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The most severe possible consequences of the climate emergency for the UK have been detailed by scientists, ranging from a blistering 4C rise in mean temperatures to a 6.5-foot surge in sea level. Another scenario predicts a decline of 6C in temperature following the shutdown of crucial Atlantic Ocean currents, which would massively disrupt farming and energy demand.

Credible Events, Not Forecasts

These impacts, some of which are linked to irreversible climate shifts, are viewed as unlikely but credible. The team said that these models addressed a shortfall in forecasting that had rendered the UK ill-prepared for catastrophic possibilities.

"The climate extremes we have detailed are not forecasts, but they are possible," said Nigel Arnell who led the analysis. "The nation has been formulating policy in the absence of the frameworks to test against extreme possibilities. We've now given decision-makers what they need to prepare for environmental results they hope never happen, but can’t afford to ignore."

A Suite of Severe Conditions Projections

A further series of dire projections sets out the possible scale of extreme weather between now and the year 2100. These show that thermometer readings could skyrocket in particular seasons by up to 6C above the norm, while precipitation could be triple standard measurements.

The likelihood of the worst-case outcomes happening remains uncertain due to uncertainty about what steps will be taken to tackle climate change and how the climate system will behave. This research was described as similar to defence planning or resilience checks for financial systems.

Major Scenario Details

• 4C Temperature Rise: Global temperatures rising well above 4C by 2100 could occur if climate action breaks down or if vicious cycles including the tropical forests dying emit its huge repository of carbon. This would result in severe and lengthy heat domes and water shortages hitting the UK in summer.
• Atlantic Current Collapse: A critical sea current, the North Atlantic current, is losing strength and losing stability due to climate change. A collapse starting around 2030 would lead to a six-degree temperature drop in the UK. "Farming would be severely challenged and freshwater availability would be fundamentally reshaped," said the professor.
• Accelerated Coastal Flooding: A surge of 2.0-2.2 metres around the UK by 2100 could happen if polar ice in Greenland and Antarctica collapse rapidly, submerging seaside communities. Differing from the other scenarios, this risk is already understood by planners.
• Aerosol Effect: Temperatures could also be pushed higher by about 0.75C if airborne particulates is rapidly cleaned up. This counterintuitive effect occurs because pollution particles from industrial activity scatter sunlight from warming the ground.

Planning for the Extreme

The severe models could be applied to guide the building of critical national infrastructure, for example expanded cities, energy plants and flood defences. Awareness of these future hazards could also hasten the drive to reduce carbon pollution.

The research created the worst-case scenarios using a mix of observed and historical experience, computer simulations and climate physics. The possibilities omit international cascading effects including the agricultural collapse and increased conflict.

The research was requested as part of a government’s risk assessment framework. Earlier reports have cautioned that not enough attention was being given to low‐likelihood but high‐impact risks.

The government's climate advisors has said that the UK must "prepare for two degrees of warming" and assess the threats for severe warming. Past policy documents have been criticised as being "insufficient" by certain experts.

A national response commented: "Global warming is at the core of this government’s agenda, both building resilience and becoming a low-carbon economy. It is crucial that the UK is resilient for the impacts."