Which one of those pathways we pick is entirely a political question.
British pensioners are increasingly at risk from the growing threat of heatwaves as the climate changes, health experts have warned.
And children are particularly at risk from air pollution, mostly caused by transport and burning coal, which stunts their lung development and has lifelong impacts on physical and mental health.
But taking action to tackle climate change is "sensible" to deliver public health and economic benefits and relieve pressure on the NHS.
The 2019 Lancet Countdown On Health And Climate Change report from 35 global institutions examines 41 indicators up to the present day across areas including the impacts of rising temperatures, resilience, and economics.
It finds climate change is already damaging the health of the world's children, and threatens them throughout their lives unless action is taken to limit temperature rises to well below 2C in line with global commitments.
Professor Elizabeth Robinson, from the University of Reading, said there was a "clear upward trend" in the number of people exposed to heatwaves.
Heat extremes cause heat stress, heat stroke, acute damage to kidneys, exacerbate congestive heart failure and increase the risk of violence and suicide, the experts said.
And Europe was the most vulnerable out of all the regions the scientists tracked, because many people live in cities, the population is older and their health problems such as heart disease and obesity put them at risk.
The UK saw heatwaves in 2018, which resulted in 863 excess deaths in England alone, and record high temperatures in 2019, putting older people who are increasingly suffering ill health at a growing risk.
Air pollution is also a major health problem to the UK, with fine particulate air pollution contributing to over 20,500 premature deaths in 2016, and coal linked to 60 deaths a week, the study said.
The UK faces increasing threats to crop production, and the spread of infectious diseases as temperatures rise - with the presence of tick-borne encephalitis virus confirmed in the UK for the first time.
The report also warns of rising exposure to threats such as wildfires globally. To illustrate the future health risks and opportunities, the report sets out two pathways for the health of a child born today.
On the one hand a child born today could live into the 2090s, potentially experiencing a world with temperatures 4C above pre-industrial levels, and affected by increasing infectious diseases, malnutrition and ongoing air pollution.
But in a world that takes steps to keep temperature rises to 2C, a child born today would see the phase-out of all coal in the UK by their sixth birthday and see the UK reach net-zero emissions by the time they are 31.
Dr Nicholas Watts, executive director, Lancet Countdown, said: "When you look at the technology available, the economics of the issue, the finance available, the technical questions, you realise that (depending on) which of these two pathways we pick, climate change is an enormous threat to public health or the response to climate change is the greatest public health opportunity we have in front of us.
"Which one of those pathways we pick is entirely a political question."
Prof Robinson said there were many things the UK could do to improve the health of the population and tackle climate emissions, starting with reducing air pollution.
"Then changing to a more plant-based diet, a balanced diet which can easily include meat products but is a more balanced plant-based diet," she said.
"And producing an environment where people can undertake active transport without threatening their health, so safer roads, less pollution on the roads, more opportunities for cycling, much more integrated transport.
"All countries need to take action, and in particular high-income countries, and the actions we are proposing make sense for our health and make sense economically."
Dr Watts added: "They are just sensible things for the UK's health, they will ease off pressure on the NHS which is incredibly overburdened."
Emily Beament is the PA environment correspondent.