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The gravest potential impacts of the global heating for the UK have been detailed by researchers, ranging from a searing 4C rise in average temperatures to a two-metre rise in sea levels. A separate possibility envisions a decline of 6C in temperatures following the collapse of key Atlantic Ocean currents, which would massively disrupt food production and power requirements.

Possible Extremes, Not Guarantees

These effects, a number of which are linked to irreversible climate shifts, are seen as low probability but possible. The researchers explained that these scenarios plugged a hole in planning that had rendered the UK not ready for worst-case results.

"The climate extremes we have outlined are not forecasts, but they are credible," said the professor who oversaw the analysis. "The UK has been formulating policy without the methods to test against catastrophic outcomes. We've now provided policymakers what they need to plan for environmental results they pray never occur, but can’t afford to ignore."

A Suite of Severe Conditions Scenarios

Another collection of dire projections details the potential extent of harsh climate between now and the end of the century. These indicate that thermometer readings could soar in particular seasons by up to 6C higher than historical levels, while precipitation could be three times typical amounts.

The probability of the catastrophic events coming to pass remains uncertain due to unknown factors about what steps will be taken to address global warming and how the planet's climate will respond. This approach was described as similar to defence planning or stress tests for the banking sector.

Major Scenario Highlights

• Severe Heating: Worldwide averages rising well above 4C by 2100 could happen if emissions cuts fails or if powerful amplifying effects such as the Amazon rainforest becoming a net carbon source unleash its huge repository of carbon. This would result in extreme and prolonged heatwaves and water shortages affecting the UK in summer.
• Ocean Circulation Failure: A major ocean current, the North Atlantic current, is weakening and becoming unstable due to global heating. A collapse starting around 2030 would lead to a 6C cooling in the UK. "Farming would be severely challenged and freshwater availability would be drastically changed," noted Arnell.
• Rapid Sea Level Rise: A increase of 2.0-2.2 metres around the UK by 2100 could take place if glaciers in Greenland and Antarctica disintegrate quickly, submerging coastal cities and towns. Unlike the other scenarios, this risk is already understood by infrastructure experts.
• Pollution Paradox: Temperatures could also be increased by about 0.75C if airborne particulates is drastically reduced. This counterintuitive effect occurs because pollution particles from fossil fuel combustion reflect sunlight from reaching the ground.

Preparing for the Worst

The severe models could be applied to guide the construction of critical national infrastructure, for example housing developments, energy plants and water management networks. Understanding these climate risks could also speed the push to reduce greenhouse gas emissions.

The analysis created the worst-case scenarios using a mix of past data and records, computer simulations and scientific theory. These outcomes exclude broader worldwide issues like the global food shortages and geopolitical instability.

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

An independent advisory body has stated that the UK must "become resilient to moderate heating" and examine the threats for 4C. Previous adaptation plans have been described as being "very weak" by environmental groups.

An official statement noted: "Climate change is at the heart of our national priorities, both adapting for the future and establishing itself as a low-carbon economy. It is essential that the UK is resilient for the effects."