The worst-case impacts of the climate crisis for the UK have been laid bare by scientists, ranging from a scorching 4C rise in temperatures to a 2-metre rise in sea level. Another scenario sees a plunge of 6C in temperature after the collapse of key Atlantic Ocean currents, massively disrupting farming and energy needs.
The impacts, some of which are linked to climate tipping points, are seen as low probability but plausible. The researchers said the scenarios filled a gap in forecasting that had left the UK unprepared for extreme outcomes.
A second set of worst-case scenarios sets out the potential extent of extreme weather between now and the end of the century. These indicate that temperatures could soar in some months by up to 6C above average, while rainfall could be triple normal levels.
“The climate extremes we have mapped aren’t predictions, but they are plausible,” said Nigel Arnell, a University of Reading professor who led the study. “The UK has been planning without the tools to test against worst-case scenarios. We’ve now given decision-makers what they need to prepare for climate outcomes they hope never happen, but can’t afford to ignore.”
The probability of the extreme scenarios happening could not be calculated because of uncertainty about what action will be taken to tackle global heating and how the climate system will respond. Arnell said this made the analysis similar to national security risk assessments or the Bank of England’s stress tests for the financial system.
“You couldn’t have known what the probability of Russia invading Ukraine was, but you could have said this is what the consequences might be,” he added.
The worst-case scenarios could be used to inform the building of long-term infrastructure, such as new towns, nuclear power stations and urban drainage systems, Arnell said, adding that awareness of climate risks could speed the drive to cut fossil fuel emissions.
Published in the journal Earth’s Future, the analysis developed the worst-case scenarios using a combination of observed and historical experience, computer simulations and theory.
Global temperatures rising well above 4C by 2100 could happen if climate action collapses or if there are strong feedback loops such as the Amazon rainforest dying and releasing its enormous store of carbon. This would result in extreme and prolonged heatwaves and droughts hitting the UK in summer. Thousands of early deaths have already occurred in England in heatwaves with just a 1.3C increase in global temperatures.
Temperatures could also be driven up by about 0.75C if pollution from industry is sharply cut, the scientists said. This is because aerosol particles from burning coal and heavy fuels block sunlight from reaching the ground.
A major ocean current, the Atlantic meridional overturning circulation (Amoc) is weakening and losing stability because of global heating. It is one of the tipping points that most concerns scientists. A collapse beginning in 2030 would lead to 6C cooling in the UK.
“Agriculture would struggle enormously and water resources would be completely altered,” said Arnell. “Our heat and energy system would be completely caught out by changing winter energy demand. It wouldn’t happen overnight, but it would be massively disruptive.”
A collapse of even one part of Amoc, the sub-polar gyre, would reduce UK temperatures by 2.5C, the scientists said.
Global sea levels are already higher because of global heating but a rise of 2.0-2.2 metres around the UK by 2100 could occur if glaciers in Greenland and Antarctica collapse rapidly, flooding coastal cities and towns. Unlike the other scenarios, this possibility was already known to planners.
The worst-case scenarios do not take into account potential global problems including the devastation of food supplies and conflict.
The research was commissioned by the Met Office as part of the government’s climate resilience programme. A House of Lords report warned in 2021 that not enough attention was being given to low‐likelihood but high‐impact risks.
The Climate Change Committee, the government’s independent advisory body, has said the UK needs to “adapt to 2C and assess the risks for 4C”. Adaptation plans published in 2023 were criticised as being “very weak”.
A government spokesperson said: “Climate change is at the heart of this government’s agenda, both adapting for the future and becoming a clean energy superpower. It is vital that the UK is ready for the impacts and we are helping local communities become more resilient, including by building nine new reservoirs, as well as a record £10.5bn in government funding for new flood defences to protect nearly 900,000 properties by 2036.”
The government has requested an evidence review of climate risks from the CCC, which will be published in the spring. It has also asked for guidance on climate scenarios to be used in planning.
