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The most severe possible consequences of the global heating for the UK have been outlined by researchers, encompassing a scorching 4C rise in average temperatures to a two-metre surge in sea levels. Another scenario foresees a decline of 6C in temperature following the collapse of key Atlantic Ocean currents, which would severely impact farming and power requirements.

Credible Events, Not Guarantees

These effects, some of which are linked to critical climate thresholds, are seen as low probability but plausible. The researchers said that these projections filled a gap in forecasting that had rendered the UK ill-prepared for extreme outcomes.

"The worst-case scenarios we have outlined are not forecasts, but they are credible," said the professor who directed the research. "The nation has been formulating policy lacking the tools to plan for extreme possibilities. We've now provided decision-makers what they need to get ready for climate outcomes they hope never happen, but must not overlook."

A Suite of Harsh Climate Projections

Another collection of dire projections maps the potential extent of severe conditions between now and the year 2100. These suggest that temperatures could rocket in certain periods by up to 6C above the norm, while precipitation could be 300% of standard measurements.

The chance of the extreme scenarios coming to pass remains uncertain due to a lack of certainty about what measures will be taken to combat climate change and how the planet's climate will respond. This research was likened to defence planning or scenario planning for financial systems.

Key Possibility Elements

• 4C Temperature Rise: Planetary heating soaring past 4C by 2100 could occur if emissions cuts fails or if vicious cycles such as the Amazon basin becoming a net carbon source unleash its enormous store of carbon. This would lead to extreme and prolonged heat domes and water shortages plaguing the UK in summer.
• Ocean Circulation Failure: A critical sea current, the North Atlantic current, is weakening and showing signs of collapse due to ocean warming. A complete failure commencing by 2030 would lead to a drastic cooling in the UK. "Farming would be severely challenged and water supplies would be completely altered," said Arnell.
• Rapid Sea Level Rise: A rise of 2.0-2.2 metres around the UK by 2100 could happen if glaciers in the polar regions disintegrate quickly, flooding low-lying areas. Differing from the other scenarios, this risk is already understood by planners.
• Aerosol Effect: Temperatures could also be pushed higher by about 0.75C if industrial aerosols is rapidly cleaned up. This counterintuitive effect occurs because aerosol particles from fossil fuel combustion scatter solar radiation from reaching the ground.

Building Resilience Against the Unthinkable

The worst-case scenarios could be applied to guide the building of long-term infrastructure, including expanded cities, energy plants and flood defences. Awareness of these climate risks could also hasten the transition to reduce greenhouse gas emissions.

The research formulated the severe projections using a mix of past data and records, digital projections and climate physics. The possibilities omit potential global problems like the devastation of food supplies and increased conflict.

The project was initiated as part of a official 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 needs to "prepare for two degrees of warming" and assess the risks for severe warming. Past policy documents have been described as being "inadequate" by certain experts.

A government spokesperson noted: "Global warming is at the heart of our national priorities, both adapting for the future and establishing itself as a low-carbon economy. It is crucial that the UK is prepared for the impacts."