of warming.

Australian Associated Press