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The most severe possible consequences of the global heating for the UK have been outlined by scientists, including a searing 4C jump in temperatures to a 6.5-foot increase in ocean levels. An alternative projection envisions a decline of 6C in temperature following the failure of critical Atlantic Ocean currents, which would massively disrupt agriculture and energy demand.

Possible Extremes, Not Guarantees

These consequences, a number of which are linked to critical climate thresholds, are seen as unlikely but credible. The team said that these models plugged a hole in planning that had rendered the UK ill-prepared for extreme outcomes.

"The worst-case scenarios we have outlined are not forecasts, but they are possible," commented Nigel Arnell who oversaw the research. "Britain has been formulating policy without the methods to test against worst-case scenarios. We've now given planners what they need to get ready for environmental results they hope never happen, but dare not dismiss."

A Range of Severe Conditions Possibilities

A further series of worst-case scenarios details the possible scale of severe conditions between now and the close of the 2100s. These show that temperatures could soar in particular seasons by up to 6C higher than average, while precipitation could be 300% of normal levels.

The probability of the extreme scenarios coming to pass remains uncertain due to uncertainty about what measures will be enacted to combat global heating and how the Earth's systems will react. This approach was compared to defence planning or stress tests for financial systems.

Critical Scenario Details

• 4C Temperature Rise: Worldwide averages exceeding 4C by 2100 could occur if climate action fails or if vicious cycles such as the Amazon basin dying emit its vast reserves of carbon. This would cause extreme and prolonged heat domes and water shortages hitting the UK in summer.
• AMOC Shutdown: A major ocean current, the Atlantic meridional overturning circulation (Amoc), is weakening and showing signs of collapse due to climate change. A collapse beginning in 2030 would lead to a six-degree temperature drop in the UK. "Farming would be severely challenged and water resources would be completely altered," noted the lead researcher.
• 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 disintegrate quickly, inundating coastal cities and towns. Unlike the other scenarios, this possibility is already understood by coastal engineers.
• Clean Air Warming: Global averages could also be pushed higher by about 0.75C if pollution from industry is drastically reduced. This counterintuitive effect occurs because tiny specks from industrial activity scatter sunlight from warming the ground.

Preparing for the Extreme

The worst-case scenarios could be used to inform the building of long-term infrastructure, for example housing developments, nuclear power stations and water management networks. Awareness of these climate risks could also hasten the push to reduce carbon pollution.

The research formulated the severe projections using a blend of real-world events, digital projections and climate physics. The scenarios exclude potential global problems including the agricultural collapse and geopolitical instability.

The project was initiated as part of a official risk assessment framework. Previous assessments have warned that too little consideration was being given to low‐likelihood but high‐impact risks.

An independent advisory body has recommended that the UK needs to "prepare for two degrees of warming" and evaluate the risks for 4C. Earlier government strategies have been described as being "very weak" by certain experts.

An official statement said: "Climate change is at the core of our national priorities, both building resilience and becoming a clean energy superpower. It is essential that the UK is prepared for the effects."