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The gravest projected effects of the climate emergency for the UK have been mapped by scientists, ranging from a searing 4C rise in average temperatures to a two-metre surge in ocean levels. Another scenario envisions a decline of 6C in temperature following the failure of critical Atlantic Ocean currents, which would massively disrupt farming and energy demand.

Possible Outcomes, Not Forecasts

These impacts, some of which are linked to critical climate thresholds, are seen as unlikely but credible. The researchers said that these models addressed a shortfall in forecasting that had rendered the UK ill-prepared for extreme outcomes.

"The worst-case scenarios we have mapped are not forecasts, but they are credible," said the professor who led the analysis. "Britain has been making preparations without the tools to assess worst-case scenarios. We've now given decision-makers what they need to prepare for future conditions they hope never happen, but dare not dismiss."

A Range of Harsh Climate Scenarios

A second set of worst-case scenarios details the potential extent of extreme weather between now and the end of the century. These indicate that thermometer readings could soar in certain periods by up to 6C higher than the norm, while rainfall could be 300% of standard measurements.

The probability of the catastrophic events coming to pass remains uncertain due to a lack of certainty about what measures will be implemented to tackle global warming and how the Earth's systems will respond. This analysis was likened to security threat evaluations or scenario planning for financial systems.

Key Outcome Elements

• Severe Heating: Worldwide averages soaring past 4C by 2100 could occur if climate action breaks down or if powerful amplifying effects such as the Amazon rainforest becoming a net carbon source emit its huge repository of carbon. This would result in severe and lengthy heatwaves and dry spells hitting the UK in summer.
• Ocean Circulation Failure: A critical sea current, the North Atlantic current, is slowing down and showing signs of collapse due to ocean warming. A full shutdown commencing by 2030 would lead to a drastic cooling in the UK. "Farming would struggle enormously and water resources would be fundamentally reshaped," said Arnell.
• Sudden Ocean Inundation: A increase of 2.0-2.2 metres around the UK by 2100 could take place if polar ice in the polar regions disintegrate quickly, submerging low-lying areas. Unlike the other scenarios, this possibility is already understood by planners.
• Aerosol Effect: Temperatures could also be increased by about 0.75C if industrial aerosols is sharply cut. This unexpected result occurs because aerosol particles from industrial activity block sunlight from reaching the ground.

Building Resilience Against the Worst

These dire projections could be utilized in planning the building of long-term infrastructure, for example housing developments, energy plants and flood defences. Knowledge about these future hazards could also speed the drive to slash fossil fuel emissions.

The analysis developed the severe projections using a combination of past data and records, computer simulations and scientific theory. These outcomes do not take into account broader worldwide issues including the devastation of food supplies and resource wars.

This work was initiated as part of a national climate resilience programme. Earlier reports have stated that insufficient focus was being allocated 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 severe warming. Earlier government strategies have been described as being "very weak" by certain experts.

A national response commented: "Global warming is at the heart of this government’s agenda, both adapting for the future and establishing itself as a renewable energy leader. It is essential that the UK is resilient for the consequences."