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The gravest potential impacts of the climate emergency for the UK have been detailed by experts, including a blistering 4C increase in mean temperatures to a two-metre rise in sea levels. Another scenario predicts a decline of 6C in temperature following the failure of critical Atlantic Ocean currents, which would severely impact food production and energy demand.

Possible Extremes, Not Predictions

These consequences, a number of which are associated with climate tipping points, are seen as unlikely but credible. The scientists said that these projections addressed a shortfall in planning that had rendered the UK ill-prepared for catastrophic possibilities.

"The severe weather events we have mapped aren’t predictions, but they are possible," commented the lead researcher who oversaw the study. "The UK has been planning lacking the methods to plan for extreme possibilities. We've now supplied planners what they need to get ready for future conditions they hope never happen, but dare not dismiss."

A Suite of Extreme Weather Possibilities

A further series of dire projections details the possible scale of severe conditions between now and the end of the century. These indicate that thermometer readings could skyrocket in certain periods by up to 6C higher than historical levels, while rainfall could be three times normal levels.

The probability of the catastrophic events happening cannot be precisely determined due to a lack of certainty about what action will be enacted to combat global heating and how the planet's climate will behave. This analysis was likened to defence planning or scenario planning for economic stability.

Critical Possibility Highlights

• Four-Degree Warming: Global temperatures exceeding 4C by 2100 could occur if emissions cuts breaks down or if strong feedback loops such as the Amazon rainforest dying unleash its huge repository of carbon. This would cause extreme and prolonged heat domes and water shortages affecting the UK in summer.
• Ocean Circulation Failure: A key circulation system, the Atlantic meridional overturning circulation (Amoc), is losing strength and becoming unstable due to global heating. A collapse starting around 2030 would lead to a 6C cooling in the UK. "Crop production would be severely challenged and water supplies would be fundamentally reshaped," explained Arnell.
• Rapid Sea Level Rise: A rise of 2.0-2.2 metres around the UK by 2100 could occur if polar ice in the polar regions collapse rapidly, flooding low-lying areas. Unlike the other scenarios, this threat is already understood by coastal engineers.
• Aerosol Effect: Temperatures could also be driven up by about 0.75C if airborne particulates is sharply cut. This unexpected result occurs because tiny specks from fossil fuel combustion scatter sunlight from warming the ground.

Planning for the Unthinkable

These dire projections could be used to inform the development of critical national infrastructure, such as housing developments, power generation facilities and urban drainage systems. Awareness of these environmental threats could also hasten the push to reduce carbon pollution.

The research developed the worst-case scenarios using a blend of observed and historical experience, digital projections and established principles. The scenarios omit potential global problems like the agricultural collapse and resource wars.

The project was initiated as part of a national risk assessment framework. Earlier reports have stated that insufficient focus was being given to catastrophic yet improbable risks.

The government's climate advisors has stated that the UK needs to "become resilient to moderate heating" and examine the dangers for four degrees. Previous adaptation plans have been described as being "very weak" by environmental groups.

An official statement said: "Climate change is at the heart of our national priorities, both adapting for the future and becoming a clean energy superpower. It is crucial that the UK is resilient for the impacts."