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The gravest potential impacts of the climate crisis for the UK have been detailed by experts, including a scorching 4C jump in average temperatures to a 6.5-foot surge in sea level. A separate possibility predicts a drop of 6C in temperature following the failure of crucial Atlantic Ocean currents, which would massively disrupt agriculture and energy needs.

Possible Outcomes, Not Predictions

These consequences, certain ones are associated with critical climate thresholds, are viewed as improbable but credible. The scientists stated that these projections addressed a shortfall in risk assessment that had rendered the UK unprepared for worst-case results.

"The worst-case scenarios we have detailed aren’t predictions, but they are credible," commented Nigel Arnell who led the research. "Britain has been making preparations lacking the frameworks to test against extreme possibilities. We've now supplied decision-makers what they need to prepare for climate outcomes they wish to avoid, but can’t afford to ignore."

A Spectrum of Severe Conditions Projections

A second set of worst-case scenarios sets out the potential extent of extreme weather between now and the end of the century. These show that temperatures could rocket in some months by up to 6C above average, while rainfall could be 300% of typical amounts.

The chance of the extreme scenarios happening remains uncertain due to uncertainty about what action will be enacted to tackle climate change and how the Earth's systems will behave. This analysis was described as similar to security threat evaluations or resilience checks for the banking sector.

Key Outcome Highlights

• Four-Degree Warming: Global temperatures exceeding 4C by 2100 could occur if climate action fails or if strong feedback loops such as the tropical forests perishing unleash its vast reserves of carbon. This would cause severe and lengthy heatwaves and droughts affecting the UK in summer.
• AMOC Shutdown: A key circulation system, the Atlantic meridional overturning circulation (Amoc), is weakening 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 face immense difficulties and water supplies would be drastically changed," said Arnell.
• Accelerated Coastal Flooding: A rise of 2.0-2.2 metres around the UK by 2100 could happen if glaciers in Greenland and Antarctica disintegrate quickly, flooding coastal cities and towns. Unlike the other scenarios, this risk is already recognized by planners.
• Aerosol Effect: Global averages could also be driven up by about 0.75C if pollution from industry is sharply cut. This paradoxical outcome occurs because tiny specks from industrial activity scatter sunlight from warming the ground.

Planning for the Unthinkable

The worst-case scenarios could be applied to guide the construction of major projects, for example housing developments, nuclear power stations and flood defences. Knowledge about these environmental threats could also speed the transition to cut fossil fuel emissions.

The study created the worst-case scenarios using a blend of observed and historical experience, climate models and scientific theory. The scenarios do not take into account broader worldwide issues such as the agricultural collapse and resource wars.

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

An independent advisory body has recommended that the UK needs to "adapt to 2C" and assess the dangers for four degrees. Earlier government strategies have been labeled being "inadequate" by environmental groups.

A national response noted: "The climate emergency is at the heart of this government’s agenda, both adapting for the future and becoming a low-carbon economy. It is crucial that the UK is ready for the impacts."