"We are now able to put a number on the deaths caused by climate change in a heat wave," Dann Mitchell of the University of Oxford, UK, told environmentalresearchweb. "This has never been done before. Previous studies have attributed changes in heat waves to climate change, or related increased heat stress to human deaths, but none have combined the two."

Mitchell and colleagues found that in Greater London, where the heatwave was less extreme, 64 of the 315 deaths were caused by manmade climate change. The researchers used personal computing power donated by the public under the Weather@home citizen science scheme to conduct simulations of a 25 km resolution regional climate model. The total computing time was equivalent to around 250 years on a single home PC. This modelling revealed the influence of manmade climate change on the 2003 heatwave. The team input the resulting temperature data into a health impact assessment model of human mortality.

"Deaths are usually recorded as being due to a specific cause such as cardiovascular disease or cancer, which makes it difficult to make a direct connection between deaths recorded during a heatwave and exposure to heat specifically," said Clare Heaviside of Public Health England. "So in order to calculate the number of deaths that can be attributed to the heatwave, we use a statistical relationship which relates increases in temperature above a certain threshold to an increase in the number of deaths typically expected on a particular day of the year."

Heat can exacerbate existing medical conditions, according to the team, contributing to the increased risk of mortality. Factors such as social status, individual behaviour, the extent of urbanization, and increased air pollution during hot periods all affect the risk. As a result, the number of deaths caused by each heatwave depends on its location, timing and the past experiences of the local population.

From June to August 2003, the seasonal heat-related mortality rate was around 4.5 per 100,000 people in London and 34 per 100,000 people in Paris. The daily mortality rate in Paris peaked at 50 per 100,000 population at the height of the heat wave, the team writes in Environmental Research Letters (ERL). Across Europe the heatwave caused up to 70,000 excess deaths.

"Climate change is impacting us in a negative way already," said Mitchell. "With heat wave intensity and frequency set to increase, our results show a possible emergent trend that policy makers need to be aware of. It shows that nations need to have very strong emergency plans in place to combat the effects of future increased magnitude heat waves."

Mitchell worked with colleagues from the University of Oxford, Public Health England, Centre for Ecology and Hydrology (CEH) in Wallingford and Reading University, all in the UK, and the Union of Concerned Scientists, US.

"The study took a long time to develop, and was a combination of climate modellers, health modellers, land surface modellers and experts in climate policy," said Mitchell. "To answer the question ‘how many people died in this heat wave as a consequence of climate change?’ we have had to utilize expertise in chemistry, radiation, land surface processes, meteorology, sociology and human physiology."

Now the team plans to apply the same techniques to future climate projections. "Specifically we are interested in what the impacts of a 1.5 degree warmer world might look like," said Mitchell. "This comes from the recent Paris Agreement on climate change, where policy makers have directly asked us this question."

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