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The direst projected effects of the climate emergency for the UK have been detailed by scientists, ranging from a blistering 4C jump in mean temperatures to a 6.5-foot surge in sea level. An alternative projection envisions a drop of 6C in temperature following the shutdown of critical Atlantic Ocean currents, which would profoundly upset agriculture and energy needs.

Possible Events, Not Guarantees

These impacts, certain ones are connected to irreversible climate shifts, are seen as unlikely but credible. The team said that these projections plugged a hole in risk assessment that had left the UK unprepared for extreme outcomes.

"The severe weather events we have outlined are not forecasts, but they are plausible," commented Nigel Arnell who led the research. "The UK has been formulating policy lacking the methods to plan for extreme possibilities. We've now provided planners what they need to prepare for climate outcomes they hope never happen, but must not overlook."

A Spectrum of Harsh Climate Possibilities

A second set of severe models details the possible scale of harsh climate between now and the year 2100. These show that thermometer readings could rocket in some months by up to 6C higher than the norm, while rainfall could be 300% of normal levels.

The likelihood of the worst-case outcomes happening remains uncertain due to unknown factors about what steps will be implemented to combat global warming and how the climate system will respond. This approach was likened to defence planning or scenario planning for the banking sector.

Critical Possibility Elements

• Severe Heating: Planetary heating exceeding 4C by 2100 could occur if emissions cuts breaks down or if powerful amplifying effects such as the Amazon basin dying emit its enormous store of carbon. This would lead to intense and enduring heat domes and dry spells plaguing the UK in summer.
• AMOC Shutdown: A major ocean current, the North Atlantic current, is weakening and showing signs of collapse due to climate change. A collapse commencing by 2030 would lead to a drastic cooling in the UK. "Farming would face immense difficulties and water supplies would be fundamentally reshaped," said Arnell.
• Rapid Sea Level Rise: A increase of 2.0-2.2 metres around the UK by 2100 could occur if ice sheets in the polar regions collapse rapidly, flooding seaside communities. Differing from the other scenarios, this risk is already known by infrastructure experts.
• Pollution Paradox: Temperatures could also be driven up by about 0.75C if airborne particulates is sharply cut. This counterintuitive effect occurs because pollution particles from fossil fuel combustion block solar radiation from reaching the ground.

Planning for the Unthinkable

These dire projections could be used to inform the construction of critical national infrastructure, such as new towns, nuclear power stations and urban drainage systems. Knowledge about these future hazards could also speed the drive to cut carbon pollution.

The study created the severe projections using a combination of observed and historical experience, climate models and scientific theory. The possibilities exclude broader worldwide issues including the devastation of food supplies and geopolitical instability.

The project was commissioned as part of a government’s adaptation planning initiative. Earlier reports have cautioned that not enough attention was being given to low‐likelihood but high‐impact risks.

An independent advisory body has recommended that the UK must "adapt to 2C" and assess the threats for 4C. Earlier government strategies have been described as being "very weak" by some observers.

A national response noted: "The climate emergency is at the center of our national priorities, both preparing for impacts and becoming a clean energy superpower. It is essential that the UK is resilient for the consequences."