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The direst possible consequences of the climate crisis for the UK have been outlined by experts, encompassing a blistering 4C jump in average temperatures to a 2-metre increase in sea level. Another scenario foresees a drop of 6C in temperature following the collapse of critical Atlantic Ocean currents, which would severely impact food production and power requirements.

Plausible Outcomes, Not Forecasts

These impacts, a number of which are linked to climate tipping points, are considered as improbable but credible. The team stated that these models filled a gap in planning that had rendered the UK not ready for catastrophic possibilities.

"The climate extremes we have outlined are not forecasts, but they are possible," said the professor who directed the study. "Britain has been planning without the methods to test against extreme possibilities. We've now provided decision-makers what they need to get ready for climate outcomes they wish to avoid, but must not overlook."

A Spectrum of Harsh Climate Scenarios

Another collection of worst-case scenarios maps the projected severity of severe conditions between now and the close of the 2100s. These indicate that temperatures could soar in some months by up to 6C above historical levels, while rainfall could be triple normal levels.

The probability of the extreme scenarios happening cannot be precisely determined due to unknown factors about what action will be enacted to address global heating and how the Earth's systems will react. This approach was described as similar to security threat evaluations or resilience checks for the banking sector.

Major Possibility Highlights

• Severe Heating: Global temperatures soaring past 4C by 2100 could transpire if emissions cuts breaks down or if powerful amplifying effects such as the tropical forests dying emit its huge repository of carbon. This would cause extreme and prolonged heatwaves and dry spells affecting the UK in summer.
• AMOC Shutdown: A major ocean current, the North Atlantic current, is slowing down and showing signs of collapse due to climate change. A complete failure beginning in 2030 would lead to a drastic cooling in the UK. "Agriculture would struggle enormously and water resources would be fundamentally reshaped," said Arnell.
• Sudden Ocean Inundation: A surge of 2.0-2.2 metres around the UK by 2100 could take place if glaciers in the polar regions melt abruptly, submerging coastal cities and towns. In contrast to the other scenarios, this possibility is already known by planners.
• Aerosol Effect: Temperatures could also be pushed higher by about 0.75C if airborne particulates is rapidly cleaned up. This unexpected result occurs because aerosol particles from industrial activity block sunlight from reaching the ground.

Preparing for the Extreme

These dire projections could be used to inform the building of critical national infrastructure, such as new towns, nuclear power stations and urban drainage systems. Knowledge about these future hazards could also speed the push to slash greenhouse gas emissions.

The analysis formulated the worst-case scenarios using a combination of observed and historical experience, climate models and climate physics. The scenarios exclude international cascading effects such as the devastation of food supplies and increased conflict.

The research was requested as part of a national risk assessment framework. Previous assessments have warned that too little consideration was being given to low‐likelihood but high‐impact risks.

The government's climate advisors has recommended that the UK needs to "become resilient to moderate heating" and examine the dangers for severe warming. Past policy documents have been criticised as being "insufficient" by environmental groups.

A national response said: "Global warming is at the core of our national priorities, both building resilience and transitioning to a low-carbon economy. It is vital that the UK is prepared for the consequences."