When the International Panel on Climate Change (IPCC) tells us that the planet is in trouble and we need to act urgently to address the issue, they are using a rather conservative assessment of the risks. They focus their assessment on the climate risks that are most likely to occur and will have the greatest impact on the most significant parts of our lives if they do occur.
Climate forecasts matter: they are used by governments and planners as part of their climate change risk management planning. The latest forecasts indicate the world is now headed for roughly 2.3–2.5°C of warming by 2100, and that’s assuming every country actually delivers its current pledges. If current pledges are not met, then 2.8°C by the end of the century is more likely. This is massively above the 1.5°C target set in the Paris Agreement in 2015, with profound implications, as every 0.1°C of warming brings with it greater risks for the planet: longer heatwaves, more intense storms and wildfires, higher sea levels, increased flooding, threats to food security, more disease, and more extinctions.
But the impacts of climate change could be much worse than conventionally assessed. If worst-case scenarios combine, they could lead to even more damaging outcomes. The likelihood of such an event may be low, but its impact could be enormous. Until now, High Impact, Low Likelihood (HILL) events have not been assessed sufficiently to allow planners to take them into account.
Tipping points
Climate tipping points are critical thresholds in the Earth’s climate system that, once exceeded, lead to large-scale, often abrupt and irreversible changes that may also be self-reinforcing. They happen quickly, cannot easily return to their original state, at least not on human timescales, and once triggered, feedback loops keep the process moving forward or even accelerating.
30 major tipping points have been identified, nine of which may already have started to tip! Climate tipping points include the following:
- Ice Sheet Collapse (Greenland & West Antarctica. The melting of the Greenland or West Antarctic ice sheets, which is already underway, would cause centuries of significant globalsea-level rise, possibly of over 7 metres.
- Ocean Circulation Slowdown/Collapse. The Atlantic Meridional Overturning Circulation (AMOC), which regulates weather patterns around the world, is a large system of ocean currents that brings warm water from the tropics to the northern hemisphere and influences climate around the world – it’s what keeps the UK climate more temperate than other countries at similar latitudes. The slowing or collapse of this system would have profound effects with regional climate shifts, altered rainfall patterns and abrupt temperature changes in Europe and beyond.
- Permafrost Thawing. Arctic permafrost holds vast amounts of carbon from plants and animals that died and decomposed over thousands of years. As it thaws, it can release large quantities of methane and CO₂, which amplify warming and further destabilise the climate system.
- Amazon Rainforest Dieback. The Amazon could shift fromrainforest to savanna if the drying and warming effects of climate change pass key thresholds, changing the Amazon from a major carbon sink to a source that releases more CO₂ into the atmosphere.
If a single tipping point isn’t worrying enough, there is a real possibility that if just one is crossed, it could cause a cascade of other tipping points, leading to severe, potentially catastrophic impacts.
HILLs that can’t be ignored
Climate scientists have been warning us about the threats posed by anthropogenic climate change since the early 19th century. With mounting data, the evidence is undeniable. Governments around the world have been slow in responding but have, in recent years, been making some efforts to reduce carbon emissions and prepare for the inevitable.
Preparations and investments have been based on the most likely, highest impact, climate change events, and climate scientists have provided the forecast scenarios to support appropriate planning. What they haven’t been able to do, until now, is to plan for HILL events, those events that are less likely to occur but would have devastating consequences if they did.
New research led by University of Reading professor Nigel Arnell, published in Earth’s Future, aims to rectify that gap in forecasting by giving “…decision-makers what they need to prepare for climate outcomes they hope never happen, but can’t afford to ignore.”
The research defines two types of “physically plausible” HILL scenario for the UK: transient changes to 2100 that are outside or at the edge of the range of “conventional” climate scenarios and shorter‐term extreme monthly and seasonal climate anomalies, which can occur at any time. Many HILL scenarios are linked to one or more climate tipping points, or compound weather extremes, higher rates of change than anticipated, and the consequences of unexpected human actions.
There are ten HILL scenarios in total for the UK that together paint a troubling picture of massive disruption with a plausible 4°C rise in temperature (6°C rise in some months), and a 2-metre rise in sea levels. According to Professor Arnell, “Agriculture would struggle enormously, and water resources would be completely altered …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.”
HILLs have consequences – and they would be devastating
A 4°C rise signifies a fundamental, long-term transformation of the planet’s climate, making a return to today’s conditions extremely difficult for millennia.
A 2-metre rise in sea levels would be catastrophic for the UK, potentially putting millions of people at risk of permanent or frequent flooding. While global sea levels have risen by about 15–20 cm since 1900, a 2-metre increase would fundamentally redraw the British coastline.
According to the House of Lords Select Committee on Risk Assessment and Planning, there is a bias against HILL risks, and the Institute for Public Policy Research says that ignoring the impacts of climate tipping points represents a “security blind spot” and threatens national security. The UK is therefore unprepared for climate risks outside the range implied by conventional climate scenarios.
Climate complacency
A significant proportion of the UK public does not take climate change seriously, even to the point of denying it is happening. Reform UK mayor of Greater Lincolnshire, Andrea Jenkyns, said in July 2025 “climate change doesn’t exist”, so is it any wonder there is public scepticism? We have already exceeded the 1.5°C threshold seen as critical to avoiding the most damaging impacts, and yet there is a realistic possibility that a HILL event will push us into an unknowable, devastating and unstoppable spiral of consequences. Rather than questioning the existence of climate change, as is common among scientifically illiterate populists, we should surely be demanding our politicians take more urgent action while there is still time.
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