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The most severe potential impacts of the climate emergency for the UK have been mapped by scientists, including a blistering 4C rise in average temperatures to a two-metre rise in ocean levels. Another scenario envisions a plunge of 6C in temperature following the collapse of critical Atlantic Ocean currents, which would profoundly upset farming and power requirements.

Credible Outcomes, Not Guarantees

These consequences, some of which are associated with critical climate thresholds, are considered as low probability but possible. The scientists explained that these scenarios filled a gap in planning that had left the UK not ready for worst-case results.

"The climate extremes we have detailed are not forecasts, but they are credible," said Nigel Arnell who directed the study. "The UK has been making preparations in the absence of the methods to assess worst-case scenarios. We've now provided planners what they need to get ready for environmental results they wish to avoid, but dare not dismiss."

A Spectrum of Harsh Climate Scenarios

A second set of dire projections details the potential extent of harsh climate between now and the end of the century. These show that thermometer readings could skyrocket in certain periods by up to 6C above average, while precipitation could be triple typical amounts.

The likelihood of the catastrophic events coming to pass cannot be precisely determined due to a lack of certainty about what steps will be taken to address global warming and how the climate system will behave. This analysis was described as similar to national security risk assessments or resilience checks for economic stability.

Major Outcome Details

• 4C Temperature Rise: Planetary heating rising well above 4C by 2100 could occur if emissions cuts fails or if vicious cycles like the tropical forests dying emit its vast reserves of carbon. This would cause extreme and prolonged heat domes and droughts hitting the UK in summer.
• AMOC Shutdown: A major ocean current, the Atlantic meridional overturning circulation (Amoc), is weakening and showing signs of collapse due to climate change. A collapse starting around 2030 would lead to a 6C cooling in the UK. "Agriculture would face immense difficulties and freshwater availability would be fundamentally reshaped," said Arnell.
• Sudden Ocean Inundation: A increase of 2.0-2.2 metres around the UK by 2100 could happen if polar ice in the Arctic and Antarctic melt abruptly, submerging low-lying areas. Differing from the other scenarios, this threat is already recognized by coastal engineers.
• Pollution Paradox: Global averages could also be pushed higher by about 0.75C if pollution from industry is sharply cut. This paradoxical outcome occurs because pollution particles from fossil fuel combustion block sunlight from warming the ground.

Planning for the Extreme

The worst-case scenarios could be used to inform the development of critical national infrastructure, such as housing developments, power generation facilities and urban drainage systems. Awareness of these climate risks could also speed the transition to reduce carbon pollution.

The analysis created the worst-case scenarios using a combination of real-world events, climate models and established principles. The scenarios exclude broader worldwide issues like the agricultural collapse and increased conflict.

This work was commissioned as part of a national risk assessment framework. Independent analyses have warned that insufficient focus was being paid to catastrophic yet improbable risks.

The government's climate advisors has recommended that the UK must "become resilient to moderate heating" and examine the risks for severe warming. Past policy documents have been criticised as being "very weak" by some observers.

A government spokesperson noted: "Climate change is at the core of this government’s agenda, both preparing for impacts and establishing itself as a clean energy superpower. It is crucial that the UK is prepared for the effects."