Tipping points in the climate system

• 1. Introduction

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  Tipping points in the climate system are critical thresholds that, once crossed, set off changes that are large, accelerating, and often impossible to reverse. Picture a threshold you can step over but cannot step back from. That is the essential terror of these points. And the question haunting climate scientists is not whether such thresholds exist, but how close humanity already stands to them.

  Some of these tipping points could be crossed at today's level of warming, which sits at just over 1 degree Celsius above preindustrial temperatures. Others are estimated to trigger somewhere between 1.5 and 2 degrees. A few would not unleash their worst until the planet reaches temperatures well beyond anything in current projections. But what makes this especially alarming is that crossing one tipping point can set off another, and another, in a sequence scientists describe as a cascade. A domino falls, and it knocks over the next.

  As of September 2022, researchers had identified nine global core tipping elements and seven regional impact tipping elements across the climate system. What they share is a quality that sets them apart from ordinary climate change: once they go, they do not come back. Not in any human lifetime. Sometimes not for millennia. The chapters ahead examine what these points are, where they are hidden, how scientists detect their approach, and what happens when they fall.

• 2. Defining the Threshold

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  The IPCC Sixth Assessment Report defines a tipping point as a critical threshold beyond which a system reorganizes, often abruptly and often irreversibly. That reorganization can be triggered by a surprisingly small disturbance. A minor push, in the right place, at the right moment, can produce a disproportionately large change in the whole system.

  What gives tipping points their dangerous character is a property called self-reinforcing feedback. Once a threshold is crossed, the system begins driving its own change. The shift can become irreversible on a human timescale, even if the original cause is removed. For any particular climate component, the move from one stable state to a new stable state may take many decades or centuries to complete, but the fate of the system is sealed at the moment of crossing.

  The 2019 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate adds a key clarification: the system does not return to its initial state even if the drivers of the change are reduced or stopped. That is the distinction between ordinary change and a tipping point. Ordinary change can be reversed by removing its cause. A tipped system stays in its new configuration.

  In the early 2000s, the IPCC first began considering the possibility of tipping points, calling them large-scale discontinuities at the time. The early assessment was relatively optimistic, concluding that such discontinuities would only be likely at global warming of 4 degrees Celsius or more. That estimate has shifted substantially since then. By 2021, scientists considered tipping points to have significant probability at today's warming of just over 1 degree Celsius, and high probability above 2 degrees.

• 3. Ice at the Edge

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  The Greenland ice sheet is the second largest ice sheet in the world. If all the water it holds were to melt, global sea levels would rise by 7.2 metres. Right now, the sheet is losing ice at an accelerating rate, adding almost 1 millimetre to global sea levels every year. About half of that loss comes from surface melting; the rest occurs where the ice meets the sea, through the calving of icebergs.

  Greenland's tipping point is driven by what is called the melt-elevation feedback. As surface ice melts, the sheet gets lower. Air at lower altitudes is warmer. The ice is then exposed to higher temperatures, which accelerates melting further. A 2021 analysis of sediment from a 1.4-kilometre ice core showed that the Greenland ice sheet melted away at least once during the last million years, strongly suggesting its tipping point sits below the 2.5 degrees Celsius maximum temperature recorded over that period. Even if it tips, the full melt would unfold over millennia, not decades.

  The West Antarctic Ice Sheet presents a different kind of vulnerability. In places it is more than 4 kilometres thick, and it rests on bedrock that lies mostly below sea level. That subglacial position puts it in direct contact with ocean heat, making it susceptible to fast and irreversible ice loss. Its tipping mechanism is called the Marine Ice Sheet Instability: once the sheet's grounding lines retreat beyond the edge of the subglacial basin, the retreat becomes self-sustaining, pulling the sheet into ever-deeper water. A complete melt of the West Antarctic Ice Sheet would contribute around 3.3 metres of sea level rise over thousands of years. Some outlet glaciers are estimated to be close to or possibly already beyond the point of self-sustaining retreat.

  The East Antarctic Ice Sheet is the largest and thickest on Earth, reaching a maximum thickness of 4,800 metres. A complete disintegration would raise sea levels by 53.3 metres, though that outcome may require warming of 10 degrees Celsius and would take no less than 10,000 years to complete. However, subglacial basin portions of the East Antarctic sheet, particularly the Wilkes Basin, may be vulnerable at lower levels of warming.

