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The most severe projected effects of the global heating for the UK have been mapped by scientists, encompassing a blistering 4C rise in mean temperatures to a 6.5-foot increase in sea level. A separate possibility predicts a decline of 6C in temperatures following the shutdown of crucial Atlantic Ocean currents, which would severely impact agriculture and energy demand.

Credible Outcomes, Not Predictions

These consequences, some of which are connected to critical climate thresholds, are seen as low probability but possible. The team stated that these models plugged a hole in risk assessment that had left the UK ill-prepared for extreme outcomes.

"The worst-case scenarios we have mapped aren’t predictions, but they are possible," stated Nigel Arnell who directed the analysis. "The nation has been making preparations without the frameworks to assess worst-case scenarios. We've now given planners what they need to plan for climate outcomes they hope never happen, but can’t afford to ignore."

A Spectrum of Harsh Climate Projections

A further series of severe models sets out the projected severity of extreme weather between now and the close of the 2100s. These show that thermometer readings could skyrocket in certain periods by up to 6C higher than historical levels, while precipitation could be 300% of standard measurements.

The likelihood of the worst-case outcomes occurring remains uncertain due to a lack of certainty about what action will be implemented to address global warming and how the Earth's systems will behave. This approach was likened to security threat evaluations or resilience checks for the banking sector.

Critical Scenario Highlights

• Four-Degree Warming: Worldwide averages rising well above 4C by 2100 could transpire if international cooperation fails or if vicious cycles like the tropical forests becoming a net carbon source release its vast reserves of carbon. This would cause intense and enduring heatwaves and droughts plaguing the UK in summer.
• Ocean Circulation Failure: A key circulation system, the AMOC, is slowing down and showing signs of collapse due to ocean warming. A full shutdown commencing by 2030 would lead to a 6C cooling in the UK. "Crop production would be severely challenged and water resources would be drastically changed," noted the lead researcher.
• Rapid Sea Level Rise: A rise of 2.0-2.2 metres around the UK by 2100 could occur if ice sheets in Greenland and Antarctica collapse rapidly, inundating seaside communities. In contrast to the other scenarios, this threat is already recognized by coastal engineers.
• Aerosol Effect: Temperatures could also be driven up by about 0.75C if airborne particulates is drastically reduced. This counterintuitive effect occurs because tiny specks from industrial activity reflect solar radiation from reaching the ground.

Planning for the Worst

The severe models could be utilized in planning the construction of major projects, including housing developments, energy plants and flood defences. Understanding these future hazards could also hasten the push to reduce fossil fuel emissions.

The analysis developed the extreme models using a mix of observed and historical experience, computer simulations and climate physics. The possibilities omit broader worldwide issues such as the agricultural collapse and increased conflict.

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

An independent advisory body has stated that the UK must "prepare for two degrees of warming" and examine the dangers for four degrees. Past policy documents have been criticised as being "inadequate" by certain experts.

A national response said: "Climate change is at the heart of our national priorities, both preparing for impacts and becoming a low-carbon economy. It is crucial that the UK is resilient for the consequences."