"This short paper discusses what people have known for quite some time but that no one to our knowledge has shown – that low-income countries due to their typically tropical location are experiencing faster changes in temperature extremes than high-income countries," Nicholas Herold of the University of New South Wales, Australia, told environmentalresearchweb. "This is based on observationally constrained data and not modelling, so we can say now that this is really happening and has in fact already been happening for more than two decades."

Herold would like to see this direct evidence that low-income countries have been suffering from heat extremes more than high-income countries help inform policy makers and negotiators regarding the aid that certain low-income countries receive.

"There is always a lot of discussion and contention around this issue – and indeed it is a complicated one," he said, "but for discussions and negotiations to take place on a platform of fairness, it's important that such facts as we've shown here are taken into consideration."

Herold and colleagues examined trends in the TX90p hot-days frequency index – the percentage of days each year where maximum temperature exceeds the calendar day 90th percentile for a given base period (in this case 1961–1990) – and the TN90p warm-nights frequency index for daily minimum temperatures. They calculated the trends using data from three global atmospheric reanalyses for 1958–2010, which integrate available observations into numerical weather forecast models to provide global climate data coverage.

"I was actually looking for some data that compared climate trends in low-income versus high-income countries and was surprised when I couldn't find a simple plot showing this," said Herold. "It's been talked about for a long time that low-income countries will suffer more from climate change than high-income countries."

Low-income countries have seen an increase in their percentage of hot days each year from 10% (37 days) for 1961–1990 to 22% (80 days) in 2010. High-income countries experienced a rise from 10% to 15% (55 days). That means low -income countries saw more than twice the increase in hot days each year compared to rich nations.

Low-income countries tend to occur at lower latitudes, where natural temperature variability is smaller, exposing them to more rapid changes in temperature extremes as climate warms.

"Given that it's through climate extremes that we tend to feel the impact of climate change, for example, through heatwaves, droughts and storms, it was even more surprising that a comparison of extremes based on observational evidence hadn't been done before," said Herold. "It's because of the impacts that climate extremes have on communities that we chose to focus on temperature extremes in our study."

If temperature extremes continue to change at the same rate, low-income countries could within two decades see a tripling in the number of hot days they experience each year compared to 1961–1990. "In regions where temperatures are already near the threshold for human comfort, and where there is a large dependence on outdoor labour and agriculture, such changes may be socially and economically devastating," writes the team in Environmental Research Letters (ERL).

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