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The most severe potential impacts of the climate emergency for the UK have been mapped by experts, including a searing 4C jump in average temperatures to a 2-metre increase in sea levels. An alternative projection foresees a plunge of 6C in temperatures following the shutdown of critical Atlantic Ocean currents, which would severely impact food production and power requirements.

Possible Extremes, Not Guarantees

These impacts, some of which are connected to critical climate thresholds, are viewed as unlikely but possible. The scientists explained that these scenarios plugged a hole in planning that had rendered the UK unprepared for extreme outcomes.

"The climate extremes we have detailed aren’t predictions, but they are plausible," commented the professor who oversaw the study. "The UK has been making preparations without the frameworks to assess worst-case scenarios. We've now provided policymakers what they need to prepare for future conditions they wish to avoid, but must not overlook."

A Spectrum of Severe Conditions Projections

A second set of worst-case scenarios maps the potential extent of extreme weather between now and the end of the century. These show that temperatures could rocket in some months by up to 6C above historical levels, while rainfall could be three times standard measurements.

The chance of the extreme scenarios coming to pass cannot be precisely determined due to unknown factors about what measures will be implemented to tackle climate change and how the Earth's systems will behave. This approach was described as similar to defence planning or scenario planning for financial systems.

Major Outcome Highlights

• 4C Temperature Rise: Planetary heating soaring past 4C by 2100 could occur if climate action collapses or if strong feedback loops like the Amazon basin becoming a net carbon source release its enormous store of carbon. This would cause severe and lengthy heat domes and water shortages affecting the UK in summer.
• Atlantic Current Collapse: A major ocean current, the Atlantic meridional overturning circulation (Amoc), is losing strength and showing signs of collapse due to global heating. A collapse starting around 2030 would lead to a drastic cooling in the UK. "Crop production would struggle enormously and freshwater availability would be fundamentally reshaped," noted the lead researcher.
• Rapid Sea Level Rise: A surge of 2.0-2.2 metres around the UK by 2100 could take place if ice sheets in Greenland and Antarctica disintegrate quickly, submerging low-lying areas. Unlike the other scenarios, this possibility is already known by planners.
• Clean Air Warming: Global averages could also be driven up by about 0.75C if industrial aerosols is rapidly cleaned up. This counterintuitive effect occurs because aerosol particles from industrial activity reflect solar radiation from warming the ground.

Planning for the Unthinkable

The worst-case scenarios could be utilized in planning the construction of major projects, such as expanded cities, nuclear power stations and flood defences. Knowledge about these environmental threats could also accelerate the transition to reduce carbon pollution.

The study formulated the extreme models using a mix of observed and historical experience, digital projections and scientific theory. The scenarios exclude potential global problems like the devastation of food supplies and resource wars.

The research was commissioned as part of a national risk assessment framework. Earlier reports have stated that too little consideration was being allocated to catastrophic yet improbable risks.

An independent advisory body has recommended that the UK needs to "prepare for two degrees of warming" and evaluate the risks for 4C. Previous adaptation plans have been criticised as being "insufficient" by certain experts.

A national response said: "Global warming is at the core of this government’s agenda, both adapting for the future and becoming a low-carbon economy. It is essential that the UK is prepared for the impacts."