Welcome to the inquiry on the BBC World Service.

I'm Helen Grady.

Each week four expert witnesses answer cities all over the world. In London they built a huge wooden

pyramid right in the middle of the city's busiest junction. On a beach in Sydney they buried

their heads in the sand, and in Dublin they marched as a human forest

covered in twigs and leaves.

All the protests were organised by Extinction Rebellion, a group that wants to stop climate change

by cutting one of its biggest drivers, carbon dioxide.

When it's released into the atmosphere, CO2 traps heat from the sun causing the planet's temperature to rise.

Extinction Rebellion wants governments to commit to a target of zero carbon by 2025.

It's ambitious.

Most people think it's impossible.

The UK government has passed a law promising to reach carbon zero by 2050. But how quickly could the whole world get there? How

soon can the planet reach carbon zero.

Part one, keeping the lights on. 25 years ago climate change was not the hot environmental topic it was a niche. You know people thought oh here's a strange academic studying a backwater topic.

Roger Pilkey Jr. started researching climate change before most of us had even heard of it.

Now a professor at the University of Colorado, he's frustrated by our failure to tackle one of the

biggest causes of climate change.

Since early 1990s the world has emitted as much carbon dioxide as it did before in all of

history. To help focus minds, scientists have developed a goal, a way of measuring

our progress in cutting carbon emissions. It's called Net Zero. Net Zero is a concept that we should just take our emissions all the way down to

zero and have none of them. So where do we start? The biggest source of

...