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The direst projected effects of the global heating for the UK have been mapped by experts, encompassing a blistering 4C rise in average temperatures to a two-metre surge in ocean levels. An alternative projection envisions a decline of 6C in temperature following the collapse of key Atlantic Ocean currents, which would profoundly upset agriculture and energy demand.

Possible Events, Not Predictions

These impacts, some of which are connected to climate tipping points, are viewed as low probability but possible. The team explained that these scenarios filled a gap in forecasting that had rendered the UK unprepared for worst-case results.

"The climate extremes we have mapped should not be seen as certainties, but they are plausible," stated the lead researcher who directed the research. "The nation has been making preparations in the absence of the methods to test against extreme possibilities. We've now provided planners what they need to get ready for climate outcomes they pray never occur, but dare not dismiss."

A Suite of Extreme Weather Scenarios

A second set of worst-case scenarios maps the possible scale of extreme weather between now and the close of the 2100s. These show that temperatures could skyrocket in particular seasons by up to 6C above the norm, while rainfall could be 300% of standard measurements.

The probability of the extreme scenarios coming to pass cannot be precisely determined due to unknown factors about what action will be taken to tackle climate change and how the climate system will react. This research was likened to defence planning or stress tests for financial systems.

Major Scenario Highlights

• Four-Degree Warming: Global temperatures exceeding 4C by 2100 could transpire if climate action collapses or if powerful amplifying effects like the tropical forests perishing unleash its vast reserves of carbon. This would result in extreme and prolonged heatwaves and droughts plaguing the UK in summer.
• Ocean Circulation Failure: A major ocean current, the Atlantic meridional overturning circulation (Amoc), is slowing down and becoming unstable due to ocean warming. A complete failure beginning in 2030 would lead to a drastic cooling in the UK. "Crop production would be severely challenged and water resources would be completely altered," said Arnell.
• Sudden Ocean Inundation: A surge of 2.0-2.2 metres around the UK by 2100 could happen if glaciers in the polar regions disintegrate quickly, flooding low-lying areas. Unlike the other scenarios, this possibility is already known by planners.
• Clean Air Warming: Global averages could also be driven up by about 0.75C if pollution from industry is drastically reduced. This paradoxical outcome occurs because pollution particles from fossil fuel combustion scatter sunlight from warming the ground.

Building Resilience Against the Extreme

The worst-case scenarios could be utilized in planning the building of critical national infrastructure, including housing developments, nuclear power stations and water management networks. Knowledge about these future hazards could also speed the drive to slash greenhouse gas emissions.

The research developed the severe projections using a blend of past data and records, computer simulations and climate physics. These outcomes do not take into account potential global problems like the agricultural collapse and geopolitical instability.

The research was commissioned as part of a government’s adaptation planning initiative. Previous assessments have cautioned that insufficient focus was being given to high-consequence, low-probability risks.

The government's climate advisors has recommended that the UK needs to "prepare for two degrees of warming" and evaluate the threats for 4C. Past policy documents have been criticised as being "insufficient" by environmental groups.

A national response commented: "Climate change is at the core of the country's focus, both building resilience and becoming a low-carbon economy. It is crucial that the UK is ready for the consequences."