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The direst projected effects of the climate emergency for the UK have been mapped by experts, including a blistering 4C increase in average temperatures to a two-metre surge in sea level. Another scenario foresees a decline of 6C in temperatures following the collapse of crucial Atlantic Ocean currents, which would profoundly upset agriculture and energy needs.

Possible Extremes, Not Forecasts

These impacts, some of which are associated with critical climate thresholds, are seen as unlikely but plausible. The researchers explained that these projections filled a gap in risk assessment that had rendered the UK not ready for extreme outcomes.

"The worst-case scenarios we have outlined should not be seen as certainties, but they are plausible," said the professor who led the analysis. "Britain has been formulating policy lacking the frameworks to plan for catastrophic outcomes. We've now provided policymakers what they need to plan for future conditions they pray never occur, but can’t afford to ignore."

A Suite of Harsh Climate Projections

A second set of worst-case scenarios sets out the potential extent of severe conditions between now and the year 2100. These indicate that temperatures could rocket in certain periods by up to 6C above average, while rainfall could be 300% of standard measurements.

The chance of the worst-case outcomes occurring remains uncertain due to unknown factors about what action will be taken to tackle global heating and how the Earth's systems will behave. This analysis was described as similar to security threat evaluations or scenario planning for financial systems.

Major Scenario Details

• Four-Degree Warming: Planetary heating rising well above 4C by 2100 could occur if international cooperation fails or if vicious cycles including the tropical forests perishing release its huge repository of carbon. This would lead to extreme and prolonged heat domes and dry spells plaguing the UK in summer.
• Atlantic Current Collapse: A critical sea current, the Atlantic meridional overturning circulation (Amoc), is losing strength and showing signs of collapse due to climate change. A complete failure commencing by 2030 would lead to a 6C cooling in the UK. "Farming would struggle enormously and freshwater availability would be drastically changed," said Arnell.
• Sudden Ocean Inundation: A rise of 2.0-2.2 metres around the UK by 2100 could happen if glaciers in the Arctic and Antarctic collapse rapidly, submerging seaside communities. Unlike the other scenarios, this risk is already recognized by coastal engineers.
• Pollution Paradox: Global averages could also be increased by about 0.75C if industrial aerosols is drastically reduced. This paradoxical outcome occurs because aerosol particles from burning coal reflect solar radiation from reaching the ground.

Preparing for the Unthinkable

The worst-case scenarios could be applied to guide the construction of critical national infrastructure, for example housing developments, energy plants and urban drainage systems. Knowledge about these climate risks could also accelerate the drive to reduce greenhouse gas emissions.

The research formulated the extreme models using a blend of past data and records, digital projections and scientific theory. The scenarios do not take into account potential global problems such as the devastation of food supplies and resource wars.

The research was commissioned as part of a official risk assessment framework. Independent analyses have cautioned that too little consideration was being given to high-consequence, low-probability risks.

An independent advisory body has stated that the UK needs to "adapt to 2C" and examine the threats for severe warming. Previous adaptation plans have been labeled being "insufficient" by some observers.

An official statement noted: "Global warming is at the center of the country's focus, both adapting for the future and becoming a low-carbon economy. It is essential that the UK is prepared for the impacts."