• 4. Frozen Ground, Warming Gas

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  Permafrost covers large fractions of land, mainly in Siberia, Alaska, northern Canada, and the Tibetan Plateau, and in some places it extends up to a kilometre deep. Scientists believe there is nearly twice as much carbon stored in permafrost as is present in the entire atmosphere today. That makes permafrost not just a climate concern but what scientists call a threat multiplier.

  As the climate warms and this frozen ground begins to thaw, it releases carbon dioxide and methane. Methane is a particularly powerful greenhouse gas. With higher temperatures, microbes become active and break down the biological material locked in the permafrost, and some of that carbon is irreversibly lost. Most thaw is gradual, unfolding over centuries, but in places where the permafrost is rich in large ice masses, abrupt thaw can occur within years to decades. When that ice melts, the ground above slumps or forms what are known as thermokarst lakes.

  These processes can become self-sustaining at a local level, and scientists estimate they could increase greenhouse gas emissions by around 40%. The gases released then act as a self-reinforcing feedback, warming the climate further, which thaws more permafrost. However, researchers believe this cycle is unlikely to drive a global tipping point or runaway warming process on its own. Subsea permafrost, up to 100 metres thick, also exists on the seafloor under parts of the Arctic Ocean, adding another reservoir of frozen carbon that warming waters can reach.

  Boreal permafrost abrupt thaw is among the four tipping elements estimated to likely cross a threshold if global warming reaches 1.5 degrees Celsius, alongside the Greenland ice sheet collapse, the West Antarctic ice sheet collapse, and tropical coral reef die-off.

• 5. The Ocean Conveyor

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  The Atlantic Meridional Overturning Circulation, also known as the Gulf Stream System, moves warm surface water from the tropics northward and returns cold, dense water southward in a vast conveyor belt. The engine that drives it is the difference in water density: cold, salty water is heavier and sinks; warm, fresh water is lighter and stays near the surface. As warm water moves north, some evaporates and increases salinity, and the water cools in contact with the air, making it dense enough to sink several kilometres.

  Global warming disrupts this engine. Increased rainfall and melting ice pour fresh water into the North Atlantic, diluting the salty surface water and making it lighter. Lighter water cannot sink as effectively, slowing the whole circulation. A weakening of between 24% and 39% is expected depending on greenhouse gas emissions, even without the system actually tipping.

  Theory, simplified models, and paleoclimate reconstructions all suggest the AMOC has a tipping point. If freshwater input reaches a critical threshold, the circulation could collapse into a state of reduced flow, and even after melting stops, the AMOC may not return to its present state. Such a collapse is unlikely in the 21st century but may occur before 2300 under high emission scenarios. A new stable state, once established, could persist for thousands of years.

  If the AMOC does collapse, the consequences would extend far beyond ocean temperatures. Cooling of more than 10 degrees Celsius is projected for parts of Europe. Drying would affect Europe, Central America, West Africa, and southern Asia. Sea levels in the North Atlantic would rise by around 1 metre. One projection found that arable agriculture could become economically unviable in Britain. In October 2024-44 climate scientists published an open letter arguing that the risk of AMOC collapse has been greatly underestimated and could occur within the next few decades. An August 2025 study concluded the collapse could begin as early as the 2060s.

• 6. Forests and Reefs at the Limit

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  The Amazon rainforest is twice the size of India and spans nine countries. It produces roughly half of its own rainfall by recycling moisture through evaporation and transpiration as air moves across the canopy. Without that internal moisture recycling, one model suggests around 40% of the current forest area would be too dry to sustain rainforest at all.

  When forest is lost to drought, wildfire, or deforestation, there is less rain in downwind regions, which increases stress on trees there. If enough forest is removed, a threshold can be crossed beyond which large parts of the remaining rainforest die off and convert into drier, degraded forest or savanna, particularly in the drier south and east. In 2022, a study reported that the rainforest has been losing resilience since the early 2000s, measured by how quickly it recovers from short-term disturbances. Modelling published in Nature by Wunderling and colleagues found that large-scale Amazon transitions would require around 3.7-4.0 degrees Celsius of warming in the absence of deforestation. But combining warming of 1.5-1.9 degrees with 22-28% deforestation could trigger a near system-wide transition across 62-77% of the forest. Between 17% and 18% of the Amazon has already been cleared.

