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The direst possible consequences of the global heating for the UK have been detailed by scientists, including a blistering 4C jump in average temperatures to a 2-metre surge in ocean levels. A separate possibility foresees a drop of 6C in temperature following the collapse of key Atlantic Ocean currents, which would severely impact farming and energy demand.

Plausible Extremes, Not Guarantees

These impacts, a number of which are linked to critical climate thresholds, are seen as unlikely but credible. The team explained that these models plugged a hole in planning that had left the UK ill-prepared for catastrophic possibilities.

"The severe weather events we have mapped aren’t predictions, but they are possible," said Nigel Arnell who led the analysis. "The nation has been planning lacking the tools to assess catastrophic outcomes. We've now supplied decision-makers what they need to get ready for climate outcomes they wish to avoid, but can’t afford to ignore."

A Suite of Severe Conditions Projections

A second set of dire projections sets out the possible scale of harsh climate between now and the year 2100. These suggest that temperatures could skyrocket in particular seasons by up to 6C higher than historical levels, while rainfall could be 300% of typical amounts.

The chance of the extreme scenarios occurring could not be calculated due to a lack of certainty about what action will be taken to combat climate change and how the Earth's systems will react. This analysis was described as similar to security threat evaluations or stress tests for financial systems.

Key Possibility Highlights

• Severe Heating: Global temperatures exceeding 4C by 2100 could transpire if climate action collapses or if powerful amplifying effects such as the Amazon rainforest perishing emit its enormous store of carbon. This would result in severe and lengthy heat domes and droughts affecting the UK in summer.
• Atlantic Current Collapse: A major ocean current, the Atlantic meridional overturning circulation (Amoc), is slowing down and losing stability due to climate change. A collapse starting around 2030 would lead to a six-degree temperature drop in the UK. "Crop production would struggle enormously and water resources would be drastically changed," explained Arnell.
• Accelerated Coastal Flooding: A rise of 2.0-2.2 metres around the UK by 2100 could happen if polar ice in Greenland and Antarctica disintegrate quickly, inundating low-lying areas. In contrast to the other scenarios, this possibility is already understood by coastal engineers.
• Clean Air Warming: Global averages could also be increased by about 0.75C if pollution from industry is drastically reduced. This paradoxical outcome occurs because pollution particles from fossil fuel combustion reflect solar radiation from reaching the ground.

Preparing for the Worst

The worst-case scenarios could be applied to guide the construction of major projects, such as expanded cities, power generation facilities and water management networks. Knowledge about these climate risks could also speed the drive to slash greenhouse gas emissions.

The analysis formulated the extreme models using a blend of real-world events, digital projections and established principles. The scenarios exclude international cascading effects including the devastation of food supplies and increased conflict.

The research was commissioned as part of a national climate resilience programme. Previous assessments have cautioned that too little consideration was being allocated to catastrophic yet improbable risks.

An independent advisory body has said that the UK needs to "adapt to 2C" and assess the dangers for severe warming. Previous adaptation plans have been criticised as being "very weak" by some observers.

A government spokesperson said: "The climate emergency is at the core of our national priorities, both building resilience and becoming a renewable energy leader. It is essential that the UK is prepared for the effects."