Global health stands threatened by climate change, with most of the associated risks from flooding, droughts and heat waves highly underestimated.
It is a medical emergency demanding an emergency response, says a Health and Climate Change Lancet commission involving experts from University College London, the World Health Organization and other research organisations.
The effects of climate change could undermine the medical advances of the last 50 years, says the report.
A rapid phase-out of coal from the global energy mix is among the recommendations of the commission.
Numbers could exceed the 250,000 deaths predicted annually as a fallout of climate change by WHO with food shortage, diseases and civil wars adding to the woes, says the report.
People forced to move, whether by food shortages, floods or extreme storms, can suffer serious physical and mental health problems.
Dengue fever is likely to spread, and malaria can rise in new areas while falling in others. Cholera outbreaks following hurricanes and extreme weather are increasing.
However, on a positive note the commission notes that the benefits to health resulting from acting on climate change are too large to be ignored.
Cutting air pollution by reducing transport emissions or coal-fired power stations in the EU alone could save €38bn (£27bn) a year by 2050 due to reduced deaths, the report said.
It calls for major policy changes to cut pollution and investing in green cities, energy and transport.
Carbon pricing to push up the price of high carbon goods and services to make people change their behaviour and cutting the price of low-carbon technology have also been suggested.
"Climate change has the potential to reverse the health gains from economic development that have been made in recent decades – not just through the direct effects on health but through indirect means such as increased migration and reduced social mobility," said Professor Anthony Costello, Director of the UCL Institute for Global Health
The Lancet report cites research estimating that reducing carbon emissions would cut premature deaths from air pollution by 500,000 a year in 2030, 1.3 million in 2050 and 2.2 million in 2100, particularly in the heavily polluted cities of India and China.
Boosts to human health can be worth 10 times the costs of cutting emissions.
An estimated $1tn (£630bn) would be needed each year up to 2050 to tackle climate emissions from energy, on top of the $105tn (£66tn) which would be required anyway for the energy system up to mid-century.
But with people spending $6.8tn (£4.3tn) a year on health, and with the "kind of health risks coming down the line if we don't stick to 2C, that is a very good precautionary investment," said Paul Ekins, director of the Institute for Sustainable Resources, University College London.
The report sees political will as the major barrier to delivering a low-carbon economy, not finance or technology.
The latest Lancet report on health and climate change, which follows on from a previous commission convened on the issue in 2009, is expected to speed up action in the run-up to the UN climate talks in Paris in December.
Keeping the global average temperature rise to less than 2°C to avoid the risk of potentially catastrophic climate change impacts requires total anthropogenic carbon dioxide (CO2) emissions to be kept below 2,900 billion tonnes by the end of the century.
As of 2011, total emissions since 1870 were a little over half of this, with current trends expected to exceed 2,900 GtCO2 in the next 15–30 years.