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The most severe possible consequences of the global heating for the UK have been detailed by scientists, ranging from a blistering 4C jump in average temperatures to a 2-metre surge in sea level. A separate possibility foresees a drop of 6C in temperature following the failure of key Atlantic Ocean currents, which would profoundly upset agriculture and energy needs.

Plausible Outcomes, Not Guarantees

These effects, a number of which are connected to critical climate thresholds, are viewed as unlikely but credible. The scientists stated that these projections filled a gap in risk assessment that had rendered the UK not ready for catastrophic possibilities.

"The climate extremes we have detailed should not be seen as certainties, but they are credible," stated Nigel Arnell who directed the research. "Britain has been formulating policy in the absence of the methods to test against extreme possibilities. We've now supplied planners what they need to get ready for future conditions they pray never occur, but can’t afford to ignore."

A Range of Severe Conditions Scenarios

A further series of worst-case scenarios details the projected severity of harsh climate between now and the close of the 2100s. These show that temperatures could soar in some months by up to 6C above average, while rainfall could be three times normal levels.

The probability of the worst-case outcomes occurring could not be calculated due to uncertainty about what measures will be implemented to combat global heating and how the planet's climate will behave. This analysis was described as similar to security threat evaluations or resilience checks for the banking sector.

Key Scenario Highlights

• 4C Temperature Rise: Planetary heating exceeding 4C by 2100 could happen if climate action collapses or if powerful amplifying effects including the Amazon rainforest dying emit its vast reserves of carbon. This would cause extreme and prolonged heat domes and dry spells plaguing the UK in summer.
• Atlantic Current Collapse: A key circulation system, the Atlantic meridional overturning circulation (Amoc), is weakening and showing signs of collapse due to global heating. A complete failure beginning in 2030 would lead to a drastic cooling in the UK. "Farming would struggle enormously and freshwater availability would be completely altered," explained Arnell.
• Rapid Sea Level Rise: A surge of 2.0-2.2 metres around the UK by 2100 could take place if glaciers in the Arctic and Antarctic melt abruptly, submerging low-lying areas. In contrast to the other scenarios, this risk is already recognized by planners.
• Aerosol Effect: Temperatures could also be increased by about 0.75C if industrial aerosols is drastically reduced. This unexpected result occurs because aerosol particles from industrial activity scatter sunlight from warming the ground.

Planning for the Extreme

The severe models could be used to inform the development of long-term infrastructure, for example new towns, energy plants and water management networks. Knowledge about these climate risks could also speed the transition to slash fossil fuel emissions.

The study formulated the worst-case scenarios using a combination of past data and records, climate models and scientific theory. The scenarios do not take into account potential global problems including the agricultural collapse and geopolitical instability.

The project was initiated as part of a official adaptation planning initiative. Earlier reports have cautioned that too little consideration was being given to low‐likelihood but high‐impact risks.

Official experts has recommended that the UK must "become resilient to moderate heating" and assess the threats for 4C. Past policy documents have been criticised as being "inadequate" by environmental groups.

A government spokesperson said: "Climate change is at the core of our national priorities, both building resilience and transitioning to a low-carbon economy. It is essential that the UK is prepared for the consequences."