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The direst projected effects of the climate crisis for the UK have been mapped by experts, ranging from a searing 4C increase in mean temperatures to a 6.5-foot increase in sea levels. A separate possibility envisions a plunge of 6C in temperature following the collapse of key Atlantic Ocean currents, which would severely impact food production and energy needs.

Plausible Events, Not Guarantees

These effects, some of which are associated with critical climate thresholds, are considered as unlikely but credible. The researchers explained that these projections filled a gap in risk assessment that had rendered the UK ill-prepared for extreme outcomes.

"The climate extremes we have outlined are not forecasts, but they are plausible," said the lead researcher who directed the analysis. "Britain has been formulating policy without the methods to plan for extreme possibilities. We've now provided planners what they need to plan for climate outcomes they hope never happen, but can’t afford to ignore."

A Suite of Severe Conditions Projections

A further series of severe models maps the potential extent of severe conditions between now and the year 2100. These indicate that thermometer readings could skyrocket in some months by up to 6C above average, while precipitation could be three times typical amounts.

The probability of the catastrophic events occurring remains uncertain due to a lack of certainty about what steps will be implemented to combat global heating and how the Earth's systems will react. This analysis was compared to security threat evaluations or scenario planning for economic stability.

Critical Scenario Details

• Four-Degree Warming: Planetary heating exceeding 4C by 2100 could happen if climate action fails or if powerful amplifying effects such as the tropical forests becoming a net carbon source unleash its vast reserves of carbon. This would lead to severe and lengthy heatwaves and water shortages affecting the UK in summer.
• Atlantic Current Collapse: A critical sea current, the Atlantic meridional overturning circulation (Amoc), is losing strength and becoming unstable due to global heating. A full shutdown starting around 2030 would lead to a drastic cooling in the UK. "Crop production would struggle enormously and freshwater availability would be completely altered," said Arnell.
• Rapid Sea Level Rise: A rise of 2.0-2.2 metres around the UK by 2100 could occur if glaciers in Greenland and Antarctica melt abruptly, inundating coastal cities and towns. In contrast to the other scenarios, this threat is already known by coastal engineers.
• Clean Air Warming: Global averages could also be increased by about 0.75C if pollution from industry is sharply cut. This paradoxical outcome occurs because tiny specks from burning coal block solar radiation from reaching the ground.

Planning for the Unthinkable

These dire projections could be used to inform the development of critical national infrastructure, such as new towns, power generation facilities and flood defences. Awareness of these environmental threats could also accelerate the transition to cut fossil fuel emissions.

The study created the worst-case scenarios using a combination of real-world events, computer simulations and scientific theory. The possibilities exclude broader worldwide issues like the devastation of food supplies and resource wars.

The research was commissioned as part of a government’s climate resilience programme. Independent analyses have warned that not enough attention was being allocated to low‐likelihood but high‐impact risks.

An independent advisory body has stated that the UK needs to "adapt to 2C" and assess the dangers for four degrees. Previous adaptation plans have been described as being "very weak" by environmental groups.

An official statement noted: "Global warming is at the core of this government’s agenda, both building resilience and establishing itself as a low-carbon economy. It is essential that the UK is resilient for the effects."