The risk that climate change will create a nightmarish world of unbearable heat, food crises and swamped cities is so material that world leaders must urgently start trying to quantify it, a UK Foreign Office study warns.

The report by Sir David King, the foreign secretary’s chief climate envoy, calls for worst-case global warming scenarios to be assessed as seriously as risks such as nuclear proliferation.

The climate threats it identifies include searing temperatures that would kill people lying down in the shade, widespread crop failures, rising sea levels, “historically unprecedented” levels of migration and the failure of even wealthy states.

Sir David said ministers had taken worst-case risks into account when he dealt with bird flu almost a decade ago in his former role as the UK government’s chief science adviser.

At that time, the worst scenario was deemed to be the virus becoming transmittable between humans, causing 1m fatalities in less than six months.

Although the chance of human transmission was thought to be less than 1 per cent over a 20-year period, the government decided that even this was unacceptable, and diverted “considerable resources” to a public vaccination programme.

“There seems to have been much less deliberate consideration of worst-case scenarios when it comes to long-term climate change,” said Sir David, adding that this may be because of “the success of the climate denial community and the worry that people shouldn’t be scaremongering”.

Even the UN’s Intergovernmental Panel on Climate Change reports tend to focus on the impact of temperature rises of 2C to 4C from pre-industrial times, the Foreign Office report shows.

In fact, there is a chance of much higher warming if greenhouse gas emissions continue to rise, according to the report, which was produced over many months, with contributions from experts from China, India and the US.

The study was launched on Monday at the London and Mumbai stock exchanges.

However, reporters from leading national newspapers were not formally invited to the UK event, and the report said its contents “should not be taken to represent the views of the UK government”.

David Cameron has angered climate campaigners by reportedly saying he wanted to cut the “green crap” to reduce energy bills.

His ministers ditched onshore wind farm subsidies and scrapped tax relief on renewable generators.

But the Foreign Office report reveals a sense of urgency in some parts of government before the Paris UN conference in December that is due to seal a global climate change agreement.

Baroness Anelay, Foreign Office minister, told the London report launch event she believed that the Paris conference would succeed, but that “there is still no definition of what constitutes success”.

The Foreign Office study concedes that the worst-case climate risks remain “highly uncertain”.

However, Professor Martin Rees, the astronomer royal, compared the situation to astronomers discovering that there was a 10 per cent chance that an asteroid would hit the Earth in 2080.

“Would we relax, saying that this is a problem that can be set on one side for 50 years — as people will by then be richer and it may turn out that it misses the Earth anyway? I do not think we would,” he wrote in the report.

“There would surely be a consensus that we should start straight away and do our damnedest to find ways to deflect it, or mitigate its effects.”

Letter in response to this report:

Show collective leadership on climate change / From Rear Admiral Neil Morisetti

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