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The most severe possible consequences of the climate emergency for the UK have been mapped by experts, encompassing a blistering 4C increase in temperatures to a two-metre rise in sea levels. Another scenario predicts a decline of 6C in temperatures following the shutdown of critical Atlantic Ocean currents, which would severely impact food production and energy demand.

Plausible Extremes, Not Forecasts

These consequences, a number of which are linked to irreversible climate shifts, are viewed as unlikely but possible. The researchers stated that these models addressed a shortfall in risk assessment that had rendered the UK not ready for worst-case results.

"The climate extremes we have outlined aren’t predictions, but they are plausible," said Nigel Arnell who directed the research. "The UK has been making preparations lacking the tools to plan for catastrophic outcomes. We've now given policymakers what they need to plan for future conditions they hope never happen, but must not overlook."

A Range of Harsh Climate Projections

A further series of severe models maps the possible scale of extreme weather between now and the close of the 2100s. These suggest that temperatures could rocket in certain periods by up to 6C above historical levels, while precipitation could be three times normal levels.

The likelihood of the extreme scenarios happening cannot be precisely determined due to uncertainty about what steps will be taken to combat climate change and how the Earth's systems will respond. This research was described as similar to national security risk assessments or stress tests for financial systems.

Major Outcome Elements

• 4C Temperature Rise: Worldwide averages soaring past 4C by 2100 could transpire if climate action collapses or if powerful amplifying effects such as the Amazon basin perishing release its enormous store of carbon. This would cause extreme and prolonged heatwaves and droughts hitting the UK in summer.
• AMOC Shutdown: A key circulation system, the Atlantic meridional overturning circulation (Amoc), is weakening and becoming unstable due to ocean warming. A collapse beginning in 2030 would lead to a drastic cooling in the UK. "Farming would struggle enormously and water supplies would be drastically changed," noted Arnell.
• Accelerated Coastal Flooding: A increase of 2.0-2.2 metres around the UK by 2100 could occur if polar ice in Greenland and Antarctica melt abruptly, flooding seaside communities. Differing from the other scenarios, this possibility is already known by coastal engineers.
• Clean Air Warming: Global averages could also be driven up by about 0.75C if industrial aerosols is rapidly cleaned up. This counterintuitive effect occurs because tiny specks from industrial activity reflect solar radiation from warming the ground.

Planning for the Unthinkable

The severe models could be used to inform the development of critical national infrastructure, such as new towns, energy plants and flood defences. Awareness of these climate risks could also accelerate the transition to reduce greenhouse gas emissions.

The analysis created the severe projections using a combination of observed and historical experience, computer simulations and climate physics. These outcomes do not take into account international cascading effects such as the global food shortages and increased conflict.

This work was initiated as part of a official adaptation planning initiative. Independent analyses have stated that insufficient focus was being paid to high-consequence, low-probability risks.

An independent advisory body has stated that the UK must "prepare for two degrees of warming" and assess the dangers for 4C. Previous adaptation plans have been criticised as being "insufficient" by some observers.

A national response said: "Climate change is at the heart of this government’s agenda, both building resilience and establishing itself as a low-carbon economy. It is essential that the UK is ready for the consequences."