• Video report by ITV News Science Editor Tom Clarke

Sir David Attenborough has issued a stark warning that the collapse of civilisations and extinction of much of the natural world is “on the horizon” without action to tackle climate change.

The naturalist and TV presenter delivered a message from people around the world to United Nations climate talks in Katowice, Poland, calling for leaders and decision-makers to take charge on driving down greenhouse gas emissions.

Sir David told ITV News that action is urgent because "something can still be done".

"It couldn't be more urgent because there is still the possibility to do something about it.

"This isn't a tragedy, which is already written in stone. Something can be done about it, that's why it's urgent."

Sir David Attenborough addresses America's stance on climate change.

He also addressed "the mystery" of America's stance on climate change.

Speaking to ITV News he said: "The American people are being more gravely damaged than anybody at the moment.

"This has been predicted by all the evidence about climate change...and you still say 'I don't believe it.'

"I mean, what is this, do two and two not make four any more?"

Sir David was speaking on behalf of the UN’s “People’s Seat” initiative to give ordinary people a voice at the international talks by gathering their thoughts, ideas and concerns through social media and polling in the past two weeks.

He said the world is facing its “greatest threat in thousands of years: climate change”.

“If we don’t take action, the collapse of our civilisations and the extinction of much of the natural world is on the horizon.”

And he said: “The world’s people have spoken, their message is clear, time is running out, they want you, the decision-makers, to act now.

“They’re supporting you in making tough decisions but they’re also willing to make sacrifices in their daily lives.”

The UN has launched an “ActNow.bot” which helps people discover simple everyday actions they can take to tackle climate change.

Sir David said: “The people have spoken: leaders of the world, you must lead, the continuation of our civilisations and the natural world upon which we depend are in your hands.”

United Nations Secretary-General Antonio Guterres warned negotiators at the meeting that the world was “in deep trouble” with climate change.

“Climate change is running faster than we are and we must catch up sooner rather than later, before it’s too late.

“For many people, regions and even countries, this is already a matter of life and death.”

He also said that “climate action is not just the right thing to do, it makes social and economic sense”, pointing to how action to cut emissions will curb air pollution deaths and generate millions of jobs and trillions of dollars.

The speeches come after four former presidents of the annual UN climate talks warned the “world is at a crossroads” and decisive action in the next two years would be crucial to tackle the threat of climate change.

In a joint statement, France’s Laurent Fabius, Frank Bainimarama, from Fiji, Salaheddine Mezouar, from Morocco, and Peru’s Manuel Pulgar Vidal said: “The challenges are there, as are the solutions.

“We require deep transformations of our economies and societies to build a better world for all. This must be powered by multilateral co-operation.”

They called for ambitious decisions which are sufficiently detailed and comprehensive to enable the effective operation of the Paris Agreement, secured three years ago in the French capital to curb global warming.

A process to enable countries to announce efforts by 2020 to ramp up their domestic ambition on cutting greenhouse gas emissions must be launched, they said, as current efforts are not enough to prevent dangerous temperature rises.

And there needs to be progress on the goal of mobilising 100 billion US dollars (£78 billion) a year for poorer countries to drive clean growth, they urged.

The World Bank has announced it is doubling investments in climate action with 200 billion US dollars (£157 billion) for 2021-2025, including 50 billion US dollars (£39 billion) towards helping countries adapt to the impacts of global warming.

Negotiators at the talks will hear details of a global review on climate change-tackling efforts, and the pressure is on to work towards increasing commitments from countries by 2020.

The talks in Poland also aim to draw up the rulebook for making the Paris deal operational, and poorer countries will be looking for a boost to the finance being made available to help them develop cleanly.