The world is breaching a key warming threshold at a rate that has scientists concerned, a BBC analysis has found.

On about a third of days in 2023, the average global temperature was at least 1.5C higher than pre-industrial levels.

Staying below that marker long-term is widely considered crucial to avoid the most damaging impacts of climate change.

But 2023 is "on track" to be the hottest year on record, and 2024 could be hotter.

"It is a sign that we're reaching levels we haven't been before," says Dr Melissa Lazenby, from the University of Sussex.