The warning came as data showed last month was the world's warmest March on record, extending monthly temperature records to 10 in a row. It sparked fears among some that the world could be tipped into a new phase of even more rapid climate change.

A weather system called El Niño is behind some of the recent heatwaves.

Temperatures should temporarily drop after El Niño subsides in the coming months, but some scientists fear they won't. (Matt McGrath and Mark Poynting, more at BBC.com)