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The most severe possible consequences of the global heating for the UK have been outlined by researchers, ranging from a scorching 4C jump in mean temperatures to a two-metre increase in sea levels. An alternative projection foresees a plunge of 6C in temperature following the failure of key Atlantic Ocean currents, which would massively disrupt agriculture and power requirements.

Credible Events, Not Forecasts

These impacts, some of which are associated with critical climate thresholds, are considered as improbable but plausible. The researchers stated that these models addressed a shortfall in planning that had rendered the UK unprepared for extreme outcomes.

"The climate extremes we have outlined aren’t predictions, but they are possible," commented Nigel Arnell who led the research. "The UK has been formulating policy in the absence of the frameworks to assess worst-case scenarios. We've now provided decision-makers what they need to prepare for climate outcomes they hope never happen, but dare not dismiss."

A Spectrum of Extreme Weather Possibilities

A second set of worst-case scenarios details the possible scale of severe conditions between now and the close of the 2100s. These show that thermometer readings could soar in certain periods by up to 6C higher than average, while rainfall could be three times normal levels.

The probability of the catastrophic events happening remains uncertain due to uncertainty about what measures will be enacted to tackle global warming and how the planet's climate will behave. This research was likened to national security risk assessments or stress tests for financial systems.

Critical Possibility Elements

• 4C Temperature Rise: Planetary heating soaring past 4C by 2100 could transpire if climate action breaks down or if powerful amplifying effects including the tropical forests dying emit its enormous store of carbon. This would result in extreme and prolonged heatwaves and droughts plaguing the UK in summer.
• Ocean Circulation Failure: A key circulation system, the North Atlantic current, is losing strength and showing signs of collapse due to global heating. A complete failure starting around 2030 would lead to a six-degree temperature drop in the UK. "Farming would struggle enormously and water resources would be completely altered," noted Arnell.
• Accelerated Coastal Flooding: A surge of 2.0-2.2 metres around the UK by 2100 could take place if ice sheets in Greenland and Antarctica melt abruptly, inundating low-lying areas. In contrast to the other scenarios, this threat is already known by infrastructure experts.
• Pollution Paradox: Global averages could also be increased by about 0.75C if pollution from industry is sharply cut. This counterintuitive effect occurs because tiny specks from industrial activity scatter solar radiation from warming the ground.

Planning for the Unthinkable

The worst-case scenarios could be utilized in planning the building of critical national infrastructure, for example expanded cities, nuclear power stations and water management networks. Understanding these climate risks could also hasten the transition to reduce greenhouse gas emissions.

The analysis developed the worst-case scenarios using a mix of observed and historical experience, computer simulations and established principles. The scenarios exclude international cascading effects such as the agricultural collapse and increased conflict.

The research was commissioned as part of a government’s risk assessment framework. Previous assessments have cautioned that not enough attention was being allocated to low‐likelihood but high‐impact risks.

Official experts has said that the UK needs to "prepare for two degrees of warming" and assess the dangers for four degrees. Past policy documents have been labeled being "inadequate" by certain experts.

A national response commented: "The climate emergency is at the center of this government’s agenda, both building resilience and transitioning to a low-carbon economy. It is essential that the UK is ready for the effects."