International media have featured quite a few stories about tipping points in the climate system. Some of these reports have been worrying. Others are less disturbing. In this blog, you can read about tipping points, why they cause concern among experts, and how much we know about the risk that we will experience climate tipping due to anthropogenic global warming. If you prefer to first understand how and why elements in the climate system such as ice sheets, wind systems, ocean currents, and rainforests can have tipping points, you can start with the section: What does it mean that a climate subsystem can tip?
Which parts of the climate system are possibly at risk of tipping?
Beneath the South Pole’s ice lies a varied landscape with valleys, mountains, plains, and even deep-frozen seabeds. A large part of western Antarctica’s ice masses indeed rests on the frozen seabed. This part of the ice sheet is particularly vulnerable to global warming and may reach a tipping point, after which large parts of West Antarctica’s ice sheet will melt and drift away, regardless of whether we later cool the climate again.
How an ice sheet develops, depends on its mass balance. As long as at least as much ice is added via snowfall as the ice sheet loses annually, it will persist. Antarctica’s ice has been formed over millions of years by snow that is compressed under its own weight. The weight forces the ice masses out towards the edges of the ice cap. Out there, ice melts away or breaks off like icebergs, a process called calving. How much ice West Antarctica loses each year depends on how much regional temperatures will rise and on how easily the ice can break free at the coasts and drift away.
Melts more ice
With global warming, ocean temperatures are rising around the coasts of Antarctica. Relatively warm water reaches the ice edge. Warmer water melts more ice. In West Antarctica, this also means that the ice that rests on the seabed is undermined and drifts away in huge pieces. When ice no longer accumulates near the coasts, the flow of the inland ice masses toward the margins of the ice sheets accelerates. The loss of ice in West Antarctica, therefore, increases sharply during global warming as the ice passes this tipping point.
Watch this video from the scientific collaboration TiPACCs for a more detailed explanation of the risk of tipping of the Antarctic ice masses.
The ice masses of West Antarctica were formed during the ice ages. If they melt away on a large scale due to global warming, we risk a sea rise of around 5-6 meters. The ice may not recover in West Antarctica until we are back in an ice age.
The Greenland ice sheet
Greenland’s ice sheet is in some places 3 kilometers high. That much ice doesn’t just melt away, even under severe global warming. But Greenland’s ice sheet may hit a tipping point. If that happens, we run the risk that the ice, or large parts of it, will begin a slow meltdown that we cannot stop again.
The Greenland ice sheet was formed during the ice ages. It survived the last 12,000 years of warm interglacial times because of its height. Newly fallen snow on the surface of the ice cap never melts away, in the same way, that the “eternal snow” on the top of the world’s highest mountains remains throughout the summer. Over the years, the snow is compressed into ice under its own weight. After this, the ice slides slowly toward the edges. As long as the summer temperatures at the ice sheet’s surface are above 0 degrees Celsius for sufficiently few days during summer, new ice will be added every year.
Loss and gain
However, the ice cap also annually loses large amounts of mass to its surroundings. This happens partly because ice melts in lower-lying areas, and partly because icebergs break off and float away along Greenland’s coasts. We talk about the mass balance of the ice: If the mass of snow that is added each year exceeds the mass of ice that is lost to the environment, the mass balance is positive, and then the ice cap grows. If the mass balance is negative, the ice sheet slowly disappears.
The tipping point of the ice sheet is where the climate has become so warm, or the ice has lost so much height, that the mass balance becomes negative due to the summer temperatures.
Committing to change
With climate change, every summer is getting warmer in Greenland. In 2021, it rained on the central ice sheet. In this way, part of the newly fallen snow disappeared. The ice sheet loses height in such a year, tnot only due to the loss along the coasts but also from the very top of the ice masses. If we do not tackle climate change, Greenland’s ice sheet may enter a state of continuous and accelerating mass loss.
A complete meltdown of the Greenland ice sheet would lead to a sea level rise of at least 7m. Even if this would take many millennia, it is important to take this into account, since we might already commit to this change with current greenhouse gas emissions, without realistic options to stop the process again in the future.
The Amazon rainforest is the world’s largest rainforest and a global biodiversity hotspot. Thousands and thousands of endemic species live here, which means they only live here. The Amazon rainforest has existed for millions of years, so long that its species are biologically adapted to the humid environment. Few of the plants or animals will be able to live under other conditions.
