Civilisations are going to collapse and much of nature will be wiped out to extinction if humanity doesn't take urgent action on climate change, Sir David Attenborough has warned.

The naturalist gave a terrifying picture of the future of humanity during climate talks being held in Poland. He said that humanity was facing a major threat and that leaders had to drive down harmful emissions if they wanted to preserve the natural world.

The warning came as Sir David took the "People's Seat" at the opening of the conference, as part of a UN initiative that aims to give normal people a voice at the international climate talks.

10 photographs to show to anyone who doesn't believe in climate change Show all 10 1 /10 10 photographs to show to anyone who doesn't believe in climate change 10 photographs to show to anyone who doesn't believe in climate change A group of emperor penguins face a crack in the sea ice, near McMurdo Station, Antarctica Kira Morris 10 photographs to show to anyone who doesn't believe in climate change Floods destroyed eight bridges and ruined crops such as wheat, maize and peas in the Karimabad valley in northern Pakistan, a mountainous region with many glaciers. In many parts of the world, glaciers have been in retreat, creating dangerously large lakes that can cause devastating flooding when the banks break. Climate change can also increase rainfall in some areas, while bringing drought to others. Hira Ali 10 photographs to show to anyone who doesn't believe in climate change Smoke – filled with the carbon that is driving climate change – drifts across a field in Colombia. Sandra Rondon 10 photographs to show to anyone who doesn't believe in climate change Amid a flood in Islampur, Jamalpur, Bangladesh, a woman on a raft searches for somewhere dry to take shelter. Bangladesh is one of the most vulnerable places in the world to sea level rise, which is expected to make tens of millions of people homeless by 2050. Probal Rashid 10 photographs to show to anyone who doesn't believe in climate change Sindh province in Pakistan has experienced a grim mix of two consequences of climate change. “Because of climate change either we have floods or not enough water to irrigate our crop and feed our animals,” says the photographer. “Picture clearly indicates that the extreme drought makes wide cracks in clay. Crops are very difficult to grow.” Rizwan Dharejo 10 photographs to show to anyone who doesn't believe in climate change Hanna Petursdottir examines a cave inside the Svinafellsjokull glacier in Iceland, which she said had been growing rapidly. Since 2000, the size of glaciers on Iceland has reduced by 12 per cent. Tom Schifanella 10 photographs to show to anyone who doesn't believe in climate change A river once flowed along the depression in the dry earth of this part of Bangladesh, but it has disappeared amid rising temperatures. Abrar Hossain 10 photographs to show to anyone who doesn't believe in climate change A shepherd moves his herd as he looks for green pasture near the village of Sirohi in Rajasthan, northern India. The region has been badly affected by heatwaves and drought, making local people nervous about further predicted increases in temperature. Riddhima Singh Bhati 10 photographs to show to anyone who doesn't believe in climate change A factory in China is shrouded by a haze of air pollution. The World Health Organisation has warned such pollution, much of which is from the fossil fuels that cause climate change, is a “public health emergency”. Leung Ka Wa 10 photographs to show to anyone who doesn't believe in climate change Water levels in reservoirs, like this one in Gers, France, have been getting perilously low in areas across the world affected by drought, forcing authorities to introduce water restrictions. Mahtuf Ikhsan

He said the world was 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," 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," he said.

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

He also said that climate action was not just the right thing to do, it made social and economic sense – pointing to how action to cut emissions would 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 that the world was 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 that were 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 were not enough to prevent dangerous temperature rises.

And there needed to be progress on the goal of mobilising $100bn (£78bn) a year for poorer countries to drive clean growth, they urged.

The World Bank has announced that it is doubling investments in climate action with $200bn for 2021-2025, including $50bn towards helping countries adapt to the impacts of global warming.

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

The talks 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.