Hosting the United Nations' COP26 summit, Johnson is hoping to keep alive a target of capping global warming at 1.5 degrees Celsius above pre-industrial levels - the limit scientists say would avoid its most destructive consequences.
But it will be no easy task as Johnson found at a meeting of leaders from 20 of the word's biggest economies over the weekend. The Group of 20 agreed on the need for "meaningful and effective" action to limit global warming but offered few concrete commitments.
"Humanity has long since run down the clock on climate change. It's one minute to midnight and we need to act now," Johnson said ahead of the summit.
Saying Britain wanted to lead by example, Johnson will make a commitment to increase the finance the country provides to developing countries, which often face the worst environmental impacts, by 1 billion pounds ($1.36 million) by 2025 to tackle climate change.