Why does the Paris climate change conference matter?

It has prompted the biggest demonstration about climate change ever seen in the UK as tens of thousands braved the rain and the wind to march through the streets to Westminster.

The conference on climate change in Paris today marks the latest step on a long - and often tortuous - road to securing international agreement on cutting greenhouse gas emissions.

World leaders hope to reach agreement in a bid to prevent global temperatures causing “dangerous” climate change.

Here we look at the challenges that will be uppermost in the minds of the politicians hoping to secure a historic deal.

1. How big is the conference?

More than 40,000 delegates from 195 countries are attending COP21 - an annual meeting of all nations that make up the United Nations Framework on Climate Change.

2. Why is it important?

The conference is crucial in framing the international response to man-made climate change.

Experts say by 2100, the Middle East may uninhabitable and a warming climate will lead to more frequent and extreme heat waves, droughts and flash floods.

3. How strongly is the UK Government pushing for reductions in carbon emissions?

Prime Minister David Cameron is expected to tell gathered leaders and negotiators: “The UK has a strong domestic framework to deal with the issue of climate change - a model which is being exported to other countries.

“I want to see a similarly robust system at an international level - one that recognises that an agreement in Paris is just the start.

“We must include a five-yearly review mechanism to increase ambition in the future. Whilst emission reductions should always be pledged country by country, we must review our ambition regularly if we are to hit our final two degree goal.

“I also want to see a long-term goal in the agreement. This will give certainty to businesses and the public across the world that governments are serious about de-carbonising.”

4. Does this mean we’re leading the way?

Not really. The Government has come in for criticism from many quarters in recent months on the home front for curbing support for renewables and energy efficiency measures.

Just days before the conference began, ministers also announced they were axing a £1bn scheme to develop technology to capture and store technology from power stations, despite backing new gas plants.

5. Who are the biggest offenders on carbon emissions?

Unsurprisingly the world’s two biggest economies - those of the US and China - are the largest producers of greenhouse gasses. They will be the main players when it comes to either reaching a meaningful deal or ending with little more than warm words and waffle.

6. What is the scale of support on the ground for a deal?

Ahead of the conference, which kicks off amid tight security in the wake of the terror attacks in the French capital, more than half a million people are estimated to have marched in cities around the world calling for ambitious action to boost renewable energy and protect vulnerable people.

Welsh singer Charlotte Church was among the celebrities who joined a march in London.

7. What are the obstacles to a deal?

India’s prime minister Narendra Modi has said advanced countries should shoulder more responsibility in the fight against climate change to allow other nations to complete their “first steps on the development ladder”. Developing nations, which according to some had “powered their way to prosperity on fossil fuel when humanity was unaware of its impact”, had to shoulder more of the responsibility, he said.

8. How much common ground have world leaders been able to find in the past?

Previous meetings include COP3 in Kyoto, Japan, which brought about the Kyoto Protocol, COP11, which generated the Montreal Action Plan and COP15 in Copenhagen, Denmark, which was deemed a failure because a binding agreement wasn’t reached. Then, as now, developing nations like India and China were unwilling to accept what they saw as the hypocrisy of the world’s most affluent nations telling them to focus on the environment as well as growth when they had millions of poor people to feed.

9. How will success be measured as the result of any deal?

The aim is to prevent global temperatures going more than 2C above pre-industrial levels and causing “dangerous” climate change.

10. How achievable is this?

Meteorologists say this will not be an easy goal to reach, since the planet already has been warmed by 0.85C since 1880, according to the latest Intergovernmental Panel on Climate Change (IPCC) report in 2014. Many scientists say the gases we have already emitted into the atmosphere will “lock us in” to around 2C of warming.

11. How convincing is the evidence for man-made as opposed to naturally occurring climate change?

The United Nation’s IPCC report has said it’s extremely likely, or 95% to 100% certain, human influence has been the dominant cause of global warming since the mid-20th century. The IPCC publishes its work every six years and last time round in 2007 said it was 90% certain humans were at fault over global warming.

Lord Nicholas Stern, one of the UK’s most influential climate change experts, said the evidence was now conclusive.

12. Does that mean the argument has been settled?

No - some sceptics remain as convinced as ever that it’s all fantasy. They insist scientists benefit through large research grants and Governments go along with it because it’s an excuse to increase taxes. They argue the earth’s climate has always changed - long before we started pumping carbon into the air.

13. What happens if there is no agreement?

Expert say the average temperature has not been 2C above pre-industrial levels for about 115,000 years, when the ice-caps were smaller and global sea level was at least five metres higher than today.

Some fear a temperature rise on this scale could cause mass migrations of hundreds of millions of people away from the worst-affected areas. That would lead to conflict and war, rather than peace and prosperity, it’s claimed.