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The gravest potential impacts of the climate crisis for the UK have been mapped by scientists, ranging from a blistering 4C rise in mean temperatures to a 6.5-foot rise in sea levels. Another scenario foresees a drop of 6C in temperature following the collapse of key Atlantic Ocean currents, which would profoundly upset farming and energy needs.

Possible Outcomes, Not Predictions

These effects, certain ones are associated with climate tipping points, are considered as unlikely but plausible. The researchers said that these models addressed a shortfall in planning that had left the UK ill-prepared for catastrophic possibilities.

"The severe weather events we have detailed aren’t predictions, but they are plausible," said the professor who led the research. "Britain has been formulating policy lacking the methods to test against extreme possibilities. We've now given decision-makers what they need to plan for environmental results they pray never occur, but dare not dismiss."

A Suite of Extreme Weather Possibilities

A further series of worst-case scenarios details the possible scale of extreme weather between now and the close of the 2100s. These suggest that temperatures could soar in some months by up to 6C above historical levels, while precipitation could be 300% of typical amounts.

The likelihood of the worst-case outcomes occurring remains uncertain due to uncertainty about what measures will be taken to combat climate change and how the Earth's systems will respond. This analysis was likened to national security risk assessments or resilience checks for financial systems.

Major Possibility Highlights

• Four-Degree Warming: Worldwide averages rising well above 4C by 2100 could happen if international cooperation fails or if strong feedback loops such as the Amazon basin perishing emit its enormous store of carbon. This would lead to severe and lengthy heat domes and water shortages hitting the UK in summer.
• AMOC Shutdown: A major ocean current, the Atlantic meridional overturning circulation (Amoc), is losing strength and showing signs of collapse due to ocean warming. A complete failure starting around 2030 would lead to a 6C cooling in the UK. "Farming would be severely challenged and freshwater availability would be fundamentally reshaped," said the lead researcher.
• Rapid Sea Level Rise: A rise of 2.0-2.2 metres around the UK by 2100 could happen if ice sheets in Greenland and Antarctica melt abruptly, inundating low-lying areas. In contrast to the other scenarios, this risk is already recognized by planners.
• Aerosol Effect: Temperatures could also be driven up by about 0.75C if industrial aerosols is sharply cut. This counterintuitive effect occurs because tiny specks from burning coal reflect solar radiation from warming the ground.

Planning for the Unthinkable

The severe models could be applied to guide the development of major projects, such as housing developments, energy plants and urban drainage systems. Knowledge about these future hazards could also speed the transition to reduce greenhouse gas emissions.

The analysis developed the extreme models using a combination of observed and historical experience, computer simulations and established principles. These outcomes do not take into account international cascading effects such as the devastation of food supplies and increased conflict.

This work was initiated as part of a official risk assessment framework. Independent analyses have cautioned that not enough attention was being paid to catastrophic yet improbable risks.

An independent advisory body has said that the UK must "adapt to 2C" and assess the dangers for 4C. Past policy documents have been criticised as being "inadequate" by environmental groups.

A national response commented: "Global warming is at the heart of this government’s agenda, both preparing for impacts and becoming a renewable energy leader. It is crucial that the UK is resilient for the impacts."