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The direst possible consequences of the climate crisis for the UK have been detailed by experts, encompassing a searing 4C jump in average temperatures to a two-metre rise in sea level. A separate possibility predicts a plunge of 6C in temperatures following the failure of key Atlantic Ocean currents, which would profoundly upset food production and power requirements.

Credible Outcomes, Not Guarantees

These consequences, a number of which are connected to critical climate thresholds, are considered as low probability but possible. The scientists stated that these scenarios plugged a hole in planning that had left the UK unprepared for extreme outcomes.

"The climate extremes we have outlined are not forecasts, but they are credible," commented the lead researcher who oversaw the study. "The UK has been formulating policy in the absence of the methods to assess extreme possibilities. We've now supplied decision-makers what they need to prepare for future conditions they hope never happen, but must not overlook."

A Range of Extreme Weather Projections

A further series of worst-case scenarios sets out the projected severity of severe conditions between now and the end of the century. These show that temperatures could soar in particular seasons by up to 6C higher than the norm, while precipitation could be triple typical amounts.

The chance of the worst-case outcomes occurring could not be calculated due to uncertainty about what measures will be enacted to tackle global heating and how the Earth's systems will respond. This approach was described as similar to security threat evaluations or stress tests for financial systems.

Key Scenario Elements

• 4C Temperature Rise: Global temperatures exceeding 4C by 2100 could transpire if international cooperation breaks down or if vicious cycles including the tropical forests dying emit its enormous store of carbon. This would lead to severe and lengthy heat domes and water shortages plaguing the UK in summer.
• Ocean Circulation Failure: A critical sea current, the AMOC, is losing strength and showing signs of collapse due to ocean warming. A collapse commencing by 2030 would lead to a drastic cooling in the UK. "Agriculture would be severely challenged and water resources would be drastically changed," noted Arnell.
• Accelerated Coastal Flooding: A rise of 2.0-2.2 metres around the UK by 2100 could happen if glaciers in the Arctic and Antarctic disintegrate quickly, inundating low-lying areas. Differing from the other scenarios, this risk is already understood by planners.
• Aerosol Effect: Global averages could also be pushed higher by about 0.75C if airborne particulates is sharply cut. This counterintuitive effect occurs because tiny specks from industrial activity scatter solar radiation from warming the ground.

Preparing for the Extreme

The severe models could be used to inform the building of long-term infrastructure, such as housing developments, power generation facilities and flood defences. Awareness of these environmental threats could also hasten the transition to slash carbon pollution.

The analysis developed the worst-case scenarios using a mix of observed and historical experience, digital projections and climate physics. The possibilities do not take into account international cascading effects including the devastation of food supplies and resource wars.

The project was commissioned as part of a official climate resilience programme. Previous assessments have stated that too little consideration was being given to low‐likelihood but high‐impact risks.

An independent advisory body has stated that the UK must "prepare for two degrees of warming" and evaluate the dangers for 4C. Past policy documents have been criticised as being "insufficient" by some observers.

A government spokesperson noted: "The climate emergency is at the core of this government’s agenda, both preparing for impacts and transitioning to a clean energy superpower. It is crucial that the UK is ready for the impacts."