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The most severe possible consequences of the global heating for the UK have been detailed by experts, including a blistering 4C rise in average temperatures to a 6.5-foot increase in sea levels. Another scenario foresees a decline of 6C in temperature following the collapse of crucial Atlantic Ocean currents, which would massively disrupt agriculture and energy demand.

Plausible Outcomes, Not Forecasts

These consequences, a number of which are associated with climate tipping points, are seen as low probability but possible. The team explained that these projections filled a gap in forecasting that had rendered the UK unprepared for extreme outcomes.

"The worst-case scenarios we have outlined should not be seen as certainties, but they are possible," said Nigel Arnell who directed the research. "The UK has been making preparations without the tools to plan for extreme possibilities. We've now provided planners what they need to get ready for future conditions they pray never occur, but can’t afford to ignore."

A Suite of Extreme Weather Possibilities

A further series of severe models sets out the possible scale of extreme weather between now and the close of the 2100s. These show that temperatures could soar in some months by up to 6C higher than historical levels, while precipitation could be 300% of normal levels.

The chance of the worst-case outcomes coming to pass could not be calculated due to unknown factors about what steps will be implemented to address global warming and how the planet's climate will react. This approach was likened to defence planning or scenario planning for the banking sector.

Major Scenario Details

• 4C Temperature Rise: Planetary heating exceeding 4C by 2100 could happen if international cooperation fails or if vicious cycles like the Amazon rainforest dying release its vast reserves of carbon. This would cause extreme and prolonged heat domes and water shortages plaguing the UK in summer.
• Atlantic Current Collapse: A critical sea current, the Atlantic meridional overturning circulation (Amoc), is losing strength and losing stability due to global heating. A collapse commencing by 2030 would lead to a drastic cooling in the UK. "Agriculture would be severely challenged and water resources would be drastically changed," noted Arnell.
• Rapid Sea Level Rise: A rise of 2.0-2.2 metres around the UK by 2100 could take place if polar ice in Greenland and Antarctica melt abruptly, submerging seaside communities. Differing from the other scenarios, this possibility is already known by planners.
• Pollution Paradox: Global averages could also be increased by about 0.75C if pollution from industry is drastically reduced. This counterintuitive effect occurs because pollution particles from industrial activity reflect solar radiation from reaching the ground.

Planning for the Extreme

The severe models could be used to inform the building of major projects, for example expanded cities, nuclear power stations and water management networks. Awareness of these climate risks could also accelerate the drive to cut greenhouse gas emissions.

The analysis created the worst-case scenarios using a combination of real-world events, computer simulations and established principles. The possibilities exclude international cascading effects such as the global food shortages and geopolitical instability.

The project was requested as part of a official risk assessment framework. Independent analyses have stated that not enough attention was being allocated to catastrophic yet improbable risks.

An independent advisory body has said that the UK needs to "become resilient to moderate heating" and evaluate the dangers for severe warming. Previous adaptation plans have been labeled being "insufficient" by some observers.

A government spokesperson said: "Global warming is at the core of our national priorities, both preparing for impacts and establishing itself as a renewable energy leader. It is essential that the UK is prepared for the consequences."