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The most severe projected effects of the global heating for the UK have been detailed by scientists, ranging from a scorching 4C increase in average temperatures to a 2-metre rise in sea level. Another scenario predicts a decline of 6C in temperature following the failure of key Atlantic Ocean currents, which would profoundly upset food production and energy demand.

Possible Outcomes, Not Forecasts

These consequences, some of which are linked to irreversible climate shifts, are considered as improbable but plausible. The team said that these models plugged a hole in planning that had left the UK ill-prepared for catastrophic possibilities.

"The worst-case scenarios we have mapped aren’t predictions, but they are plausible," said the professor who directed the research. "The nation has been planning without the tools to plan for extreme possibilities. We've now given decision-makers what they need to prepare for climate outcomes they hope never happen, but dare not dismiss."

A Suite of Severe Conditions Projections

A further series of worst-case scenarios details the possible scale of extreme weather between now and the end of the century. These suggest that temperatures could rocket in certain periods by up to 6C above average, while precipitation could be triple standard measurements.

The likelihood of the worst-case outcomes happening could not be calculated due to uncertainty about what steps will be enacted to address climate change and how the planet's climate will react. This approach was likened to defence planning or stress tests for the banking sector.

Major Outcome Details

• Four-Degree Warming: Global temperatures rising well above 4C by 2100 could occur if international cooperation fails or if strong feedback loops such as the tropical forests becoming a net carbon source emit its huge repository of carbon. This would lead to intense and enduring heat domes and dry spells affecting the UK in summer.
• Ocean Circulation Failure: A major ocean current, the Atlantic meridional overturning circulation (Amoc), is weakening and becoming unstable due to global heating. A full shutdown beginning in 2030 would lead to a 6C cooling in the UK. "Farming would be severely challenged and water supplies would be fundamentally reshaped," said the lead researcher.
• Accelerated Coastal Flooding: A surge of 2.0-2.2 metres around the UK by 2100 could take place if ice sheets in Greenland and Antarctica melt abruptly, flooding seaside communities. Unlike the other scenarios, this threat is already known by infrastructure experts.
• Pollution Paradox: Temperatures could also be pushed higher by about 0.75C if pollution from industry is rapidly cleaned up. This counterintuitive effect occurs because aerosol particles from industrial activity block solar radiation from warming the ground.

Preparing for the Extreme

The severe models could be applied to guide the building of critical national infrastructure, for example new towns, energy plants and water management networks. Understanding these environmental threats could also accelerate the drive to slash fossil fuel emissions.

The analysis developed the worst-case scenarios using a blend of past data and records, digital projections and climate physics. The possibilities omit international cascading effects like the global food shortages and geopolitical instability.

The research was requested as part of a national climate resilience programme. Earlier reports have stated that insufficient focus was being allocated to catastrophic yet improbable risks.

Official experts has stated that the UK needs to "prepare for two degrees of warming" and examine the threats for severe warming. Past policy documents have been labeled being "inadequate" by environmental groups.

A government spokesperson said: "Climate change is at the heart of the country's focus, both building resilience and establishing itself as a clean energy superpower. It is essential that the UK is prepared for the consequences."