Teenage climate activist Greta Thunberg has blasted global leaders for their inaction on climate change, warning them that younger generations “will be watching” them.

Speaking at the UN Climate Action Summit in New York, Ms Thunberg gave an emotional speech calling out governments for their sluggish adoption of climate policies and focus on economic, rather than environmental concerns.

“People are suffering, people are dying, entire ecosystems are collapsing,” she said.

“We are in the beginning of a mass extinction and all you can talk about is money and fairytales of eternal economic growth. How dare you?”

Ms Thunberg added she did not believe governments truly understand the scale of the problem, because if they did their inaction so far “would be evil”.

Members of the audience appeared captivated by the 16-year-old activist’s earnest comments, only occasionally punctuating their own reverent silence with rounds of applause.

Ms Thunberg has galvanised a new wave of climate change activism through her weekly Fridays for Future school strikes, which she began with her weekly, solitary protests outside of the Swedish parliament.

UN Secretary-General Antonio Guterres had warned governments ahead of the event that they would have to offer action plans to qualify to speak at the summit, which is aimed at boosting the 2015 Paris Agreement to combat global warming.

In his opening remarks, he tried to capture the urgency of climate change and called out the fossil fuel industry.

“Nature is angry. And we fool ourselves if we think we can fool nature because nature always strikes back, and around the world nature is striking back with fury,” Mr Guterres said.

“There is a cost to everything. But the biggest cost is doing nothing. The biggest cost is subsidising a dying fossil fuel industry, building more and more coal plants, and denying what is plain as day: that we are in a deep climate hole, and to get out we must first stop digging.”

US President Donald Trump, a climate change denier who has undone every major US regulation aimed at combating climate change, made a brief appearance in the audience of the summit along with Vice President Mike Pence and Secretary of State Mike Pompeo.

He did not give remarks but he listened to remarks by German Chancellor Angela Merkel and Indian Prime Minister Narendra Modi.

Former New York mayor Michael Bloomberg, who serves as the UN special envoy on climate action, called out Mr Trump’s stealth appearance before he spoke on Monday:

“Hopefully our deliberations will be helpful to you as you formulate climate policy,” he said to audience laughter.

Among the day’s other initial announcements was one from the Marshall Islands, whose president Hilda Heine said she would seek parliamentary approval to declare a climate crisis on the low-lying atoll, already grappling with sea level rise.

Heine said her country and New Zealand, Sweden, Norway, Denmark and others who form the “High Ambition” bloc at UN climate negotiations, will commit to achieving net-zero greenhouse gas emissions by 2050.

Over 60 world leaders and CEOs of energy and financial companies are expected to address the conference and announce climate finance measures and transitioning from coal power.

" … representatives from other countries had noticed Australia's absence …" "It is very obvious who is absent from the room. Everyone is well aware Australia has not made good on its promises in Paris to scale up its commitment to climate action."https://t.co/2qidryeoPv — Benjamin Law 羅旭能 (@mrbenjaminlaw) September 23, 2019

While some countries have made progress, some of the biggest emitting countries remain far behind, even as wildfires, heat waves and record temperatures have provided glimpses of the devastation that could lie in store in a warmer world.

In a measure of the gap between government action and the ever-louder alarms sounded by climate scientists, the United Nations Development Program said 14 nations representing a quarter of global emissions have signalled that they do not intend to revise current climate plans by 2020.

– with AAP