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The gravest potential impacts 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. An alternative projection foresees a plunge of 6C in temperature following the shutdown of crucial Atlantic Ocean currents, which would profoundly upset agriculture and energy demand.

Possible Outcomes, Not Forecasts

These effects, some of which are connected to irreversible climate shifts, are seen as low probability but possible. The scientists said that these models addressed a shortfall in forecasting that had left the UK unprepared for extreme outcomes.

"The worst-case scenarios we have detailed should not be seen as certainties, but they are credible," commented Nigel Arnell who directed the research. "The nation has been planning without the tools to test against catastrophic outcomes. We've now supplied policymakers what they need to prepare for climate outcomes they wish to avoid, but can’t afford to ignore."

A Range of Harsh Climate Projections

Another collection of worst-case scenarios maps the potential extent of severe conditions between now and the year 2100. These show that thermometer readings could soar in some months by up to 6C above the norm, while precipitation could be triple typical amounts.

The probability of the catastrophic events happening remains uncertain due to a lack of certainty about what measures will be taken to address global warming and how the planet's climate will respond. This approach was described as similar to defence planning or resilience checks for the banking sector.

Critical Outcome Highlights

• Severe Heating: Worldwide averages rising well above 4C by 2100 could transpire if international cooperation collapses or if powerful amplifying effects including the Amazon basin perishing release its vast reserves of carbon. This would result in severe and lengthy heat domes and droughts hitting the UK in summer.
• Atlantic Current Collapse: A major ocean current, the Atlantic meridional overturning circulation (Amoc), is weakening and becoming unstable due to ocean warming. A collapse beginning in 2030 would lead to a six-degree temperature drop in the UK. "Farming would face immense difficulties and water resources would be fundamentally reshaped," explained Arnell.
• Rapid Sea Level Rise: A increase of 2.0-2.2 metres around the UK by 2100 could occur if ice sheets in the Arctic and Antarctic disintegrate quickly, submerging seaside communities. Unlike the other scenarios, this possibility is already recognized by infrastructure experts.
• Aerosol Effect: Global averages could also be pushed higher by about 0.75C if airborne particulates is drastically reduced. This paradoxical outcome occurs because tiny specks from burning coal scatter solar radiation from reaching the ground.

Planning for the Extreme

These dire projections could be applied to guide the building of major projects, such as expanded cities, nuclear power stations and urban drainage systems. Understanding these climate risks could also hasten the push to slash fossil fuel emissions.

The study developed the extreme models using a mix of real-world events, climate models and established principles. These outcomes omit potential global problems such as the devastation of food supplies and resource wars.

The project was requested as part of a government’s risk assessment framework. Earlier reports have stated that too little consideration was being paid to catastrophic yet improbable risks.

Official experts has recommended that the UK needs to "become resilient to moderate heating" and examine the risks for 4C. Past policy documents have been criticised as being "inadequate" by environmental groups.

A government spokesperson noted: "Global warming is at the heart of this government’s agenda, both building resilience and transitioning to a renewable energy leader. It is essential that the UK is ready for the impacts."