A rainforest crucially depends on the amount of rain and its seasonal distribution over the year. In tropical South America, the easterly tradewinds transport moisture from the Atlantic Ocean to the coastal regions where it rains out. From here, the trees feed the moisture back into the atmosphere through evapotranspiration. Evapotranspiration is the process where water transpires through pores in the green parts of the plants.
Once in the air, the moisture is picked up again by the trade winds and transported further west, deeper into the forest, where it contributes to the formation of rain clouds. This, in turn, creates an updraft in the air masses, which reinforces the winds traveling from the east to the west over the Amazon. The result is that the forest reuses its water as moisture evapotranspires, blows further west, and rains out in a repeated process.
The evapotranspiration, however, is also the Amazon’s Achilles’ heel. Because obviously, the moisture that the rainforest depends on only exists because the forest is already there. If it disappears, evapotranspiration rates will be substantially lower, and the region will likely not be wet enough for a rainforest to grow back.
The Amazon rainforest is threatened from two sides. Deforestation leaves an open landscape that contributes little to evapotranspiration and makes the vegetation more susceptible to fires. And, anthropogenic climate change is predicted to lead to an overall dryer South America in the coming century. If the levels of moisture in the Amazon fall below a certain threshold, the region is expected to tip to a savanna-like state.
Analyses of satellite data have revealed that the Amazon is indeed approaching this tipping point.
Ocean currents – the AMOC
In the northern Atlantic area, in particular, in Western Europe, Iceland, and southern Greenland, the climate is considerably warmer than in other regions at the same latitudes. This is due to Atlantic ocean currents that transport heat from the equatorial regions north towards the coasts of Europe, where the energy is released into the atmosphere. You could say that Europe has central heating installed.
The central heating system should not be confused with the Gulf Stream, a wind-driven current mostly along the eastern coast of the US. It is rather what scientists refer to as the Atlantic Meridional Overturning Circulation (AMOC), which transports heat at the ocean’s surface northward across the entire Atlantic, but then sinks to the bottom in the very north of the ocean, and runs southwards as a much colder current. The driving mechanism behind this is that the AMOC transports warm salty water masses northward, where they cool down and thus become heavier and start sinking to deeper ocean levels. It is above all the AMOC that gives Europe a relatively warm climate, with the Gulf Stream only forming a comparably small part of the total circulation system. Unfortunately, climate change risks weakening the AMOC.
Two stable states
From so-called paleo data, i.e. information about the climate of the past found in, for example, drill samples from ice and the seabed, it is known that the AMOC can change abruptly between two different stable states. At the moment, the ocean current system is in its strong mode, where large amounts of heat are transported northwards. In the opposite, much weaker mode, the central heating in the northern latitudes is essentially switched off.
Data analyses have revealed that the AMOC may have approached the tipping point at which a collapse from the strong to the weak mode might occur. If that happens, the climate will change in large parts of the world. Not only because the North Atlantic receives less heat. Also the heat will remain in the Southern Hemisphere and heat its surroundings there, thus contributing to the destabilization of the Antarctic Ice Sheets. The winds that blow are precisely controlled by where it is cold and where it is warm. So when global heat distribution changes, the Earth’s weather systems are influenced.
How close we are to AMOC’s tipping point, i.e. at which levels of global warming a collapse could happen, is very difficult to predict and currently highly uncertain.
Other parts of the climate system can tip
Other parts of the climate system that have been proposed to possibly have the potential to tip due to man-made global warming are the tropical monsoon system in Africa and Asia. The monsoons provide rainfall for many of the planet’s most important agricultural areas and disturbances would affect around 3 billion people.
A collapse of the AMOC would lead to shifts in the global tropical rainbands that would have severe impacts on the monsoon systems, possibly even leading to their failure. But also without an external trigger such as an AMOC collapse, anthropogenic forcing in terms of CO2 and aerosols could affect the monsoon systems.
Most of the subsystems that have been suggested to potentially tip in the future are coupled in some way. For example, meltwater runoff from the Greenland Ice Sheet will slow down the AMOC because it would make the North Atlantic water masses fresher and hence lighter; an AMOC weakening would change rainfall patterns in the Amazon and lead to enhanced melting in Antarctica, and so on.
