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The gravest potential impacts of the climate emergency for the UK have been detailed by experts, ranging from a blistering 4C rise in average temperatures to a 6.5-foot rise in sea levels. Another scenario predicts a plunge of 6C in temperatures following the collapse of critical Atlantic Ocean currents, which would severely impact food production and power requirements.

Plausible Extremes, Not Forecasts

These consequences, certain ones are associated with critical climate thresholds, are seen as unlikely but possible. The team stated that these scenarios filled a gap in planning that had left the UK ill-prepared for worst-case results.

"The worst-case scenarios we have detailed should not be seen as certainties, but they are plausible," stated Nigel Arnell who directed the analysis. "The UK has been making preparations in the absence of the frameworks to assess worst-case scenarios. We've now given policymakers what they need to prepare for environmental results they hope never happen, but dare not dismiss."

A Suite of Severe Conditions Possibilities

A further series of dire projections maps the potential extent 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 historical levels, while precipitation could be three times typical amounts.

The probability of the extreme scenarios coming to pass could not be calculated due to a lack of certainty about what action will be implemented to tackle climate change and how the climate system will behave. This approach was described as similar to defence planning or stress tests for economic stability.

Critical Scenario Elements

• Four-Degree Warming: Global temperatures exceeding 4C by 2100 could occur if international cooperation fails or if vicious cycles like the tropical forests dying unleash its vast reserves of carbon. This would cause extreme and prolonged heat domes and water shortages hitting the UK in summer.
• Ocean Circulation Failure: A major ocean current, the AMOC, is slowing down and becoming unstable due to ocean warming. A full shutdown starting around 2030 would lead to a drastic cooling in the UK. "Crop production would struggle enormously and water supplies would be fundamentally reshaped," said the professor.
• Accelerated Coastal Flooding: A surge of 2.0-2.2 metres around the UK by 2100 could occur if polar ice in Greenland and Antarctica melt abruptly, submerging low-lying areas. Unlike the other scenarios, this possibility is already known by coastal engineers.
• Clean Air Warming: Global averages could also be pushed higher by about 0.75C if airborne particulates is sharply cut. This counterintuitive effect occurs because aerosol particles from fossil fuel combustion block sunlight from warming the ground.

Planning for the Worst

The severe models could be utilized in planning the building of major projects, including new towns, power generation facilities and urban drainage systems. Knowledge about these climate risks could also speed the drive to reduce greenhouse gas emissions.

The study developed the worst-case scenarios using a mix of real-world events, digital projections and established principles. The scenarios do not take into account broader worldwide issues such as the agricultural collapse and increased conflict.

This work was requested as part of a government’s risk assessment framework. Previous assessments have warned that insufficient focus was being paid to low‐likelihood but high‐impact risks.

The government's climate advisors has said that the UK needs to "prepare for two degrees of warming" and assess the dangers for four degrees. Previous adaptation plans have been labeled being "very weak" by certain experts.

An official statement noted: "Climate change is at the heart of the country's focus, both building resilience and transitioning to a low-carbon economy. It is essential that the UK is prepared for the consequences."