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The direst potential impacts of the climate emergency for the UK have been outlined by experts, including a blistering 4C jump in average temperatures to a two-metre increase in sea level. An alternative projection predicts a plunge of 6C in temperatures following the shutdown of critical Atlantic Ocean currents, which would severely impact agriculture and energy demand.

Possible Extremes, Not Guarantees

These impacts, a number of which are connected to critical climate thresholds, are considered as improbable but credible. The team said that these projections addressed a shortfall in planning that had left the UK ill-prepared for worst-case results.

"The severe weather events we have detailed should not be seen as certainties, but they are credible," commented the professor who directed the research. "Britain has been making preparations lacking the tools to assess extreme possibilities. We've now provided decision-makers what they need to plan for future conditions they pray never occur, but dare not dismiss."

A Range of Extreme Weather Projections

Another collection of worst-case scenarios sets out the possible scale of extreme weather between now and the close of the 2100s. These show that temperatures could rocket in certain periods by up to 6C above historical levels, while precipitation could be 300% of typical amounts.

The chance of the catastrophic events happening cannot be precisely determined due to uncertainty about what action will be taken to address global warming and how the Earth's systems will react. This research was likened to security threat evaluations or stress tests for financial systems.

Critical Outcome Highlights

• 4C Temperature Rise: Planetary heating exceeding 4C by 2100 could transpire if international cooperation fails or if powerful amplifying effects such as the Amazon rainforest becoming a net carbon source emit its huge repository of carbon. This would result in extreme and prolonged heatwaves and water shortages hitting the UK in summer.
• Atlantic Current Collapse: A key circulation system, the Atlantic meridional overturning circulation (Amoc), is slowing down and becoming unstable due to global heating. A complete failure commencing by 2030 would lead to a six-degree temperature drop in the UK. "Farming would be severely challenged and water supplies would be drastically changed," said Arnell.
• Accelerated Coastal Flooding: A rise of 2.0-2.2 metres around the UK by 2100 could take place if ice sheets in the polar regions melt abruptly, flooding low-lying areas. Differing from the other scenarios, this threat is already recognized by planners.
• Clean Air Warming: Global averages could also be pushed higher by about 0.75C if industrial aerosols is rapidly cleaned up. This unexpected result occurs because aerosol particles from burning coal block solar radiation from reaching the ground.

Planning for the Worst

The severe models could be applied to guide the construction of long-term infrastructure, for example expanded cities, nuclear power stations and flood defences. Knowledge about these future hazards could also speed the push to slash greenhouse gas emissions.

The research formulated the extreme models using a blend of observed and historical experience, climate models and established principles. These outcomes omit international cascading effects like the devastation of food supplies and increased conflict.

The project was requested as part of a official climate resilience programme. Independent analyses have cautioned that not enough attention was being allocated to high-consequence, low-probability risks.

The government's climate advisors has recommended that the UK needs to "prepare for two degrees of warming" and assess the risks for four degrees. Earlier government strategies have been described as being "very weak" by some observers.

A government spokesperson noted: "The climate emergency is at the heart of our national priorities, both preparing for impacts and establishing itself as a clean energy superpower. It is essential that the UK is prepared for the effects."