It is not clear if these interactions could lead to so-called tipping cascades, where subsystems tip each other like domino pieces, abruptly affecting the global climate. It is also possible that the opposite might happen. In that case, the tipping of one element might block the tipping of others.
The Earth transitioned
If tipping cascades happen, it would lead to abrupt changes to the climate system as a whole. Indeed, in paleo data, i.e. data that contains information about the climate of the past, you can see that the Earth has from time to time undergone large and abrupt climate changes. In the past 66 million years, the Earth has transitioned between a very hot, a hot, a cool, and a cold climate.
Since the last ice age, we have had a stable, cool climate. It should be noted, however, that the transitions between the different Earth system states took many millennia or even millions of years, so on the timescale of human life they were not abrupt and indeed rather slow compared to the speed at which we modify the climate system via the release of greenhouse gases to the atmosphere.
To better estimate the risks associated with climate tipping points, we need more research into tipping points and the Earth’s historical climate.
What does it mean that a part of the climate system can tip?
It is not only climate subsystems such as ice sheets, ocean currents, and wind systems, that can tip. All physical systems that have more than one stable state will naturally have the potential to tip from state to state. We use the simple system of a chair as an example.
The physical system, the chair, can stand up or lie down. These are the stable states of the chair. Now imagine that the chair is standing upright, but that the floor tilts and becomes increasingly slanted. Soon the chair’s tilt will reach a point where it is on the verge of overbalancing. A very weak push will be sufficient to knock it over. If the floor continues to slope further, the chair will suddenly fall “by itself”. The chair has passed its tipping point. Note that in the illustration we have hinged the legs of the chair so that it cannot slide.
Sudden climate change
The chair on the sloping floor can be used as an illustration of how a climate subsystem can tip due to our emissions of greenhouse gases. The condition of the climate system depends on the amount of greenhouse gas in the atmosphere in the same way that the condition of the chair depends on the increasing slope of the floor in our analogy. Increasing amounts of CO2 in the atmosphere affect temperatures and precipitation conditions worldwide. The changes can cause parts of the climate system, for example, the Greenland ice sheet, the Amazon rainforest, and ocean currents, to suddenly change state.
Tipping typically cannot simply be un-done
Once a physical system such as a chair, a forest, or an ice sheet has tipped into a new state, the system can be difficult to tip back again. In the case of the chair, the floor has to be tilted much further back in the opposite direction – almost back to vertical! – before the hinged chair rises “by itself”. For the climate system, this means that we must expect that if we change the CO2 content of the atmosphere so much that the system tips, it will likely not be sufficient to return CO2 levels to pre-industrial levels. Perhaps the Earth will have to live through another ice age before the climate again resembles what we know today.
How common is tipping in climate systems?
Actually, no one knows for sure. Data from the climate of the past indicate that parts of the climate system and indeed the entire climate system have tipped repeatedly in Earth’s history. Climate theory and measurements suggest that certain elements of the climate system are approaching their tipping points due to global warming. For example, it seems that the North Atlantic Ocean currents, the Amazon rainforest, and an area of the Greenland ice sheet have been approaching tipping points in recent decades.
Can we predict tipping early enough to prevent it?
Climate scientists are trying to develop so-called Early Warning Systems. They take advantage of certain characteristics a climate system has when it is on the verge of tipping over. Consider the chair one more time. When it is completely stable and far from its tipping point, the chair stands firmly on the floor. A gentle push will only cause it to vibrate slightly. But if the floor is sloping and the chair is close to the tipping point, a corresponding push can cause the chair to sway dangerously. The chair, therefore, does not behave the same far from and close to its tipping point.
Early Warning Systems are exactly based on the possibility of measuring this difference in the behavior of climate subsystems that are approaching their tipping points. Three parameters are being monitored: 1) Increased sensitivity (The chair reacts more easily to a push). 2) An increase in the natural variation (the chair sways more), and 3) a tendency for the system to take longer to return to a stable state. Technically, these properties are called sensitivity, variance, and autocorrelation.
Early warning signals have been recorded in data from the Amazon rainforest, a part of the Greenland Ice Sheet near Ilulissat in Central-Western Greenland, and in the AMOC, a system of ocean currents in the North Atlantic that distributes heat to the northern hemisphere from the south. These three climate sub-systems thus appear to be moving in the direction of their tipping points.
