As the world gets hotter, heatwaves become longer lasting and more intense globally, scientists say.
Hundreds of millions of people, especially in densely packed megacities, will be vulnerable to more severe illness and death, the United Nations' Intergovernmental Panel on Climate Change (IPCC) said last year in its special report on warming of 1.5 deg C against that of 2 deg C.
Australia is particularly at risk, studies show. Heatwaves will occur more often with greater spikes in temperature. In a country with an ageing population, that presents a huge health challenge.
Heatwaves could cause an additional 6,214 deaths by 2050 in Victoria state alone, the Climate Council of Australia, a non-profit organisation, said in a 2014 special report on heatwaves.
More extreme heat would also lead to loss of labour productivity and greater stress on crops and livestock, enhance conditions for bushfires, and raise the risks from vector-borne diseases.
A study published last year in the Weather And Climate Extremes journal found that the number of heatwaves in Australia and the duration of the most intense heatwaves will increase significantly in the near future (2020 to 2039) and more so in the far future (2060 to 2079), with greater increases in the tropical north than south.
All capital cities are projected to experience at least a tripling of heatwave days each year by the far future, the researchers said.
Northern Australia could face heatwaves lasting more than a month. In the south, Adelaide, Canberra, Melbourne, Perth and Sydney face more hot days and more extremely hot days.
Globally, heatwaves will become an increasing threat to people, agriculture and infrastructure.
The IPCC report said the largest increases in the number of hot days are projected to occur in the tropics, including South-east Asia. People in large cities are particularly vulnerable, the report says, because urban centres are already warmer than surrounding areas. Cities trap heat from the sun, air-conditioners, transport and industrial activity.
It says future warming and urban expansion could lead to more extreme heat. At 1.5 deg C of warming, twice as many megacities globally could become heat stressed, exposing more than 350 million additional people to deadly heat by 2050 as cities keep growing.
The IPCC and others point to the need for better urban planning, such as planting more trees, more reflective roofs, more efficient building design that relies less on air-conditioning, and cleaner transport.