Nine Critical Systems Threatened by Climate Change
Editorial Note: This article has been written for Quantock Eco by Lottie Leigh-Browne BSc, MSc, a long time QE associate. As an undergraduate at Cardiff University she received QE’s first bursary “to help with expenses” incurred in writing a dissertation on Anaerobic Digestion at the University’s School of Earth and Ocean Sciences. Having graduated with first class honours, she completed her MSc in Environmental Science at Bristol University and is now working for eCountability Ltd on international biodiversity consultancy projects.
The article describes the existential crisis faced by humanity and the multiplier effect of nine different potential tipping point crises. Lottie and QE would welcome your view on the subject. Please email it to quantockeco@btinternet.com.
Introduction
It is well known that climate change is causing records to be broken across the globe. Some of the most alarming discoveries surrounding climate change suggest that the Earth may be approaching key ‘tipping points’.
The Nine Climate ‘Tipping Points’
Where Climate Change could Push Parts of the Earth System into Irreversible Change
These tipping points are described as changes that could push a system into a completely new state. These changes are unprecedented and irreversible. The media is constantly presenting news on the changing climate across the globe, including declining arctic sea ice, record-breaking heatwaves to catastrophic storms. These changes reflect the gradual increase in anthropogenic greenhouse gas (GHG) emissions resulting in a gradual warming of the atmosphere.
Scientists estimate that for every tonne of carbon dioxide (CO2) emitted, the Arctic summer sea ice is reduced by three square metres. Other parts of the Earth System are more difficult to measure and have the potential to change more rapidly, by crossing its ‘tipping point’. Rather than gradual warming, a dramatic shift in an entire system can be seen. This shift from one state to another can take decades or even centuries, but once the tipping point is crossed, it is impossible to go back.
Tipping points can also be caused by natural fluctuations in the climate. These are referred to as ‘noise-induced’ tipping points. During the last ice age tipping points occurred repeatedly. Although not applicable to the current interglacial period, it highlights that the Earth System has previously been unstable across multiple timescales.
We are now forcing the system due to anthropogenic GHG emissions causing global temperatures to increase at a higher rate than the most recent deglaciation period. The system is highly sensitive and natural fluctuations together with anthropogenic climate change can result in that final nudge to its tipping point.
The term ‘tipping point’ was first written about by journalist Malcolm Gladwell in his book ‘The Tipping Point’, published in 2000. Gladwell describes the tipping point as ‘the moment of critical mass, the threshold, the boiling point’. Since then, the term has entered the climate change vocabulary. In 2005, Dr James Hansen, a climate scientist at Columbia University’s Earth Institute stated, ‘we are on the precipice of climate system tipping points beyond which there is no redemption’.
In 2008, Exeter University released a landmark paper about the risk of climate tipping points. The paper stated that the dangers would only arise when global warming exceeded 5oC above pre-industrial. Recently the authors have argued that the risks are much more likely and imminent, and some tipping points may be reached at 1oC warming.
The IPCC’s fifth assessment report (AR5) published a definition in 2013; ‘We define abrupt climate change as a large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems’.
This passing of an irreversible tipping point means the system cannot revert to its original state even if the forcing decreases or reverses. This is known as ‘hysteresis’, when a system undergoes a change that is difficult or impossible for the system to revert to a previous state.
A Carbon Brief article uses a Jenga analogy to describe tipping points, stating that a small nudge can result in a tower collapsing, with the amount of energy required to rebuild it being significantly greater than energy used to tip it. A known example of this is in Greenland. The Greenland ice sheet has been present for hundreds of thousands of years. If the ice sheet were to pass a tipping point that led to its collapse, any reduction in GHG emissions or decrease of global temperatures would not guarantee ice sheet regrowth.
What follows discusses nine potential tipping points in the Earth System. It is important to highlight the use of ‘potential’ here. The uncertainties associated with these tipping points is still being explored in the scientific literature. However, each of the systems discussed have shown the potential to react to small changes, risking crossing a tipping point in future.
