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The most severe possible consequences of the climate crisis for the UK have been mapped by scientists, encompassing a blistering 4C jump in average temperatures to a 2-metre rise in sea level. A separate possibility foresees a decline of 6C in temperatures following the collapse of key Atlantic Ocean currents, which would profoundly upset food production and power requirements.

Plausible Extremes, Not Forecasts

These consequences, some of which are linked to climate tipping points, are seen as unlikely but plausible. The researchers explained that these models addressed a shortfall in planning that had left the UK unprepared for catastrophic possibilities.

"The severe weather events we have mapped are not forecasts, but they are plausible," said Nigel Arnell who directed the research. "The nation has been making preparations in the absence of the frameworks to plan for catastrophic outcomes. We've now given policymakers what they need to prepare for future conditions they pray never occur, but can’t afford to ignore."

A Spectrum of Severe Conditions Possibilities

A second set of severe models sets out the projected severity of extreme weather between now and the end of the century. These indicate that thermometer readings could soar in certain periods by up to 6C above the norm, while rainfall could be triple normal levels.

The probability of the extreme scenarios coming to pass cannot be precisely determined due to unknown factors about what measures will be enacted to combat climate change and how the planet's climate will respond. This approach was described as similar to security threat evaluations or scenario planning for the banking sector.

Critical Outcome Elements

• Severe Heating: Worldwide averages rising well above 4C by 2100 could occur if emissions cuts collapses or if strong feedback loops such as the Amazon basin becoming a net carbon source emit its enormous store of carbon. This would lead to severe and lengthy heatwaves and droughts affecting the UK in summer.
• Atlantic Current Collapse: A key circulation system, the AMOC, is slowing down and becoming unstable due to ocean warming. A complete failure commencing by 2030 would lead to a drastic cooling in the UK. "Crop production would be severely challenged and freshwater availability would be fundamentally reshaped," noted Arnell.
• Accelerated Coastal Flooding: A surge of 2.0-2.2 metres around the UK by 2100 could happen if glaciers in Greenland and Antarctica collapse rapidly, submerging low-lying areas. In contrast to the other scenarios, this risk is already understood by planners.
• Aerosol Effect: Global averages could also be pushed higher by about 0.75C if industrial aerosols is sharply cut. This paradoxical outcome occurs because aerosol particles from fossil fuel combustion reflect sunlight from warming the ground.

Building Resilience Against the Extreme

These dire projections could be applied to guide the building of long-term infrastructure, such as housing developments, power generation facilities and flood defences. Awareness of these future hazards could also speed the push to cut fossil fuel emissions.

The study formulated the severe projections using a blend of observed and historical experience, digital projections and scientific theory. The possibilities exclude potential global problems like the agricultural collapse and geopolitical instability.

The research was requested as part of a national adaptation planning initiative. Earlier reports have cautioned that too little consideration was being paid to catastrophic yet improbable risks.

The government's climate advisors has said that the UK needs to "become resilient to moderate heating" and examine the threats for four degrees. Earlier government strategies have been described as being "inadequate" by environmental groups.

A government spokesperson said: "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 prepared for the impacts."