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The most severe possible consequences of the global heating for the UK have been detailed by experts, encompassing a searing 4C jump in average temperatures to a two-metre surge in sea level. Another scenario predicts a decline of 6C in temperature following the shutdown of critical Atlantic Ocean currents, which would severely impact farming and power requirements.

Possible Events, Not Forecasts

These impacts, some of which are connected to irreversible climate shifts, are considered as improbable but credible. The team said that these projections plugged a hole in planning that had left the UK not ready for catastrophic possibilities.

"The climate extremes we have detailed aren’t predictions, but they are plausible," said the professor who oversaw the study. "The UK has been planning without the tools to test against extreme possibilities. We've now given decision-makers what they need to get ready for future conditions they pray never occur, but can’t afford to ignore."

A Suite of Extreme Weather Possibilities

A further series of dire projections maps the projected severity of harsh climate between now and the year 2100. These show that temperatures could soar in certain periods by up to 6C higher than historical levels, while precipitation could be three times normal levels.

The chance of the catastrophic events coming to pass could not be calculated due to a lack of certainty about what measures will be taken to address global warming and how the planet's climate will react. This approach was described as similar to national security risk assessments or stress tests for the banking sector.

Critical Scenario Highlights

• 4C Temperature Rise: Worldwide averages soaring past 4C by 2100 could transpire if climate action breaks down or if vicious cycles including the Amazon basin perishing release its enormous store of carbon. This would cause intense and enduring heatwaves and water shortages affecting the UK in summer.
• AMOC Shutdown: A major ocean current, the North Atlantic current, is slowing down and becoming unstable due to ocean warming. A full shutdown starting around 2030 would lead to a 6C cooling in the UK. "Crop production would struggle enormously and freshwater availability would be completely altered," noted Arnell.
• Accelerated Coastal Flooding: A rise of 2.0-2.2 metres around the UK by 2100 could take place if glaciers in the Arctic and Antarctic melt abruptly, submerging coastal cities and towns. Differing from the other scenarios, this possibility is already understood by coastal engineers.
• Pollution Paradox: Global averages could also be increased by about 0.75C if pollution from industry is drastically reduced. This paradoxical outcome occurs because aerosol particles from industrial activity scatter solar radiation from reaching the ground.

Building Resilience Against the Unthinkable

The severe models could be used to inform the construction of long-term infrastructure, such as expanded cities, power generation facilities and water management networks. Understanding these environmental threats could also accelerate the push to cut fossil fuel emissions.

The study created the extreme models using a combination of observed and historical experience, digital projections and scientific theory. These outcomes exclude international cascading effects including the global food shortages and increased conflict.

This work was requested as part of a government’s climate resilience programme. Independent analyses have warned that not enough attention was being given to catastrophic yet improbable risks.

The government's climate advisors has said that the UK needs to "prepare for two degrees of warming" and assess the dangers for 4C. Earlier government strategies have been labeled being "very weak" by environmental groups.

A national response commented: "Climate change is at the center of this government’s agenda, both preparing for impacts and becoming a low-carbon economy. It is essential that the UK is prepared for the effects."