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The direst possible consequences of the climate crisis for the UK have been detailed by scientists, encompassing a searing 4C rise in temperatures to a two-metre rise in sea level. An alternative projection predicts a decline of 6C in temperatures following the collapse of critical Atlantic Ocean currents, which would profoundly upset food production and energy needs.

Possible Outcomes, Not Forecasts

These effects, some of which are associated with irreversible climate shifts, are viewed as unlikely but credible. The scientists said that these projections addressed a shortfall in risk assessment that had rendered the UK unprepared for extreme outcomes.

"The worst-case scenarios we have outlined aren’t predictions, but they are plausible," stated Nigel Arnell who directed the study. "The nation has been formulating policy lacking the frameworks to assess extreme possibilities. We've now provided planners what they need to prepare for climate outcomes they hope never happen, but dare not dismiss."

A Suite of Extreme Weather Possibilities

A further series of severe models maps the possible scale of severe conditions between now and the end of the century. These show that thermometer readings could rocket in certain periods by up to 6C above average, while precipitation could be 300% of typical amounts.

The likelihood of the extreme scenarios coming to pass could not be calculated due to uncertainty about what action will be enacted to address global heating and how the Earth's systems will behave. This analysis was described as similar to security threat evaluations or resilience checks for financial systems.

Key Scenario Details

• Severe Heating: Worldwide averages exceeding 4C by 2100 could transpire if international cooperation collapses or if powerful amplifying effects such as the Amazon rainforest perishing emit its enormous store of carbon. This would lead to severe and lengthy heat domes and dry spells plaguing the UK in summer.
• Atlantic Current Collapse: A key circulation system, the North Atlantic current, is losing strength and losing stability due to ocean warming. A full shutdown starting around 2030 would lead to a drastic cooling in the UK. "Farming would struggle enormously and water supplies would be drastically changed," said the professor.
• Accelerated Coastal Flooding: A increase of 2.0-2.2 metres around the UK by 2100 could take place if ice sheets in the polar regions collapse rapidly, inundating low-lying areas. Differing from the other scenarios, this threat is already recognized by planners.
• Aerosol Effect: Temperatures could also be increased by about 0.75C if airborne particulates is drastically reduced. This paradoxical outcome occurs because tiny specks from burning coal reflect solar radiation from warming the ground.

Preparing for the Unthinkable

The worst-case scenarios could be utilized in planning the construction of major projects, such as housing developments, energy plants and water management networks. Understanding these climate risks could also speed the push to cut fossil fuel emissions.

The study created the severe projections using a mix of real-world events, computer simulations and scientific theory. The scenarios omit international cascading effects such as the agricultural collapse and resource wars.

The project was commissioned as part of a national risk assessment framework. Earlier reports have cautioned that not enough attention was being allocated to high-consequence, low-probability risks.

The government's climate advisors has recommended that the UK must "prepare for two degrees of warming" and examine the dangers for 4C. Earlier government strategies have been described as being "inadequate" by certain experts.

An official statement said: "Global warming is at the heart of the country's focus, both adapting for the future and establishing itself as a renewable energy leader. It is essential that the UK is ready for the impacts."