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The most severe possible consequences of the climate emergency for the UK have been mapped by experts, including a searing 4C increase in temperatures to a 2-metre rise in sea levels. An alternative projection envisions a drop of 6C in temperatures following the shutdown of key Atlantic Ocean currents, which would severely impact food production and energy demand.

Plausible Outcomes, Not Guarantees

These consequences, certain ones are connected to critical climate thresholds, are considered as low probability but possible. The researchers stated that these scenarios plugged a hole in planning that had left the UK unprepared for extreme outcomes.

"The worst-case scenarios we have mapped are not forecasts, but they are plausible," stated Nigel Arnell who oversaw the research. "The UK has been planning in the absence of the methods to plan for extreme possibilities. We've now supplied decision-makers what they need to get ready for future conditions they hope never happen, but dare not dismiss."

A Range of Extreme Weather Scenarios

A second set of severe models sets out the possible scale of extreme weather between now and the close of the 2100s. These indicate that temperatures could rocket in certain periods by up to 6C above historical levels, while precipitation could be triple normal levels.

The probability of the extreme scenarios occurring remains uncertain due to uncertainty about what steps will be enacted to combat global heating and how the planet's climate will respond. This approach was likened to security threat evaluations or resilience checks for the banking sector.

Critical Outcome Details

• Four-Degree Warming: Worldwide averages exceeding 4C by 2100 could happen if climate action breaks down or if vicious cycles including the Amazon basin becoming a net carbon source release its huge repository of carbon. This would cause extreme and prolonged heat domes and droughts affecting the UK in summer.
• AMOC Shutdown: A key circulation system, the Atlantic meridional overturning circulation (Amoc), is weakening and showing signs of collapse due to ocean warming. A full shutdown beginning in 2030 would lead to a 6C cooling in the UK. "Farming would be severely challenged and freshwater availability would be completely altered," explained the professor.
• Rapid Sea Level Rise: A increase of 2.0-2.2 metres around the UK by 2100 could occur if ice sheets in the Arctic and Antarctic melt abruptly, inundating coastal cities and towns. Unlike the other scenarios, this threat is already recognized by infrastructure experts.
• Clean Air Warming: Temperatures could also be increased by about 0.75C if industrial aerosols is drastically reduced. This paradoxical outcome occurs because aerosol particles from fossil fuel combustion reflect sunlight from warming the ground.

Planning for the Unthinkable

These dire projections could be used to inform the construction of long-term infrastructure, such as new towns, energy plants and flood defences. Awareness of these environmental threats could also accelerate the push to cut carbon pollution.

The research created the extreme models using a combination of real-world events, climate models and established principles. These outcomes omit potential global problems such as the devastation of food supplies and geopolitical instability.

The project was requested as part of a government’s risk assessment framework. Earlier reports have stated that too little consideration was being given to high-consequence, low-probability risks.

The government's climate advisors has said that the UK must "become resilient to moderate heating" and examine the dangers for 4C. Previous adaptation plans have been criticised as being "inadequate" by some observers.

A government spokesperson commented: "Global warming is at the heart of the country's focus, both adapting for the future and establishing itself as a renewable energy leader. It is essential that the UK is resilient for the effects."