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The gravest potential impacts of the climate emergency for the UK have been mapped by experts, including a scorching 4C increase in average temperatures to a 6.5-foot increase in ocean levels. Another scenario envisions a decline of 6C in temperatures following the collapse of key Atlantic Ocean currents, which would profoundly upset farming and power requirements.

Credible Outcomes, Not Forecasts

These consequences, a number of which are linked to irreversible climate shifts, are considered as low probability but possible. The scientists said that these scenarios addressed a shortfall in risk assessment that had left the UK not ready for catastrophic possibilities.

"The climate extremes we have detailed aren’t predictions, but they are possible," said Nigel Arnell who oversaw the study. "Britain has been making preparations in the absence of the methods to test against worst-case scenarios. We've now supplied policymakers what they need to prepare for future conditions they wish to avoid, but must not overlook."

A Suite of Severe Conditions Possibilities

A second set of dire projections sets out the projected severity of harsh climate between now and the end of the century. These show that temperatures could skyrocket in some months by up to 6C above the norm, while precipitation could be triple typical amounts.

The probability of the catastrophic events happening cannot be precisely determined due to unknown factors about what measures will be enacted to tackle global warming and how the Earth's systems will react. This analysis was likened to national security risk assessments or scenario planning for financial systems.

Major Outcome Highlights

• 4C Temperature Rise: Worldwide averages rising well above 4C by 2100 could happen if climate action collapses or if strong feedback loops including the tropical forests dying emit its huge repository of carbon. This would lead to extreme and prolonged heatwaves and droughts hitting the UK in summer.
• Ocean Circulation Failure: A critical sea current, the Atlantic meridional overturning circulation (Amoc), is losing strength and becoming unstable due to global heating. A collapse commencing by 2030 would lead to a six-degree temperature drop in the UK. "Crop production would struggle enormously and freshwater availability would be fundamentally reshaped," explained Arnell.
• Rapid Sea Level Rise: A surge 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. Differing from the other scenarios, this risk is already understood by coastal engineers.
• Aerosol Effect: Temperatures could also be pushed higher by about 0.75C if industrial aerosols is sharply cut. This counterintuitive effect occurs because pollution particles from fossil fuel combustion reflect solar radiation from warming the ground.

Planning for the Unthinkable

The worst-case scenarios could be utilized in planning the construction of major projects, such as expanded cities, power generation facilities and urban drainage systems. Awareness of these future hazards could also speed the drive to reduce greenhouse gas emissions.

The research formulated the extreme models using a combination of observed and historical experience, climate models and established principles. These outcomes do not take into account broader worldwide issues like the global food shortages and geopolitical instability.

The research was requested as part of a official adaptation planning initiative. Independent analyses have cautioned that insufficient focus was being given to low‐likelihood but high‐impact risks.

The government's climate advisors has stated that the UK must "become resilient to moderate heating" and assess the threats for 4C. Past policy documents have been described as being "insufficient" by environmental groups.

An official statement noted: "Global warming is at the center of this government’s agenda, both preparing for impacts and becoming a low-carbon economy. It is essential that the UK is ready for the consequences."