Human populations may already be adapting to climate change, new research suggests.
The evidence comes from sunny Spain, where average summer temperatures have risen by more than 1C since 1980.
Scientists found that, contrary to expectations, numbers of heat-related deaths had declined rather than increased.
The study compared daily temperatures and death rates in 47 provincial capitals in Spain during every summer from 1980 to 2015.
It revealed a progressive increase in average summer temperature of 0.33C per decade.
Yet, to the surprise of the researchers, the risk of heat-related death had gradually decreased over the same period.
Lead scientist Dr Hicham Achebak, from the Autonomous University of Barcelona, said: "It is usually assumed that climate change will be accompanied by an increase in heat-related mortality, especially in places where the mean age of the population is increasing."
The trend is most likely explained by adaptations to higher temperatures, said the researchers.
Co-author Dr Joan Ballester, from the Barcelona Institute for Global Health, said: "We are becoming less vulnerable to heat thanks to society's adaptation to higher temperatures and also to the socioeconomic development we have seen in recent decades.
"Improvements in housing stock, the popularisation of air conditioning, advances in health services, and awareness campaigns are all factors that may have contributed to the trend we are seeing.
"However, we still don't know whether this downward trend will continue if climate change becomes more intense in the future."
An important exception to the general downward trend was increased numbers of deaths from respiratory causes, especially among women.
This could be linked to an ageing population and higher incidence of certain chronic diseases, said the researchers.
The findings are reported in the journal Public Library of Science Medicine.