The fate of the planet will be negotiated in Glasgow, Scotland

Almost every country in the world signed the 2015 Paris climate agreement, a monumental accord that aimed to limit global warming. But it was forged on a contradiction: Every signatory agreed that everyone must do something to address the urgent threat of climate change, but no one at the time pledged to do enough.

In the years since the agreement, the emissions that trap heat in Earth’s atmosphere have continued to rise. Concentrations of carbon dioxide, the main greenhouse gas emitted by humans, reached a record high of 419 parts per million in the atmosphere this year.

The Paris agreement aimed to limit global warming this century to less than 2 degrees Celsius, compared to temperatures before the Industrial Revolution, with a more optimistic goal of staying below 1.5°C. Both of these goals would require rapid and radical shifts away from fossil fuels — and eventually, zeroing out emissions of greenhouse gases entirely.

Signatories did agree that they would set more ambitious targets for themselves over time and eventually get on track to meet global climate goals. Whether they will actually do so is about to be tested over the next two weeks at COP26, the most important international climate conference in years.

“This is definitely the biggest [climate meeting] since Paris, and it has to be a turning point if we’re going to be successful,” said Helen Mountford, vice president for climate and economics at the World Resources Institute.