  Coral reefs face a tipping point driven by ocean temperature. Around 500 million people depend on coral reefs for food, income, tourism, and coastal protection. A sustained ocean temperature spike of just 1 degree Celsius above average is enough to cause bleaching. Under heat stress, corals expel the zooxanthellae, the small colourful algae living in their tissues, and without those algae the corals slowly die. The IPCC projects that coral reefs will decline by a further 70-90% by the time temperatures reach 1.5 degrees Celsius above pre-industrial levels, and that at 2 degrees they will become extremely rare. Mass bleaching events have been accelerating since the 1980s, when warming sea temperatures first began triggering them at scale.

• 7. Cascades and the Domino Problem

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  Crossing a tipping point in one part of the climate system may push another system closer to its own threshold. Scientists call these sequences cascading tipping points, and they represent the most alarming dimension of the entire subject. The mechanisms link up in concrete ways: ice loss in West Antarctica and Greenland will significantly alter ocean circulation. Sustained warming of the northern high latitudes that results from that circulation change could then activate permafrost degradation and boreal forest dieback, each of which releases additional greenhouse gases, warming the planet further.

  A 2021 study using three million computer simulations of a climate model showed that nearly one-third of those simulations resulted in domino effects, even when temperature increases were kept below 2 degrees Celsius, the upper limit set by the Paris Agreement in 2015. The authors described the possibility of cascading tipping points as an existential threat to civilisation. A separate network model analysis found that temporary overshoots of Paris Agreement goals, scenarios where warming briefly exceeds targets before coming back down, can increase the risk of tipping cascades by up to 72% compared with scenarios where temperatures stay below the target without overshoot.

  The geological record supplies evidence that such cascades have happened before. The African humid period, which lasted from around 15,000 to 5,000 years ago, ended abruptly in a drier state. The desertification that followed cascaded into the retreat of pastoral societies in North Africa and a change of dynasty in Egypt. Dansgaard-Oeschger events during the last ice age produced abrupt warming within decades, with 25 occurrences of sudden climate fluctuations over a 500-year period, and likely involved abrupt changes in major ocean currents.

  Some researchers have proposed that a threshold of around 2 degrees Celsius above pre-industrial levels could trigger multiple tipping points and self-reinforcing feedback loops in what is sometimes called the Hothouse Earth scenario. The existence and precise value of that threshold remain speculative, but the underlying concern is grounded in the interconnected structure of the systems described throughout this documentary, where decisions taken over the next decade could influence the climate of the planet for tens to hundreds of thousands of years.

• 8. Reading the Warning Signs

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  For certain kinds of tipping points, the approach of a threshold leaves measurable traces in the data. Systems that are nearing a bifurcation, the mathematical term for a shift from one stable state to another, begin to lose their ability to recover from small disturbances. Scientists call this critical slowing down: the system takes longer to bounce back, and that slowdown shows up as rising autocorrelation and increasing variance in the data over time.

  These early warning signals have been developed and tested against the paleoclimate record, using time series from sediments, ice cores, and tree rings, where past tipping events can be observed directly. They have been applied to drought stress in California forests and to the Pine Island Glacier in West Antarctica, among other systems. Using increased autocorrelation and variance in melt rate data, researchers have suggested that the Greenland ice sheet is currently losing resilience, consistent with modelled early warning signals for that ice sheet.

  But early warning signals have limitations. One 2021 study found early-warning signals in a set of AMOC indices, suggesting the circulation may be close to tipping. Another study published in the same journal the following year found a largely stable AMOC that had not been affected by climate change beyond its natural variability. Two further studies published in 2022 suggested that the modelling approaches commonly used to evaluate AMOC risk appear to overestimate its likelihood of collapse. Increased variance and autocorrelation can reflect internal variability rather than proximity to a threshold, making interpretation difficult.

  Noise-induced tipping adds another layer of difficulty. Some tipping events are triggered not by a gradual drift toward a threshold but by random fluctuations within the system, and these produce no early warning signals at all because the underlying potential does not change. The Dansgaard-Oeschger events are one example. Finally, rate-induced tipping occurs when the environment changes faster than the system's own restoring force, as may be the case with the AMOC if ice melt accelerates rapidly enough, and with peatlands where sudden destabilisation can trigger an explosive release of soil carbon. Human-induced changes in the climate system may be unfolding too fast for early warning signals to become evident before a threshold is crossed.