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The gravest potential impacts of the global heating for the UK have been mapped by experts, including a searing 4C jump in mean temperatures to a 6.5-foot rise in sea levels. A separate possibility envisions a plunge of 6C in temperatures following the collapse of critical Atlantic Ocean currents, which would massively disrupt farming and power requirements.

Plausible Outcomes, Not Predictions

These consequences, some of which are linked to critical climate thresholds, are viewed as unlikely but possible. The scientists stated that these projections filled a gap in risk assessment that had rendered the UK unprepared for worst-case results.

"The severe weather events we have detailed are not forecasts, but they are credible," said the lead researcher who directed the analysis. "The UK has been formulating policy without the tools to plan for catastrophic outcomes. We've now provided planners what they need to prepare for environmental results they wish to avoid, but dare not dismiss."

A Spectrum of Extreme Weather Scenarios

Another collection of worst-case scenarios maps the potential extent of severe conditions between now and the year 2100. These indicate that thermometer readings could soar in particular seasons by up to 6C higher than the norm, while precipitation could be 300% of standard measurements.

The probability of the extreme scenarios occurring could not be calculated due to uncertainty about what measures will be implemented to tackle global warming and how the planet's climate will respond. This approach was likened to security threat evaluations or stress tests for the banking sector.

Major Outcome Highlights

• 4C Temperature Rise: Planetary heating soaring past 4C by 2100 could occur if emissions cuts collapses or if strong feedback loops like the Amazon rainforest becoming a net carbon source emit its enormous store of carbon. This would cause severe and lengthy heat domes and dry spells affecting the UK in summer.
• Ocean Circulation Failure: A key circulation system, the Atlantic meridional overturning circulation (Amoc), is weakening and losing stability due to ocean warming. A complete failure starting around 2030 would lead to a six-degree temperature drop in the UK. "Crop production would face immense difficulties and water supplies would be drastically changed," said Arnell.
• Accelerated Coastal Flooding: A rise of 2.0-2.2 metres around the UK by 2100 could take place if ice sheets in Greenland and Antarctica melt abruptly, submerging seaside communities. Unlike the other scenarios, this threat is already recognized by coastal engineers.
• Aerosol Effect: Global averages could also be driven up by about 0.75C if airborne particulates is sharply cut. This counterintuitive effect occurs because tiny specks from industrial activity reflect solar radiation from reaching the ground.

Planning for the Extreme

The severe models could be utilized in planning the building of long-term infrastructure, for example expanded cities, nuclear power stations and water management networks. Knowledge about these environmental threats could also accelerate the transition to reduce fossil fuel emissions.

The analysis formulated the worst-case scenarios using a blend of real-world events, climate models and climate physics. These outcomes exclude potential global problems like the agricultural collapse and resource wars.

This work was initiated as part of a official adaptation planning initiative. Independent analyses have warned that insufficient focus was being paid to high-consequence, low-probability risks.

Official experts has recommended that the UK needs to "prepare for two degrees of warming" and assess the threats for severe warming. Previous adaptation plans have been criticised as being "insufficient" by environmental groups.

A national response said: "Global warming is at the core of the country's focus, both adapting for the future and establishing itself as a renewable energy leader. It is crucial that the UK is ready for the consequences."