Climate Tipping Points: Passing the Point of No Return

While climate records are being broken every year, the impacts of climate change are only just beginning. July saw an atmospheric carbon dioxide concentration of 426 ppm – over 50% higher than pre-industrial levels. This is the direct result of anthropogenic greenhouse gas emissions, and this persistent warming is driving the climate towards new states. 

However, the Earth’s system does not always exhibit a linear response to forcing and because of this it can change dramatically and irreversibly. These points of no return are climate tipping points – hidden thresholds capable of inducing sea level rises over 7 m and regional cooling of 20°C. 

Curbing emissions and ensuring that these points are not surpassed is paramount to maintaining the habitability of our planet. 

What is a climate tipping point?

The Earth as a system exists in a state of equilibrium. The atmosphere, biosphere, cryosphere, and hydrosphere are all interconnected and balance each other. However, under sufficient external forcing the system can be pushed over a critical threshold whereby feedback mechanisms perpetuate the change leading to a new equilibrium state – these thresholds are tipping points. 

A common analogy of these points is to imagine a ball in a valley. It is currently in a stable state – in other words it won’t roll due to gravity. However, if pushed with enough force it could pass over the top of one valley before rolling down into another. In this analogy the ball has passed over a tipping point and entered a new stable state. The amount of force required to send the ball into this new equilibrium defines the system’s sensitivity. 

In these stable states there is usual variability – whether this be daily, monthly, yearly, or longer. These smaller perturbations can be due to any factor ranging from planetary orbits to inherent cyclicity in regional climate systems. When disturbances come close to a tipping point, they often return to their original states more slowly – this property is often used by scientists as an early warning signal. 

Passing a tipping point typically has abrupt, widespread, and often irreversible, impacts. Thus, much scientific effort has been put into identifying tipping points, gauging their sensitivity and severity, as well as working out how close the current system is to them. 

What are the primary climate tipping points?

Studies have found as many as 25 climate tipping points, however, around 9 of these are high-confidence global thresholds. These 9 are some of the most researched and have been attributed to specific degrees of anthropogenic warming. 

With less than 2°C of warming three can still be passed. Accelerated West Antarctic Ice Sheet loss could be irreversible at this point, releasing vast amounts of freshwater and thus raise sea level by 4.3 m if fully collapsed. The Greenland Ice Sheet is a similar story but on a much larger scale, contributing 7.2 m to sea level if completely lost. All this freshwater release could be responsible for a third event: the collapse of the Labrador-Irminger Seas convection. This would lead to dramatic alterations to ocean currents and thus heat distribution in the North Atlantic leading to marine ecosystem displacement and more extreme weather events. 

Where else might be impacted?

Between 2 and 3.7°C of warming the Amazon rainforest could pass a point of no return and completely dieback. Also, the East Antarctic subglacial basins may also reach at point of permanent and complete ice loss after this amount of warming. 

Given 3.7 to 6°C of warming the boreal permafrost may collapse due to thawing and release up the equivalent of 175 billion tonnes of CO2 per 1°C of warming due to it storing vast amounts of carbon dioxide and methane. Furthermore, the Atlantic Meridional Overturning Circulation (AMOC) – the system of ocean currents in the Atlantic Ocean – could shutdown at this level. This would remove one of Northern Europe’s primary sources of heat and cause regional cooling of up to 20°C in a century. 

With more than 6°C of warming the East Antarctic Ice Sheet and Arctic winter sea ice may collapse. At this point sea levels would rise an additional 52 m and global warming would be amplified as the poles no longer reflect as much incoming solar radiation. 

Tipping points also exist for regional climates such as those for extrapolar glacier loss, low-latitude coral reef die-off, and West African monsoon collapse. Also, and most worryingly, there is the potential for tipping points to feed into each other causing cascades. For example, if the threshold for Arctic Sea ice was surpassed causing runaway ice loss, the Greenland Ice Sheet could face a similar fate. If this happens then the AMOC would likely shutdown due to the freshwater influx and cause massive Amazon Rainforest dieback, and the potential collapse of both the West Antarctic Ice Sheet and Wilkes Basin. 

