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The most severe possible consequences of the global heating for the UK have been mapped by experts, ranging from a blistering 4C increase in average temperatures to a two-metre surge in ocean levels. A separate possibility foresees a decline of 6C in temperatures following the collapse of critical Atlantic Ocean currents, which would profoundly upset food production and power requirements.

Possible Outcomes, Not Guarantees

These impacts, certain ones are linked to critical climate thresholds, are seen as low probability but credible. The team explained that these projections addressed a shortfall in forecasting that had left the UK ill-prepared for catastrophic possibilities.

"The climate extremes we have outlined aren’t predictions, but they are possible," commented the professor who oversaw the study. "The nation has been making preparations in the absence of the tools to assess catastrophic outcomes. We've now supplied decision-makers what they need to get ready for climate outcomes they pray never occur, but dare not dismiss."

A Spectrum of Harsh Climate Possibilities

Another collection of dire projections details the projected severity of harsh climate between now and the end of the century. These suggest that thermometer readings could skyrocket in some months by up to 6C higher than historical levels, while precipitation could be triple standard measurements.

The chance of the extreme scenarios happening remains uncertain due to a lack of certainty about what steps will be enacted to tackle global heating and how the Earth's systems will respond. This research was likened to national security risk assessments or scenario planning for financial systems.

Critical Outcome Elements

• Four-Degree Warming: Planetary heating exceeding 4C by 2100 could transpire if climate action fails or if vicious cycles such as the Amazon rainforest dying release its huge repository of carbon. This would lead to intense and enduring heat domes and water shortages affecting the UK in summer.
• Atlantic Current Collapse: A key circulation system, the AMOC, is losing strength and becoming unstable due to ocean warming. A complete failure starting around 2030 would lead to a drastic cooling in the UK. "Crop production would be severely challenged and water supplies 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 polar ice in the polar regions collapse rapidly, flooding coastal cities and towns. In contrast to the other scenarios, this possibility is already understood by infrastructure experts.
• Clean Air Warming: Global averages could also be pushed higher by about 0.75C if pollution from industry is rapidly cleaned up. This paradoxical outcome occurs because aerosol particles from fossil fuel combustion block solar radiation from warming the ground.

Preparing for the Extreme

The severe models could be applied to guide the building of critical national infrastructure, such as housing developments, power generation facilities and flood defences. Awareness of these climate risks could also accelerate the drive to cut carbon pollution.

The research developed the severe projections using a blend of past data and records, computer simulations and climate physics. These outcomes do not take into account potential global problems like the agricultural collapse and resource wars.

The research was commissioned as part of a national risk assessment framework. Earlier reports have warned that not enough attention was being allocated to high-consequence, low-probability risks.

Official experts has said that the UK needs to "prepare for two degrees of warming" and assess the dangers for 4C. Earlier government strategies have been described as being "inadequate" by environmental groups.

A national response said: "Climate change is at the heart of the country's focus, both building resilience and becoming a renewable energy leader. It is essential that the UK is prepared for the effects."