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The gravest possible consequences of the climate emergency for the UK have been outlined by researchers, including a blistering 4C rise in temperatures to a two-metre rise in sea level. An alternative projection predicts a plunge of 6C in temperature following the shutdown of crucial Atlantic Ocean currents, which would massively disrupt food production and power requirements.

Plausible Outcomes, Not Predictions

These effects, some of which are associated with critical climate thresholds, are seen as unlikely but credible. The team explained that these projections filled a gap in risk assessment that had rendered the UK not ready for worst-case results.

"The climate extremes we have outlined aren’t predictions, but they are credible," commented Nigel Arnell who directed the research. "The nation has been making preparations in the absence of the frameworks to assess worst-case scenarios. We've now given planners what they need to get ready for future conditions they hope never happen, but must not overlook."

A Spectrum of Severe Conditions Possibilities

A further series of severe models sets out the possible scale of harsh climate between now and the end of the century. These suggest that thermometer readings could rocket in some months by up to 6C above historical levels, while precipitation could be 300% of normal levels.

The chance of the extreme scenarios coming to pass could not be calculated due to a lack of certainty about what measures will be enacted to address climate change and how the Earth's systems will respond. This analysis was described as similar to security threat evaluations or stress tests for economic stability.

Critical Possibility Elements

• 4C Temperature Rise: Global temperatures exceeding 4C by 2100 could occur if emissions cuts breaks down or if powerful amplifying effects like the tropical forests dying unleash its enormous store of carbon. This would lead to intense and enduring heatwaves and water shortages hitting the UK in summer.
• AMOC Shutdown: A major ocean current, the North Atlantic current, is losing strength and losing stability due to ocean warming. A full shutdown commencing by 2030 would lead to a drastic cooling in the UK. "Farming would be severely challenged and water resources would be drastically changed," said Arnell.
• Accelerated Coastal Flooding: A rise of 2.0-2.2 metres around the UK by 2100 could take place if polar ice in Greenland and Antarctica melt abruptly, inundating coastal cities and towns. Unlike the other scenarios, this risk is already recognized by coastal engineers.
• Clean Air Warming: Temperatures could also be pushed higher by about 0.75C if industrial aerosols is sharply cut. This unexpected result occurs because aerosol particles from fossil fuel combustion reflect solar radiation from reaching the ground.

Preparing for the Unthinkable

These dire projections could be applied to guide the development of major projects, for example expanded cities, energy plants and flood defences. Understanding these climate risks could also speed the transition to slash greenhouse gas emissions.

The analysis created the extreme models using a combination of observed and historical experience, climate models and climate physics. These outcomes do not take into account potential global problems like the devastation of food supplies and geopolitical instability.

The research was requested as part of a official 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 stated that the UK needs to "adapt to 2C" and evaluate the threats for four degrees. Earlier government strategies have been criticised as being "very weak" by some observers.

A government spokesperson said: "Climate change is at the core of our national priorities, both adapting for the future and transitioning to a renewable energy leader. It is essential that the UK is resilient for the consequences."