When Nicholas Herold was hunting for data on how much climate change was affecting the world's poorest nations, he was surprised when he couldn't find answers in the published research.

"Everyone's spoken for years about how poor countries are going to suffer more – or are suffering more – and I couldn't find one figure actually showing this," the research fellow at the Climate Change Research Centre at the University of NSW said.

While lots of models were in use, nobody had crunched the observations. One reason was that until recent decades, large regions particularly in Africa had only limited instrumental monitoring.

Using World Bank wealth definitions and targeting the number of days and nights that fell in the top 10 per cent of temperatures for any date, Dr Herold and fellow researchers found extreme heat readings have increased much faster in low-income nations than richer ones since at least the 1980s.