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The most severe projected effects of the climate crisis for the UK have been detailed by experts, encompassing a scorching 4C rise in temperatures to a two-metre increase in sea levels. A separate possibility envisions a plunge of 6C in temperature following the collapse of critical Atlantic Ocean currents, which would profoundly upset food production and energy needs.

Possible Events, Not Guarantees

These effects, certain ones are linked to irreversible climate shifts, are considered as low probability but credible. The researchers explained that these projections filled a gap in forecasting that had rendered the UK not ready for catastrophic possibilities.

"The worst-case scenarios we have outlined aren’t predictions, but they are credible," stated the professor who led the research. "The UK has been formulating policy lacking the frameworks to plan for worst-case scenarios. We've now given policymakers what they need to prepare for climate outcomes they hope never happen, but must not overlook."

A Range of Severe Conditions Scenarios

A second set of worst-case scenarios sets out the potential extent of extreme weather between now and the end of the century. These suggest that temperatures could soar in some months by up to 6C above historical levels, while precipitation could be 300% of normal levels.

The probability of the catastrophic events occurring could not be calculated due to uncertainty about what steps will be implemented to address global heating and how the climate system will respond. This analysis was likened to defence planning or stress tests for the banking sector.

Major Scenario Elements

• 4C Temperature Rise: Worldwide averages soaring past 4C by 2100 could transpire if climate action breaks down or if powerful amplifying effects including the Amazon basin dying emit its enormous store of carbon. This would cause extreme and prolonged heatwaves and dry spells hitting the UK in summer.
• Ocean Circulation Failure: A key circulation system, the North Atlantic current, is weakening and showing signs of collapse due to ocean warming. A collapse commencing by 2030 would lead to a six-degree temperature drop in the UK. "Farming would face immense difficulties and freshwater availability would be completely altered," noted Arnell.
• Rapid Sea Level Rise: A rise of 2.0-2.2 metres around the UK by 2100 could happen if polar ice in Greenland and Antarctica collapse rapidly, flooding seaside communities. In contrast to the other scenarios, this risk is already understood by coastal engineers.
• Clean Air Warming: Global averages could also be driven up by about 0.75C if airborne particulates is sharply cut. This unexpected result occurs because aerosol particles from industrial activity scatter solar radiation from reaching the ground.

Preparing for the Unthinkable

These dire projections could be applied to guide the building of critical national infrastructure, such as new towns, nuclear power stations and flood defences. Awareness of these environmental threats could also hasten the transition to slash greenhouse gas emissions.

The study formulated the worst-case scenarios using a combination of observed and historical experience, computer simulations and scientific theory. The scenarios exclude international cascading effects like the devastation of food supplies and geopolitical instability.

This work was initiated as part of a official climate resilience programme. Independent analyses have cautioned that too little consideration was being allocated to low‐likelihood but high‐impact risks.

An independent advisory body has recommended that the UK needs to "adapt to 2C" and evaluate the risks for 4C. Earlier government strategies have been described as being "very weak" by environmental groups.

An official statement said: "Climate change is at the center of the country's focus, both adapting for the future and establishing itself as a low-carbon economy. It is vital that the UK is ready for the consequences."