The target of keeping long-term global warming within 1.5 degrees Celsius (2.7 Fahrenheit) is moving out of reach, climate experts say, with nations failing to set more ambitious goals despite months of record-breaking heat on land and sea.

As envoys gathered in Bonn in early June to prepare for this year's annual climate talks in November, average global surface air temperatures were more than 1.5C above pre-industrial levels for several days, the EU-funded Copernicus Climate Change Service (C3S) said.

Though mean temperatures had temporarily breached the 1.5C threshold before, this was the first time they had done so in the northern hemisphere summer that starts on June 1. Sea temperatures also broke April and May records.