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The direst potential impacts of the global heating for the UK have been detailed by experts, including a searing 4C increase in mean temperatures to a 6.5-foot increase in ocean levels. Another scenario envisions a decline of 6C in temperature following the collapse of key Atlantic Ocean currents, which would massively disrupt food production and energy demand.

Possible Outcomes, Not Guarantees

These effects, some of which are connected to critical climate thresholds, are viewed as unlikely but credible. The team said that these models addressed a shortfall in risk assessment that had rendered the UK ill-prepared for worst-case results.

"The worst-case scenarios we have outlined should not be seen as certainties, but they are possible," stated the professor who directed the study. "The UK has been making preparations lacking the tools to plan for worst-case scenarios. We've now supplied planners what they need to prepare for environmental results they pray never occur, but dare not dismiss."

A Range of Harsh Climate Projections

A further series of severe models maps the possible scale of severe conditions between now and the end of the century. These show that thermometer readings could rocket in certain periods by up to 6C above average, while precipitation could be three times typical amounts.

The probability of the worst-case outcomes coming to pass remains uncertain due to unknown factors about what action will be enacted to tackle global heating and how the Earth's systems will respond. This approach was described as similar to security threat evaluations or resilience checks for financial systems.

Major Possibility Details

• Four-Degree Warming: Planetary heating rising well above 4C by 2100 could happen if climate action collapses or if vicious cycles including the Amazon basin perishing emit its huge repository of carbon. This would cause severe and lengthy heatwaves and water shortages plaguing the UK in summer.
• Atlantic Current Collapse: A major ocean current, the North Atlantic current, is slowing down and becoming unstable due to global heating. A full shutdown commencing by 2030 would lead to a 6C cooling in the UK. "Agriculture would struggle enormously and water resources would be fundamentally reshaped," said Arnell.
• Rapid Sea Level Rise: A rise of 2.0-2.2 metres around the UK by 2100 could happen if glaciers in Greenland and Antarctica melt abruptly, submerging low-lying areas. Differing from the other scenarios, this possibility is already known by planners.
• Clean Air Warming: Global averages could also be increased by about 0.75C if pollution from industry is drastically reduced. This counterintuitive effect occurs because tiny specks from fossil fuel combustion block sunlight from reaching the ground.

Preparing for the Unthinkable

The severe models could be utilized in planning the development of critical national infrastructure, such as housing developments, nuclear power stations and flood defences. Awareness of these future hazards could also hasten the transition to reduce greenhouse gas emissions.

The study developed the worst-case scenarios using a mix of observed and historical experience, climate models and established principles. The scenarios omit broader worldwide issues like the devastation of food supplies and resource wars.

The project was requested as part of a national adaptation planning initiative. Earlier reports have warned that too little consideration was being allocated to catastrophic yet improbable risks.

Official experts has stated that the UK must "adapt to 2C" and examine the risks for four degrees. Earlier government strategies have been criticised as being "insufficient" by certain experts.

A government spokesperson noted: "Climate change is at the heart of our national priorities, both building resilience and establishing itself as a low-carbon economy. It is vital that the UK is ready for the effects."