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The most severe possible consequences of the climate crisis for the UK have been mapped by experts, ranging from a searing 4C rise in temperatures to a 2-metre increase in sea levels. An alternative projection predicts a plunge of 6C in temperatures following the shutdown of key Atlantic Ocean currents, which would profoundly upset agriculture and energy needs.

Possible Extremes, Not Predictions

These effects, a number of which are linked to irreversible climate shifts, are viewed as improbable but possible. The scientists stated that these scenarios plugged a hole in planning that had rendered the UK unprepared for extreme outcomes.

"The severe weather events we have outlined are not forecasts, but they are credible," stated Nigel Arnell who directed the research. "Britain has been planning without the methods to assess catastrophic outcomes. We've now provided decision-makers what they need to plan for environmental results they wish to avoid, but can’t afford to ignore."

A Suite of Harsh Climate Scenarios

A second set of dire projections sets out the potential extent of extreme weather between now and the end of the century. These show that temperatures could skyrocket in particular seasons by up to 6C above average, while rainfall could be triple normal levels.

The chance of the catastrophic events occurring remains uncertain due to unknown factors about what steps will be implemented to address climate change and how the planet's climate will react. This analysis was described as similar to defence planning or scenario planning for economic stability.

Critical Possibility Highlights

• Severe Heating: Worldwide averages exceeding 4C by 2100 could occur if emissions cuts breaks down or if powerful amplifying effects including the Amazon rainforest dying emit its enormous store of carbon. This would lead to extreme and prolonged heatwaves and droughts hitting the UK in summer.
• Atlantic Current Collapse: A key circulation system, the AMOC, is weakening and becoming unstable due to global heating. A full shutdown commencing by 2030 would lead to a six-degree temperature drop in the UK. "Crop production would struggle enormously and water resources would be fundamentally reshaped," noted Arnell.
• Rapid Sea Level Rise: A surge of 2.0-2.2 metres around the UK by 2100 could occur if polar ice in Greenland and Antarctica collapse rapidly, submerging seaside communities. Unlike the other scenarios, this threat is already known by coastal engineers.
• Pollution Paradox: Temperatures could also be driven up by about 0.75C if pollution from industry is rapidly cleaned up. This unexpected result occurs because aerosol particles from burning coal block sunlight from reaching the ground.

Planning for the Unthinkable

The worst-case scenarios could be applied to guide the development of long-term infrastructure, including new towns, nuclear power stations and water management networks. Awareness of these future hazards could also hasten the push to slash carbon pollution.

The research developed the worst-case scenarios using a mix of past data and records, digital projections and climate physics. These outcomes do not take into account broader worldwide issues such as the global food shortages and geopolitical instability.

The research was requested as part of a official risk assessment framework. Earlier reports have cautioned that not enough attention was being paid to low‐likelihood but high‐impact risks.

An independent advisory body has stated that the UK needs to "prepare for two degrees of warming" and examine the threats for 4C. Past policy documents have been labeled being "inadequate" by environmental groups.

An official statement said: "Climate change is at the heart of this government’s agenda, both adapting for the future and becoming a clean energy superpower. It is essential that the UK is ready for the consequences."