1.Amazon Rainforest Dieback
The Amazon is the largest rainforest in the world and covers nine countries in south America. It plays a vital role in the Earth’s climate system, recycling water to sustain rainfall and drive atmospheric circulation in the tropics. As well as this the forest is a major CO2 store. It is estimated that the Amazon absorbs 5-10% of anthropogenic CO2 emissions. Deforestation removes this sink of carbon and replaces it with a source of carbon. Deforestation rates have been on the rise under the Brazilian President Jair Bolsonaro’s leadership with 2019 representing the highest number of forest fires and a deforestation rate 85% higher than in 2018.
The Amazon contains about 15% of the total carbon stored worldwide in vegetation, representing 70 billion tonnes. If this is released to the atmosphere, it would combine with oxygen in the atmosphere to form CO2, reducing the oxygen concentration in the atmosphere from 21% to 0.01%.
Estimates have suggested that the Amazon tipping point could lie in the range of 40% deforestation. With 17% of the forest lost since 1970 for cattle ranching and soy plantations alone, deforestation is threatening the stability of the Amazon.
The Amazon can also drive a direct shift in the climate by either reducing the amount of rainfall or a direct shift in the forest cover, consequently shifting the climate into a drier state that cannot support a rainforest.
The three potential consequences of this include a decline in rainfall in response to warming, a reaction to reduced transpiration in response to higher CO2 or the direct impact of deforestation, with fewer trees resulting in less evapotranspiration and less moisture entering the atmosphere. A drier climate means the forest is more vulnerable to a transition to savannah and increases the risk of forest fires.
Simulation of Amazon Deforestation – The Scenario in 2050
At 20-25% deforestation, the Amazon risks crossing a tipping point and switching to a non-forest ecosystem. It is suggested that this figure could occur in 15-20 years if the current rates are continued.
It is also suggested that a 4oC warming could be the tipping point to degraded savannah for most of the Amazon. The impacts of losing part of the Amazon would be felt globally, with changes to atmospheric circulation and weather patterns.
Although the IPCC’s AR5 report describes dieback of tropical forests as ‘reversible within centuries’, there is contention in the literature if this would be achievable. Reforestation requires significant water sources and time to reach peak carbon absorption, highlighting the significant importance of the Amazon in helping act as a carbon sink for anthropogenic carbon emissions
2. Shutdown of the Atlantic Meridional Overturning Circulation
The Atlantic Meridional Overturning Circulation (AMOC) is a system of currents in the Atlantic Ocean that brings warm, salty surface water up to Europe from the tropics and returns cooler deep water to the equator. The sinking of dense, therefore heavier waters in the high latitudes drives this circulation.
AMOC contributes to the global ocean circulation patterns that transports heat around the world and is responsible for the mild UK climate. Climate change threatens this cycle by diluting salty water with freshwater from glacial melt and changing precipitation patterns, resulting in less dense water that is unable to sink, consequently weakening the cycle.
Recent scientific literature has suggested that AMOC has already declined by 15% over the past decade due to global warming.
Thermohaline Circulation
Deep ocean current driven by variations in salinity and temperature
The IPCC’s ‘Special Report on the Ocean and Cryosphere in a Changing Climate’ (SROCC) suggests that the system may weaken by one third by 2100 if emissions continue at current rates.
Although a complete shutdown is seen as a low probability, uncertainties surround the temperature at which it might cross its tipping point. If it were to be crossed, models suggest a quick decline that takes decades, followed by a slower decline taking hundreds of years, which would be irreversible on human timescales. This shutdown is a good example of hysteresis as previously explained.
The impact of a slow down or even shutdown of AMOC would cause global cooling of the northern hemisphere, particularly around western Europe and eastern North America. This could lead to changing precipitation patterns, further exacerbating the effects of climate change such as wetter winters and dryer summers. These changes could also considerably impact the agricultural industry in the UK.
3. Coral Reef Dieback
Coral reefs are one of the most well-known sensitive receptors of global warming. ‘Mass Bleaching’ events of coral have occurred more often in recent years. Bleaching is caused by prolonged exposure of coral to high sea temperatures. This heat stress results in the coral expelling algae living in their tissues known as zooxanthellae, leaving a white skeleton behind.
