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The most severe possible consequences of the climate emergency for the UK have been detailed by scientists, ranging from a searing 4C rise in mean temperatures to a two-metre increase in sea level. An alternative projection envisions a drop of 6C in temperatures following the collapse of critical Atlantic Ocean currents, which would massively disrupt farming and power requirements.

Credible Events, Not Guarantees

These consequences, some of which are associated with critical climate thresholds, are seen as improbable but possible. The researchers explained that these projections filled a gap in risk assessment that had rendered the UK ill-prepared for worst-case results.

"The severe weather events we have detailed should not be seen as certainties, but they are credible," said Nigel Arnell who oversaw the study. "The UK has been formulating policy in the absence of the tools to assess worst-case scenarios. We've now supplied planners what they need to plan for climate outcomes they pray never occur, but can’t afford to ignore."

A Range of Severe Conditions Projections

Another collection of worst-case scenarios details the potential extent of severe conditions between now and the close of the 2100s. These show that thermometer readings could skyrocket in certain periods by up to 6C above the norm, while precipitation could be three times typical amounts.

The chance of the extreme scenarios happening could not be calculated due to uncertainty about what measures will be taken to tackle global heating and how the climate system will react. This approach was likened to security threat evaluations or scenario planning for economic stability.

Critical Outcome Elements

• Four-Degree Warming: Worldwide averages rising well above 4C by 2100 could transpire if emissions cuts fails or if strong feedback loops including the Amazon basin dying release its huge repository of carbon. This would result in intense and enduring heat domes and droughts hitting the UK in summer.
• Ocean Circulation Failure: A critical sea current, the AMOC, is weakening and showing signs of collapse due to global heating. A full shutdown beginning in 2030 would lead to a six-degree temperature drop in the UK. "Agriculture would face immense difficulties and water supplies would be drastically changed," noted the lead researcher.
• Rapid Sea Level Rise: A increase of 2.0-2.2 metres around the UK by 2100 could happen if ice sheets in the polar regions melt abruptly, submerging coastal cities and towns. In contrast to the other scenarios, this threat is already known by coastal engineers.
• Aerosol Effect: Global averages could also be pushed higher by about 0.75C if airborne particulates is drastically reduced. This paradoxical outcome occurs because pollution particles from fossil fuel combustion scatter solar radiation from warming the ground.

Preparing for the Worst

The severe models could be applied to guide the building of long-term infrastructure, for example housing developments, energy plants and water management networks. Understanding these future hazards could also hasten the transition to slash carbon pollution.

The study developed the severe projections using a blend of past data and records, climate models and established principles. These outcomes omit international cascading effects such as the global food shortages and increased conflict.

The research was initiated as part of a national adaptation planning initiative. Independent analyses have warned that not enough attention was being allocated to high-consequence, low-probability risks.

The government's climate advisors has stated that the UK needs to "prepare for two degrees of warming" and examine the risks for four degrees. Earlier government strategies have been criticised as being "insufficient" by certain experts.

A government spokesperson said: "Global warming is at the core of our national priorities, both building resilience and becoming a clean energy superpower. It is crucial that the UK is ready for the impacts."