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The gravest potential impacts of the climate crisis for the UK have been mapped by experts, encompassing a blistering 4C jump in mean temperatures to a 2-metre increase in ocean levels. An alternative projection predicts a drop of 6C in temperatures following the shutdown of critical Atlantic Ocean currents, which would severely impact agriculture and energy needs.

Credible Outcomes, Not Guarantees

These effects, a number of which are connected to critical climate thresholds, are viewed as improbable but possible. The team explained that these models addressed a shortfall in forecasting that had left the UK not ready for extreme outcomes.

"The worst-case scenarios we have detailed aren’t predictions, but they are credible," commented Nigel Arnell who oversaw the research. "The nation has been formulating policy in the absence of the tools to plan for extreme possibilities. We've now given planners what they need to get ready for environmental results they hope never happen, but must not overlook."

A Suite of Severe Conditions Projections

A second set of worst-case scenarios sets out the projected severity 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 average, while rainfall could be 300% of normal levels.

The chance of the worst-case outcomes happening could not be calculated due to uncertainty about what measures will be enacted to tackle climate change and how the Earth's systems will react. This analysis was likened to national security risk assessments or resilience checks for the banking sector.

Major Outcome Details

• Four-Degree Warming: Planetary heating rising well above 4C by 2100 could occur if international cooperation breaks down or if strong feedback loops like the Amazon basin perishing emit its vast reserves of carbon. This would cause extreme and prolonged heatwaves and dry spells plaguing the UK in summer.
• Ocean Circulation Failure: A critical sea current, the North Atlantic current, is weakening and showing signs of collapse due to climate change. A complete failure beginning in 2030 would lead to a drastic cooling in the UK. "Agriculture would be severely challenged and freshwater availability would be drastically changed," said the lead researcher.
• Sudden Ocean Inundation: A rise of 2.0-2.2 metres around the UK by 2100 could take place if ice sheets in the Arctic and Antarctic melt abruptly, submerging seaside communities. Differing from the other scenarios, this possibility is already understood by infrastructure experts.
• Aerosol Effect: Temperatures could also be pushed higher by about 0.75C if pollution from industry is rapidly cleaned up. This paradoxical outcome occurs because tiny specks from fossil fuel combustion reflect sunlight from warming the ground.

Planning for the Unthinkable

The severe models could be applied to guide the building of major projects, for example expanded cities, power generation facilities and water management networks. Knowledge about these environmental threats could also hasten the push to reduce carbon pollution.

The analysis created the extreme models using a blend of past data and records, computer simulations and established principles. These outcomes do not take into account potential global problems including the devastation of food supplies and increased conflict.

The project was commissioned as part of a government’s adaptation planning initiative. Previous assessments have cautioned that insufficient focus was being paid to low‐likelihood but high‐impact risks.

An independent advisory body has stated that the UK must "prepare for two degrees of warming" and assess the dangers for 4C. Earlier government strategies have been criticised as being "inadequate" by some observers.

A national response said: "Global warming is at the core of this government’s agenda, both adapting for the future and establishing itself as a clean energy superpower. It is essential that the UK is ready for the effects."