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The most severe possible consequences of the global heating for the UK have been mapped by researchers, ranging from a blistering 4C jump in temperatures to a two-metre increase in ocean levels. An alternative projection foresees a drop of 6C in temperatures following the failure of critical Atlantic Ocean currents, which would profoundly upset food production and power requirements.

Possible Extremes, Not Guarantees

These impacts, certain ones are linked to climate tipping points, are considered as low probability but plausible. The scientists stated that these scenarios filled a gap in planning that had rendered the UK unprepared for catastrophic possibilities.

"The climate extremes we have detailed aren’t predictions, but they are credible," commented Nigel Arnell who directed the study. "The UK has been formulating policy in the absence of the frameworks to plan for extreme possibilities. We've now supplied planners what they need to prepare for future conditions they wish to avoid, but dare not dismiss."

A Spectrum of Severe Conditions Scenarios

A second set of dire projections details the potential extent of harsh climate between now and the close of the 2100s. These suggest that temperatures could skyrocket in certain periods by up to 6C higher than historical levels, while precipitation could be triple standard measurements.

The probability of the extreme scenarios coming to pass could not be calculated due to uncertainty about what action will be implemented to address climate change and how the Earth's systems will behave. This approach was described as similar to defence planning or scenario planning for economic stability.

Major Outcome Elements

• 4C Temperature Rise: Worldwide averages soaring past 4C by 2100 could occur if climate action collapses or if strong feedback loops like the Amazon rainforest becoming a net carbon source emit its vast reserves of carbon. This would result in 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 losing strength and becoming unstable due to climate change. A complete failure beginning in 2030 would lead to a 6C cooling in the UK. "Agriculture would face immense difficulties and water resources would be drastically changed," explained Arnell.
• Rapid Sea Level Rise: A surge of 2.0-2.2 metres around the UK by 2100 could occur if polar ice in the Arctic and Antarctic disintegrate quickly, inundating coastal cities and towns. Unlike the other scenarios, this threat is already recognized by infrastructure experts.
• Clean Air Warming: Global averages could also be increased by about 0.75C if pollution from industry is rapidly cleaned up. This unexpected result occurs because pollution particles from industrial activity scatter sunlight from reaching the ground.

Planning for the Extreme

The worst-case scenarios could be used to inform the building of major projects, including new towns, power generation facilities and urban drainage systems. Knowledge about these future hazards could also accelerate the transition to reduce fossil fuel emissions.

The research formulated the worst-case scenarios using a blend of past data and records, climate models and climate physics. The scenarios omit broader worldwide issues such as the agricultural collapse and resource wars.

This work was requested as part of a official climate resilience programme. Earlier reports have warned that not enough attention was being paid to high-consequence, low-probability risks.

The government's climate advisors has said that the UK must "become resilient to moderate heating" and evaluate the threats for severe warming. Earlier government strategies have been criticised as being "very weak" by certain experts.

A government spokesperson commented: "The climate emergency is at the heart of this government’s agenda, both preparing for impacts and becoming a renewable energy leader. It is vital that the UK is prepared for the effects."