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The most severe projected effects of the global heating for the UK have been detailed by scientists, encompassing a blistering 4C jump in temperatures to a 2-metre rise in sea level. An alternative projection envisions a decline of 6C in temperature following the collapse of key Atlantic Ocean currents, which would massively disrupt agriculture and power requirements.

Credible Extremes, Not Forecasts

These impacts, a number of which are associated with irreversible climate shifts, are viewed as unlikely but plausible. The scientists said that these scenarios addressed a shortfall in risk assessment that had left the UK ill-prepared for extreme outcomes.

"The worst-case scenarios we have outlined aren’t predictions, but they are credible," said the lead researcher who led the research. "Britain has been formulating policy lacking the methods to assess worst-case scenarios. We've now supplied planners what they need to prepare for future conditions they wish to avoid, but dare not dismiss."

A Suite of Extreme Weather Projections

Another collection of dire projections details the projected severity of severe conditions between now and the close of the 2100s. These suggest that thermometer readings could soar in some months by up to 6C higher than the norm, while precipitation could be triple normal levels.

The likelihood of the catastrophic events coming to pass cannot be precisely determined due to a lack of certainty about what measures will be implemented to address global warming and how the planet's climate will behave. This research was likened to security threat evaluations or stress tests for financial systems.

Critical Scenario Elements

• 4C Temperature Rise: Global temperatures exceeding 4C by 2100 could happen if climate action breaks down or if vicious cycles such as the Amazon rainforest becoming a net carbon source unleash its vast reserves of carbon. This would result in intense and enduring heat domes and dry spells plaguing the UK in summer.
• AMOC Shutdown: A key circulation system, the Atlantic meridional overturning circulation (Amoc), is losing strength and losing stability due to ocean warming. A collapse commencing by 2030 would lead to a drastic cooling in the UK. "Crop production would be severely challenged and freshwater availability would be fundamentally reshaped," explained Arnell.
• Accelerated Coastal Flooding: A surge of 2.0-2.2 metres around the UK by 2100 could happen if polar ice in the polar regions melt abruptly, inundating low-lying areas. Unlike the other scenarios, this risk is already recognized by planners.
• Clean Air Warming: Global averages could also be increased by about 0.75C if industrial aerosols is drastically reduced. This unexpected result occurs because tiny specks from fossil fuel combustion scatter sunlight from reaching the ground.

Planning for the Extreme

These dire projections could be used to inform the development of long-term infrastructure, for example housing developments, nuclear power stations and urban drainage systems. Understanding these future hazards could also speed the push to slash greenhouse gas emissions.

The research developed the worst-case scenarios using a mix of past data and records, computer simulations and climate physics. These outcomes exclude broader worldwide issues including the devastation of food supplies and geopolitical instability.

The research was initiated as part of a national risk assessment framework. Earlier reports have stated that not enough attention was being paid to low‐likelihood but high‐impact risks.

The government's climate advisors has recommended that the UK must "prepare for two degrees of warming" and evaluate the risks for four degrees. Earlier government strategies have been labeled being "inadequate" by environmental groups.

An official statement said: "Global warming is at the core of our national priorities, both preparing for impacts and establishing itself as a renewable energy leader. It is vital that the UK is resilient for the impacts."