What did they look like in the past?

These tipping events are present in the geological record. One example are Dansgaard-Oeschger events – or DO events for short. These events, typically identified in ice core records, have been tied to changes in the strength of the AMOC and exhibit warming spikes in the Northern Hemisphere on the order of 10-15°C within a few decades. Due to the enhanced warming, Northern Hemisphere ice melts causing vast amounts of freshwater to enter the oceans. When this happens the AMOC substantially weakens leading to abrupt cooling. These secondary phases are called Heinrich events. One study estimated 2.3 million km3 of freshwater entered the oceans within as little as 250 years leading to up to 3 m of sea level rise. 

The Younger Dryas is a past example of abrupt climate change. It saw a sudden cooling phase of 4-10°C in a few decades during a post glacial warming period between 12,900 and 11,700 years ago. While the cause of this event is debated there are three principal hypotheses: AMOC shutdown, a bolide impact, and large-scale volcanism. 

Our solar system also presents evidence of the effects of tipping points. During the early stages of Venus’s development its thick atmosphere caused a runaway greenhouse effect leading to the complete evaporation of its oceans, and for it to enter an irreversibly hot climate state. However, this scale of climate change on Earth would require external forcing on a celestial scale.

When could current warming surpass these thresholds?

There are many ways scientists gauge how close a system is to a tipping point. These early warning signals show lost system resilience and include analysis of recovery rates and autocorrelation. This type of analysis has shown that 75% of the Amazon rainforest has lost resilience since 2003 meaning it recovers from droughts and heatwaves at slower rates. Similarly, warm-water coral reefs are likely past their tipping point due to the committed warming that would still occur if emissions stopped today. Both events would have severe impacts on biodiversity, and the loss of the Amazon rainforest would cause droughts in South America while coral reef extinction would leave coasts less protected from storms.

The proximity to collapse and proposed impacts it would have mean that the AMOC and Labrador Current tipping points have been extensively researched. Using multiple climate archives and modelling it is thought that the AMOC is at its weakest in the last 1000 years. With weakening beginning in the mid-1800s, modelling studies predict a shutdown around 2050. It has been estimated that after collapsing, broad areas of northwestern Europe would see cooling in excess of 1°C per decade.  

Ice sheet tipping points pose a big issue

Ice sheet tipping points are another concern particularly given the proposed sea level rise their collapse would cause. However, precise estimates of their tipping point are not as well constrained as the AMOC due to the complexity of their stability. Most modelling incorporates the feedback a retrograde slope can have on ice sheet loss, but the impacts of ice cliff instability are not yet fully realised. 

It is estimated that there is significant risk of three tipping points being crossed in the 2030s, when the planet exceeds 1.5°C of warming. Therefore, more stringent and ambitious policies to address climate change are essential and COP29 could be a platform to help with the energy transition.

Are some climate tipping points beneficial to stop global warming?

While there are some natural feedback mechanisms that could help to drawdown CO2, the potential for these to reverse global warming to the extent of inducing cooling is a concern. 

However, from a human perspective many positive thresholds have already been passed that are key to producing the necessary future decarbonisation. These include: 

• Renewable energy – energy sources such as solar and wind are now cheaper than fossil fuels in most of the world. 
• Electric vehicles – global electric vehicle sales have tripled in three years and are projected to soon dominate the market. 
• Green ammonia – a zero-carbon fuel and fertiliser which is approaching being cheaper than carbon emitting alternatives.

While we rely on policymakers to address the larger-scale changes that are needed to prevent crossing these critical thresholds, if you are keen to help these vulnerable systems you can get involved in one of the many programmes WorkingAbroad have on offer. For example, you could aid Amazon rainforest conservation efforts in Peru or help protect the Great Barrier Reef in Australia.

Written by WorkingAbroad Blogger Edward Forman