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The gravest potential impacts of the climate crisis for the UK have been detailed by experts, ranging from a searing 4C rise in mean temperatures to a two-metre surge in ocean levels. Another scenario foresees a decline of 6C in temperature following the collapse of critical Atlantic Ocean currents, which would severely impact agriculture and power requirements.

Possible Outcomes, Not Forecasts

These effects, a number of which are associated with irreversible climate shifts, are considered as improbable but possible. The researchers stated that these scenarios plugged a hole in risk assessment that had left the UK not ready for worst-case results.

"The climate extremes we have detailed are not forecasts, but they are credible," stated the professor who oversaw the study. "Britain has been formulating policy in the absence of the frameworks to plan for worst-case scenarios. We've now provided planners what they need to get ready for climate outcomes they pray never occur, but must not overlook."

A Spectrum of Harsh Climate Scenarios

A second set of severe models sets out the projected severity of extreme weather between now and the end of the century. These show that thermometer readings could soar in certain periods by up to 6C higher than average, while precipitation could be triple standard measurements.

The likelihood of the worst-case outcomes happening could not be calculated due to uncertainty about what action will be enacted to combat climate change and how the climate system will react. This research was likened to defence planning or scenario planning for economic stability.

Critical Scenario Elements

• Severe Heating: Planetary heating soaring past 4C by 2100 could occur if climate action collapses or if powerful amplifying effects including the tropical forests perishing release its enormous store of carbon. This would lead to intense and enduring heatwaves and droughts affecting the UK in summer.
• Ocean Circulation Failure: A critical sea current, the AMOC, is slowing down and showing signs of collapse due to global heating. A collapse beginning in 2030 would lead to a six-degree temperature drop in the UK. "Crop production would struggle enormously and water supplies would be drastically changed," said the lead researcher.
• Rapid Sea Level Rise: A increase of 2.0-2.2 metres around the UK by 2100 could happen if glaciers in the Arctic and Antarctic melt abruptly, submerging coastal cities and towns. Differing from the other scenarios, this risk is already recognized by coastal engineers.
• Clean Air Warming: Global averages could also be increased by about 0.75C if industrial aerosols is drastically reduced. This unexpected result occurs because pollution particles from fossil fuel combustion block solar radiation from warming the ground.

Building Resilience Against the Extreme

The worst-case scenarios could be applied to guide the building of critical national infrastructure, such as expanded cities, nuclear power stations and flood defences. Knowledge about these future hazards could also speed the drive to slash carbon pollution.

The study created the extreme models using a mix of observed and historical experience, climate models and scientific theory. The scenarios omit international cascading effects like the global food shortages and geopolitical instability.

This work was initiated as part of a official climate resilience programme. Earlier reports have warned that not enough attention was being allocated to low‐likelihood but high‐impact risks.

The government's climate advisors has stated that the UK must "adapt to 2C" and examine the threats for 4C. Earlier government strategies have been labeled being "insufficient" by some observers.

An official statement noted: "Climate change is at the core of this government’s agenda, both preparing for impacts and establishing itself as a clean energy superpower. It is vital that the UK is prepared for the impacts."