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The direst possible consequences of the global heating for the UK have been mapped by scientists, encompassing a searing 4C jump in average temperatures to a 2-metre increase in ocean levels. Another scenario predicts a decline of 6C in temperature following the collapse of key Atlantic Ocean currents, which would severely impact farming and power requirements.

Credible Events, Not Guarantees

These consequences, a number of which are linked to critical climate thresholds, are viewed as low probability but credible. The team said that these scenarios filled a gap in forecasting that had rendered the UK ill-prepared for extreme outcomes.

"The climate extremes we have outlined are not forecasts, but they are plausible," commented the lead researcher who directed the analysis. "Britain has been making preparations in the absence of the methods to plan for extreme possibilities. We've now provided policymakers what they need to get ready for climate outcomes they pray never occur, but must not overlook."

A Spectrum of Harsh Climate Scenarios

A second set of worst-case scenarios sets out the possible scale of harsh climate between now and the year 2100. These show that temperatures could soar in some months by up to 6C above historical levels, while precipitation could be 300% of typical amounts.

The probability of the extreme scenarios coming to pass could not be calculated due to uncertainty about what steps will be taken to tackle global heating and how the Earth's systems will behave. This approach was likened to defence planning or stress tests for financial systems.

Critical Scenario Details

• 4C Temperature Rise: Planetary heating rising well above 4C by 2100 could transpire if international cooperation breaks down or if strong feedback loops including the Amazon rainforest becoming a net carbon source emit its huge repository of carbon. This would result in intense and enduring heatwaves and dry spells hitting the UK in summer.
• Ocean Circulation Failure: A key circulation system, the AMOC, is losing strength and showing signs of collapse due to global heating. A full shutdown beginning in 2030 would lead to a 6C cooling in the UK. "Farming would struggle enormously and freshwater availability would be completely altered," explained the professor.
• Accelerated Coastal Flooding: A rise of 2.0-2.2 metres around the UK by 2100 could happen if ice sheets in the Arctic and Antarctic melt abruptly, inundating coastal cities and towns. Unlike the other scenarios, this risk is already known by planners.
• Aerosol Effect: Global averages could also be driven up by about 0.75C if industrial aerosols is sharply cut. This paradoxical outcome occurs because aerosol particles from burning coal reflect sunlight from reaching the ground.

Planning for the Extreme

These dire projections could be used to inform the development of critical national infrastructure, for example new towns, energy plants and water management networks. Awareness of these climate risks could also accelerate the push to cut carbon pollution.

The research developed the severe projections using a mix of observed and historical experience, digital projections and scientific theory. The scenarios do not take into account broader worldwide issues like the agricultural collapse and geopolitical instability.

The research was commissioned as part of a official adaptation planning initiative. Independent analyses have stated that too little consideration was being given to high-consequence, low-probability risks.

An independent advisory body has recommended that the UK must "become resilient to moderate heating" and examine the dangers for 4C. Previous adaptation plans have been described as being "insufficient" by certain experts.

A government spokesperson noted: "The climate emergency is at the center of our national priorities, both building resilience and transitioning to a renewable energy leader. It is vital that the UK is ready for the impacts."