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The direst possible consequences of the climate emergency for the UK have been mapped by experts, including a blistering 4C rise in temperatures to a two-metre increase in sea levels. Another scenario foresees a plunge of 6C in temperatures following the failure of key Atlantic Ocean currents, which would severely impact farming and energy needs.

Plausible Extremes, Not Forecasts

These effects, some of which are linked to critical climate thresholds, are considered as unlikely but plausible. The researchers said that these scenarios plugged a hole in planning that had rendered the UK not ready for extreme outcomes.

"The severe weather events we have outlined aren’t predictions, but they are credible," said the lead researcher who oversaw the analysis. "The UK has been making preparations in the absence of the frameworks to plan for extreme possibilities. We've now supplied policymakers what they need to prepare for future conditions they hope never happen, but must not overlook."

A Spectrum of Harsh Climate Projections

A further series of dire projections maps the potential extent of harsh climate between now and the close of the 2100s. These suggest that temperatures could skyrocket in particular seasons by up to 6C higher than average, while precipitation could be triple normal levels.

The probability of the extreme scenarios occurring remains uncertain due to uncertainty about what action will be implemented to combat global warming and how the planet's climate will behave. This analysis was likened to security threat evaluations or scenario planning for financial systems.

Critical Outcome Elements

• Four-Degree Warming: Global temperatures exceeding 4C by 2100 could occur if emissions cuts collapses or if strong feedback loops including the Amazon basin dying emit its vast reserves of carbon. This would lead to intense and enduring heatwaves and water shortages hitting the UK in summer.
• Atlantic Current Collapse: A critical sea current, the Atlantic meridional overturning circulation (Amoc), is losing strength and becoming unstable due to climate change. A collapse beginning in 2030 would lead to a six-degree temperature drop in the UK. "Crop production would be severely challenged and freshwater availability would be completely altered," said Arnell.
• Accelerated Coastal Flooding: A rise of 2.0-2.2 metres around the UK by 2100 could occur if ice sheets in Greenland and Antarctica disintegrate quickly, flooding seaside communities. In contrast to the other scenarios, this possibility is already known by coastal engineers.
• Clean Air Warming: Global averages could also be increased by about 0.75C if industrial aerosols is drastically reduced. This unexpected result occurs because aerosol particles from fossil fuel combustion reflect sunlight from reaching the ground.

Preparing for the Extreme

The worst-case scenarios could be utilized in planning the construction of major projects, including housing developments, nuclear power stations and flood defences. Understanding these future hazards could also accelerate the drive to reduce fossil fuel emissions.

The analysis formulated the severe projections using a combination of real-world events, digital projections and scientific theory. These outcomes exclude broader worldwide issues such as the agricultural collapse and geopolitical instability.

This work was requested as part of a national adaptation planning initiative. Previous assessments have warned that too little consideration was being paid to low‐likelihood but high‐impact risks.

An independent advisory body has said that the UK must "adapt to 2C" and assess the dangers for severe warming. Past policy documents have been described as being "very weak" by some observers.

A government spokesperson said: "The climate emergency is at the core of this government’s agenda, both preparing for impacts and becoming a renewable energy leader. It is crucial that the UK is resilient for the impacts."