In the cynical world of politics it is important, just occasionally, to give credit where it is due. The commitments on carbon emissions announced on Wednesday by President Barack Obama of the US and President Xi Jinping of China, supported by the significant diplomacy of Secretary of State John Kerry, are both environmentally substantive and politically influential. Between them, these countries account for 44 per cent of global carbon emissions – a share that is still rising.

The US has promised that by 2025 its emissions will be at least 26 per cent lower than they were two decades earlier. China has pledged that its carbon emissions will peak around 2030 and that carbon-free energy sources will account for 20 per cent of what it consumed by the same year.

This sea change comes just five years after the setbacks of the Copenhagen summit of 2009. In the lead-up to the crucial Paris conference on climate change in December next year, these American and Chinese commitments will make a significant difference to the debate.