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The most severe projected effects of the climate emergency for the UK have been outlined by experts, ranging from a scorching 4C increase in mean temperatures to a two-metre increase in ocean levels. Another scenario predicts a decline of 6C in temperature following the shutdown of critical Atlantic Ocean currents, which would severely impact agriculture and power requirements.

Plausible Extremes, Not Guarantees

These impacts, some of which are connected to critical climate thresholds, are seen as low probability but possible. The scientists said that these models plugged a hole in forecasting that had left the UK not ready for extreme outcomes.

"The climate extremes we have outlined aren’t predictions, but they are credible," said the lead researcher who directed the study. "The UK has been formulating policy in the absence of the frameworks to test against catastrophic outcomes. We've now given policymakers what they need to plan for climate outcomes they wish to avoid, but must not overlook."

A Suite of Harsh Climate Scenarios

A second set of dire projections sets out the potential extent of severe conditions between now and the year 2100. These show that temperatures could soar in certain periods by up to 6C higher than historical levels, while rainfall could be 300% of normal levels.

The likelihood of the extreme scenarios happening could not be calculated due to unknown factors about what measures will be taken to combat climate change and how the planet's climate will behave. This analysis was described as similar to defence planning or resilience checks for financial systems.

Key Possibility Details

• Severe Heating: Planetary heating rising well above 4C by 2100 could occur if emissions cuts fails or if powerful amplifying effects such as the tropical forests dying unleash its vast reserves of carbon. This would lead to intense and enduring heat domes and droughts plaguing the UK in summer.
• Atlantic Current Collapse: A critical sea current, the AMOC, is slowing down and becoming unstable due to climate change. A collapse starting around 2030 would lead to a 6C cooling in the UK. "Farming would struggle enormously and water resources would be drastically changed," noted the professor.
• Rapid Sea Level Rise: A rise of 2.0-2.2 metres around the UK by 2100 could happen if ice sheets in Greenland and Antarctica melt abruptly, submerging seaside communities. Unlike the other scenarios, this risk is already understood by infrastructure experts.
• Pollution Paradox: Temperatures could also be increased by about 0.75C if industrial aerosols is sharply cut. This paradoxical outcome occurs because pollution particles from fossil fuel combustion scatter solar radiation from reaching the ground.

Preparing for the Unthinkable

These dire projections could be applied to guide the construction of critical national infrastructure, for example expanded cities, nuclear power stations and flood defences. Knowledge about these climate risks could also hasten the drive to reduce greenhouse gas emissions.

The study developed the worst-case scenarios using a blend of past data and records, computer simulations and established principles. These outcomes omit international cascading effects such as the global food shortages and increased conflict.

This work was commissioned as part of a government’s climate resilience programme. Previous assessments have stated that too little consideration was being paid to high-consequence, low-probability risks.

The government's climate advisors has stated that the UK needs to "adapt to 2C" and examine the threats for severe warming. Earlier government strategies have been described as being "very weak" by environmental groups.

A government spokesperson said: "The climate emergency is at the core of the country's focus, both building resilience and establishing itself as a clean energy superpower. It is essential that the UK is resilient for the consequences."