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The direst projected effects of the global heating for the UK have been outlined by scientists, encompassing a blistering 4C jump in average temperatures to a two-metre surge in ocean levels. An alternative projection envisions a decline of 6C in temperatures following the collapse of key Atlantic Ocean currents, which would profoundly upset farming and power requirements.

Plausible Outcomes, Not Predictions

These effects, some of which are linked to climate tipping points, are considered as improbable but possible. The researchers explained that these projections filled a gap in planning that had rendered the UK not ready for catastrophic possibilities.

"The worst-case scenarios we have mapped should not be seen as certainties, but they are plausible," stated Nigel Arnell who led the analysis. "Britain has been making preparations in the absence of the frameworks to test against catastrophic outcomes. We've now given policymakers what they need to plan for climate outcomes they pray never occur, but can’t afford to ignore."

A Suite of Harsh Climate Possibilities

A further series of severe models maps the projected severity of harsh climate between now and the end of the century. These show that temperatures could rocket in some months by up to 6C above the norm, while rainfall could be triple standard measurements.

The probability of the worst-case outcomes coming to pass remains uncertain due to uncertainty about what action will be enacted to combat global warming and how the climate system will respond. This approach was likened to national security risk assessments or scenario planning for the banking sector.

Major Scenario Highlights

• 4C Temperature Rise: Planetary heating rising well above 4C by 2100 could occur if climate action fails or if vicious cycles such as the Amazon rainforest dying emit its vast reserves of carbon. This would result in extreme and prolonged heatwaves and dry spells affecting the UK in summer.
• AMOC Shutdown: A critical sea current, the AMOC, is weakening and showing signs of collapse due to ocean warming. A full shutdown beginning in 2030 would lead to a 6C cooling in the UK. "Farming would face immense difficulties and water supplies would be completely altered," explained the lead researcher.
• Sudden Ocean Inundation: A rise of 2.0-2.2 metres around the UK by 2100 could happen if polar ice in Greenland and Antarctica collapse rapidly, submerging coastal cities and towns. In contrast to the other scenarios, this threat is already recognized by planners.
• Pollution Paradox: Global averages could also be increased by about 0.75C if pollution from industry is sharply cut. This counterintuitive effect occurs because aerosol particles from fossil fuel combustion reflect sunlight from reaching the ground.

Building Resilience Against the Extreme

The severe models could be utilized in planning the building of major projects, such as housing developments, energy plants and water management networks. Understanding these environmental threats could also speed the transition to cut greenhouse gas emissions.

The study formulated the extreme models using a blend of real-world events, climate models and climate physics. The possibilities omit broader worldwide issues including the agricultural collapse and resource wars.

This work was initiated as part of a national climate resilience programme. Independent analyses have warned that not enough attention was being allocated to catastrophic yet improbable risks.

An independent advisory body has stated that the UK must "become resilient to moderate heating" and examine the dangers for four degrees. Earlier government strategies have been described as being "insufficient" by some observers.

A government spokesperson said: "Global warming is at the center of our national priorities, both preparing for impacts and becoming a renewable energy leader. It is vital that the UK is ready for the effects."