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The direst possible consequences of the global heating for the UK have been mapped by researchers, encompassing a blistering 4C increase in average temperatures to a 2-metre surge in ocean levels. Another scenario envisions a drop of 6C in temperature following the collapse of key Atlantic Ocean currents, which would profoundly upset agriculture and energy demand.

Plausible Extremes, Not Guarantees

These consequences, some of which are connected to critical climate thresholds, are seen as unlikely but credible. The scientists said that these models filled a gap in forecasting that had left the UK not ready for extreme outcomes.

"The climate extremes we have outlined aren’t predictions, but they are plausible," said the lead researcher who oversaw the analysis. "The UK has been planning without the frameworks to plan for catastrophic outcomes. We've now supplied planners what they need to get ready for future conditions they hope never happen, but must not overlook."

A Spectrum of Severe Conditions Projections

A second set of worst-case scenarios maps the possible scale of harsh climate between now and the end of the century. These show that temperatures could skyrocket in certain periods by up to 6C above the norm, while precipitation could be triple normal levels.

The likelihood of the extreme scenarios coming to pass could not be calculated due to a lack of certainty about what measures will be enacted to combat global heating and how the Earth's systems will behave. This analysis was compared to security threat evaluations or stress tests for financial systems.

Key Outcome Elements

• 4C Temperature Rise: Planetary heating rising well above 4C by 2100 could occur if international cooperation collapses or if vicious cycles such as the Amazon rainforest becoming a net carbon source release its huge repository of carbon. This would lead to severe and lengthy heatwaves and dry spells affecting the UK in summer.
• AMOC Shutdown: A critical sea current, the Atlantic meridional overturning circulation (Amoc), is weakening and becoming unstable due to ocean warming. A complete failure beginning in 2030 would lead to a six-degree temperature drop in the UK. "Agriculture would face immense difficulties and water supplies would be drastically changed," noted Arnell.
• Rapid Sea Level Rise: A rise of 2.0-2.2 metres around the UK by 2100 could take place if glaciers in the polar regions melt abruptly, flooding coastal cities and towns. In contrast to the other scenarios, this possibility is already understood by coastal engineers.
• Pollution Paradox: Global averages could also be driven up by about 0.75C if industrial aerosols is drastically reduced. This paradoxical outcome occurs because tiny specks from burning coal block sunlight from warming the ground.

Planning for the Worst

The worst-case scenarios could be utilized in planning the building of long-term infrastructure, such as new towns, power generation facilities and urban drainage systems. Understanding these climate risks could also hasten the drive to reduce greenhouse gas emissions.

The analysis created the severe projections using a combination of observed and historical experience, climate models and scientific theory. The possibilities do not take into account international cascading effects including the agricultural collapse and geopolitical instability.

The project was initiated as part of a official risk assessment framework. Earlier reports have cautioned that insufficient focus was being paid to low‐likelihood but high‐impact risks.

The government's climate advisors has recommended that the UK must "become resilient to moderate heating" and evaluate the dangers for 4C. Past policy documents have been labeled being "very weak" by some observers.

A national response said: "Global warming is at the center of this government’s agenda, both preparing for impacts and transitioning to a clean energy superpower. It is essential that the UK is resilient for the consequences."