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The most severe possible consequences of the climate emergency for the UK have been outlined by experts, including a scorching 4C rise in average temperatures to a 6.5-foot rise in ocean levels. An alternative projection predicts a drop of 6C in temperature following the shutdown of key Atlantic Ocean currents, which would severely impact agriculture and power requirements.

Possible Outcomes, Not Forecasts

These effects, certain ones are associated with irreversible climate shifts, are seen as low probability but plausible. The researchers stated that these models plugged a hole in risk assessment that had left the UK not ready for worst-case results.

"The severe weather events we have detailed should not be seen as certainties, but they are credible," said the professor who oversaw the analysis. "Britain has been making preparations lacking the methods to test against extreme possibilities. We've now supplied policymakers what they need to prepare for future conditions they wish to avoid, but must not overlook."

A Spectrum of Extreme Weather Projections

A second set of worst-case scenarios sets out the potential extent of extreme weather between now and the end of the century. These show that temperatures could soar in certain periods by up to 6C above average, while rainfall could be three times typical amounts.

The likelihood of the worst-case outcomes happening remains uncertain due to unknown factors about what steps will be implemented to address climate change and how the Earth's systems will behave. This analysis was described as similar to national security risk assessments or resilience checks for financial systems.

Major Possibility Elements

• Four-Degree Warming: Global temperatures soaring past 4C by 2100 could occur if climate action collapses or if powerful amplifying effects including the Amazon basin becoming a net carbon source release its vast reserves of carbon. This would cause intense and enduring heatwaves and water shortages affecting the UK in summer.
• Atlantic Current Collapse: A critical sea current, the AMOC, is weakening and becoming unstable due to climate change. A full shutdown commencing by 2030 would lead to a drastic cooling in the UK. "Agriculture would struggle enormously and water resources would be drastically changed," explained the lead researcher.
• Sudden Ocean Inundation: A increase of 2.0-2.2 metres around the UK by 2100 could take place if glaciers in Greenland and Antarctica disintegrate quickly, flooding low-lying areas. Differing from the other scenarios, this possibility is already recognized by planners.
• Pollution Paradox: Global averages could also be pushed higher by about 0.75C if airborne particulates is drastically reduced. This paradoxical outcome occurs because tiny specks from burning coal reflect solar radiation from reaching the ground.

Planning for the Worst

The worst-case scenarios could be utilized in planning the building of critical national infrastructure, including new towns, nuclear power stations and flood defences. Knowledge about these environmental threats could also hasten the drive to cut carbon pollution.

The research formulated the extreme models using a blend of observed and historical experience, climate models and scientific theory. The scenarios exclude international cascading effects like the devastation of food supplies and resource wars.

The project was initiated as part of a national adaptation planning initiative. Earlier reports have cautioned that not enough attention was being given to high-consequence, low-probability risks.

The government's climate advisors has stated that the UK needs to "prepare for two degrees of warming" and assess the risks for four degrees. Previous adaptation plans have been criticised as being "very weak" by environmental groups.

A government spokesperson said: "Climate change is at the heart of this government’s agenda, both building resilience and establishing itself as a low-carbon economy. It is vital that the UK is ready for the consequences."