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The gravest potential impacts of the climate crisis for the UK have been outlined by experts, including a scorching 4C rise in average temperatures to a two-metre surge in sea level. An alternative projection foresees a drop of 6C in temperatures following the failure of key Atlantic Ocean currents, which would profoundly upset agriculture and energy demand.

Plausible Outcomes, Not Guarantees

These impacts, a number of which are associated with irreversible climate shifts, are considered as unlikely but plausible. The researchers said that these scenarios filled a gap in risk assessment that had rendered the UK not ready for worst-case results.

"The worst-case scenarios we have detailed should not be seen as certainties, but they are credible," said Nigel Arnell who led the study. "The nation has been planning in the absence of the methods to test against catastrophic outcomes. We've now supplied decision-makers what they need to plan for environmental results they wish to avoid, but dare not dismiss."

A Suite of Harsh Climate Scenarios

A further series of dire projections maps the potential extent of extreme weather between now and the close of the 2100s. These indicate that temperatures could skyrocket in particular seasons by up to 6C above historical levels, while rainfall could be triple typical amounts.

The chance of the catastrophic events occurring cannot be precisely determined due to uncertainty about what action will be enacted to combat climate change and how the planet's climate will behave. This analysis was compared to defence planning or stress tests for economic stability.

Key Possibility Elements

• 4C Temperature Rise: Worldwide averages soaring past 4C by 2100 could transpire if international cooperation breaks down or if powerful amplifying effects such as the Amazon basin becoming a net carbon source unleash its enormous store of carbon. This would cause extreme and prolonged heatwaves and dry spells affecting the UK in summer.
• Atlantic Current Collapse: A major ocean current, the Atlantic meridional overturning circulation (Amoc), is weakening and showing signs of collapse due to climate change. A full shutdown starting around 2030 would lead to a six-degree temperature drop in the UK. "Crop production would face immense difficulties and freshwater availability would be drastically changed," explained the professor.
• Rapid Sea Level Rise: A increase of 2.0-2.2 metres around the UK by 2100 could happen if polar ice in the Arctic and Antarctic melt abruptly, submerging seaside communities. In contrast to the other scenarios, this risk is already recognized by planners.
• Pollution Paradox: Global averages could also be driven up by about 0.75C if pollution from industry is rapidly cleaned up. This counterintuitive effect occurs because aerosol particles from fossil fuel combustion scatter solar radiation from reaching the ground.

Planning for the Extreme

These dire projections could be applied to guide the construction of major projects, including housing developments, nuclear power stations and water management networks. Knowledge about these climate risks could also accelerate the push to cut greenhouse gas emissions.

The study created the severe projections using a blend of real-world events, digital projections and scientific theory. The scenarios exclude international cascading effects such as the devastation of food supplies and increased conflict.

The project was commissioned as part of a national risk assessment framework. Previous assessments have warned that too little consideration was being allocated to catastrophic yet improbable risks.

The government's climate advisors has said that the UK needs to "adapt to 2C" and evaluate the threats for 4C. Previous adaptation plans have been described as being "very weak" by certain experts.

An official statement noted: "Climate change is at the heart of the country's focus, both preparing for impacts and transitioning to a renewable energy leader. It is essential that the UK is resilient for the impacts."