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The direst projected effects of the global heating for the UK have been outlined by scientists, ranging from a searing 4C rise in temperatures to a two-metre increase in sea levels. An alternative projection foresees a plunge of 6C in temperature following the shutdown of key Atlantic Ocean currents, which would severely impact agriculture and energy demand.

Possible Events, Not Guarantees

These consequences, some of which are associated with critical climate thresholds, are viewed as improbable but possible. The team stated that these models filled a gap in planning that had rendered the UK ill-prepared for worst-case results.

"The climate extremes we have detailed are not forecasts, but they are plausible," said Nigel Arnell who oversaw the study. "The UK has been making preparations lacking the tools to assess catastrophic outcomes. We've now provided decision-makers what they need to prepare for climate outcomes they wish to avoid, but must not overlook."

A Range of Severe Conditions Projections

A further series of worst-case scenarios sets out the possible scale of severe conditions between now and the close of the 2100s. These indicate that temperatures could rocket in certain periods by up to 6C above average, while rainfall could be 300% of normal levels.

The likelihood of the worst-case outcomes occurring could not be calculated due to uncertainty about what steps will be taken to combat global warming and how the planet's climate will respond. This analysis was compared to defence planning or scenario planning for financial systems.

Critical Possibility Highlights

• 4C Temperature Rise: Worldwide averages exceeding 4C by 2100 could happen if international cooperation fails or if powerful amplifying effects like the Amazon basin dying release its huge repository of carbon. This would result in intense and enduring heat domes and droughts plaguing the UK in summer.
• AMOC Shutdown: A major ocean current, the Atlantic meridional overturning circulation (Amoc), is weakening and showing signs of collapse due to global heating. A complete failure commencing by 2030 would lead to a six-degree temperature drop in the UK. "Farming would struggle enormously and freshwater availability would be completely altered," noted the professor.
• Rapid Sea Level Rise: A surge of 2.0-2.2 metres around the UK by 2100 could occur if glaciers in the polar regions disintegrate quickly, submerging low-lying areas. In contrast to the other scenarios, this threat is already understood by infrastructure experts.
• Pollution Paradox: Temperatures could also be increased by about 0.75C if airborne particulates is rapidly cleaned up. This paradoxical outcome occurs because pollution particles from burning coal reflect sunlight from reaching the ground.

Planning for the Worst

These dire projections could be used to inform the construction of critical national infrastructure, including new towns, power generation facilities and flood defences. Understanding these climate risks could also accelerate the transition to cut carbon pollution.

The analysis developed the worst-case scenarios using a combination of past data and records, climate models and established principles. The scenarios exclude potential global problems including the global food shortages and increased conflict.

The research was initiated as part of a national risk assessment framework. Earlier reports have warned that insufficient focus was being paid to catastrophic yet improbable risks.

The government's climate advisors has said that the UK must "adapt to 2C" and assess the risks for severe warming. Previous adaptation plans have been described as being "insufficient" by some observers.

An official statement said: "Global warming is at the heart of this government’s agenda, both building resilience and transitioning to a clean energy superpower. It is crucial that the UK is prepared for the consequences."