Tipping is typically triggered before the tipping point is reached
A physical system that is close to its tipping point has almost completely lost stability. This means that two types of events can cause the system to abruptly switch to another stable state “ahead of time”.
1) Noise causes a system to tip before it has actually reached its tipping point. The technical term is noise-induced tipping. For the chair, noise is not sound, but rather vibrations. If we imagine that the surface on which the chair is placed, is the bed of a moving truck, the unevenness of the road will cause the car to shake. The vibrations can mean that the chair suddenly crosses its tipping point and it falls over even if the bed is not actually tipped enough for this to happen “by itself”.
In the climate system, it is usually the weather that is the noise. Because, while the climate changes relatively slowly over decades, the weather, as you know, varies a great deal throughout the year. The weather can therefore move a climate system past its tipping point. This can happen, for example, if a severe storm transports warm water to the coasts of West Antarctica. Or if an unusually heavy, persistent rain over the ocean changes the salt balance at the sea surface, weakening ocean currents. It is known that a dry summer can damage the Amazon, temporarily limiting evapotranspiration (See: Amazon rainforest). Just as noise (vibrations) can cause the chair to tip over on the bed of the truck, noise (especially extreme weather events) might tip climate subsystems.
The rate of change
2) Rapid changes can also cause a climate system to tip prematurely. The technical term is rate-induced tipping. The chair on the lorry’s sloping bed is again a good example. If the truck suddenly brakes hard or perhaps accelerates strongly, the chair can pass its tipping point and topple “prematurely”. Computer calculations have shown that the system of ocean currents in the North Atlantic that helps pump heat from the Equator to Europe can tip if conditions change very quickly. In our time, we increase CO2 in the atmosphere with fossil fuels much faster than the Earth has experienced at least in many millions of years. It is therefore a risk in itself that the climate changes of our time are happening so quickly. The rate at which we are changing the climate may be enough to tip the climate system.
Listen to this podcast about how tipping can be triggered in different ways.
Is it always a disaster when a climate subsystem tips? Nah.
Researchers have found that it is not always catastrophic when a climate subsystem tips. Sometimes you can reverse the process after the system has passed its tipping point. Other times, the system tips only partially, i.e. in smaller portions. What does it mean?
Well, it means that we may not need to fear that if, for example, parts of the Greenland ice sheet hit a tipping point, the entire ice sheet will disappear. Instead, it is possibly more correct to imagine that different areas of the ice sheet each have their own tipping points and not a common one. Thereby, only part of the ice disappears when a tipping point is exceeded. The rest still have a chance to survive climate change.
Some systems may also react to climate change so slowly that we might get away with too high global mean temperatures for a limited amount of time. It has been shown that this may be the case for Greenland’s ice sheet. The reason is that the ice cap is so incomprehensibly large that even severe climate change cannot possibly manage to melt a particularly large proportion of the ice if we rush to lower the global average temperature before a few centuries have passed. That is to say: If we quickly pull CO2 out of the atmosphere in the future, we may be able to prevent Greenland’s ice sheet to collapse, thus preventing the sea to rise many meters although we might have crossed the tipping point temporarily.
The biology of a lake is another example of a system that is not necessarily lost because it is tipped. Let’s say excess fertilization has given rise to strong algae growth in the lake. The algae die and use up all the oxygen. Fish and plants suffocate. Water and lake bed rot. The lake’s biology has hit a tipping point – the amount of fertilizer where the amount of algae is so great that the lake tips to a new state.
Much of the lake’s biology may be lost forever, even if the lake becomes clear again. But typically it will only be parts of a large lake that die out, scientists point out. This means that life often spreads back into the extinct parts of the lake, the moment it is healthy again.
Avoiding the change
The three examples show that a system that has passed its tipping point can sometimes…
- Be stopped in the process.
- Be brought back on the safe side of the tipping point again.
- Quickly find its way back to its original steady state.
Calculations indicate that climate systems will sometimes respond like the lake and ice cap in the three examples. It might give us a chance to repair the damage we’ve caused. It is of course best to make certain that tipping is completely avoided by ensuring that greenhouse gas emissions and land-use change are reduced as much as possible, so climate change is slowed down as much as possible, and at best rolled back as soon as possible.