This algae is important for the coral as it provides energy through photosynthesis. Without these, the coral risks starvation. With corals taking around 10-15 years to recover from a bleaching event, the state of the global coral reef communities is under threat. Corals can reacquire algae if the temperatures return to more favourable conditions; but prolonged exposure to high temperature can permanently kill coral communities.
The first global mass bleaching event was recorded in 1998 and is now becoming a regular occurrence. Severe bleaching events occurred every 25-30 years before the 1990s. They now recur every six years. Bleaching events often coincide with marine heatwaves or extended periods of unusually high temperature caused by a combination of El Niño conditions and anthropogenic warming.
As well as warming ocean temperatures, coral are exposed to overfishing, ocean acidification, shifts in ocean circulation patterns, nutrient runoff and sedimentation.
Fast growing macroalgae can takeover dead coral skeletons preventing them from being recolonised, resulting in an altered environment. The literature refers to these as dramatic and abrupt shifts in a community state from coral dominated to ‘space holder’ dominated, often referring to macroalgae.
A Coral Reef – Before & After Death
After the 1998 mass-bleaching event, coral cover declined to less than 1% in the Seychelles, while macroalgae increased by 40%. A declining trend of 80% from 1977 to 2001 was recorded in the Caribbean, with studies suggesting Caribbean coral could disappear by 2035, depending on further overfishing, climate change, and ocean acidification.
How well a reef recovers from a bleaching event is also to do with herbivores present. A 2007 study explained that herbivores play an important role in promoting reef resilience and recovery by removing algae. This does not necessarily result in a reversal shift from space holders to coral as herbivorous fish avoid macroalgae, making it harder for corals to recolonise.
The IPCC’s special report on 1.5oC states that under a 1.5oC global warming, a result of 70-90% loss of reef-building corals will be seen, with 99% being lost under 2oC warming. Some corals are more heat resistant than others, highlighting uncertainties in these figures. However, the impact on other reef inhabitants such as fish remains to be explored. Experts say we have already reached a tipping point for corals, with the Great Barrier Reef losing half of its corals in two years from the global bleaching event in 2014-2017.
Despite reefs covering less than 0.1% of the ocean floor, they hold more than one quarter of all marine fish species. They also support over 500 million people worldwide who rely on them for daily subsistence. This highlights the importance coral play in the global system and supporting populations or marine biodiversity and human livelihoods reefs.
4. Indian Monsoon Shift
India receives 70% of its annual rainfall during the monsoon season. The monsoons are crucial for India’s agricultural sector, which makes up 20% of the economy and employs half of the country’s population. The Indian monsoon, known as the South Asian Monsoon (SAM), occurs in June moving north across India until August.
The pressure gradient between the Indian Ocean and the Asian continent determines the strength of the monsoon. Land warms faster than the oceans, resulting in a strengthening of the pressure gradient with global warming. As well as this, a warmer atmosphere can hold additional moisture, resulting in greater rainfall.
The SAM is said to have two stable states that it can be in; wet (as it is now) and a low rainfall state. In order for the monsoon to remain in its current state, the heat released when the water vapour condenses to form rain, acting as an internal amplifier. This feedback can result in the magnification of any affect to the air pressure gradient, resulting in potentially abrupt changes.
It is stated that there is no evidence that there is a tipping point from a wet to a dry monsoon under 1.5-2ºC warming will occur, but an increase in the intensity of monsoonal rainfall is likely under a 3ºC warming.
The IPCC AR5 report states that areas affected by monsoons are likely to increase, with monsoonal winds weakening alongside more extreme precipitation events due to increased atmospheric moisture.
5. West African Monsoon Shift
The West African Monsoon (WAM) brings precipitation to the Sahara Desert in Western Africa and the Sahel. The WAM is driven by insolation by the Intertropical Convergence Zone (ITCZ) (a belt of low pressure surrounding the equator). The Sahel marks the ITCZ’s northerly position and the WAM brings rain to the region from June to September.
The monsoon is unreliable, bringing seasons of significantly reduced rainfall. In the 1960s and 80s, the average rainfall from this monsoon declined by 30% over most of the region compared to 1950s, causing the region to experience drought and famine.
The cause of this decline is due to increasing sea surface temperatures around Africa. This warm temperature reduced the temperature contrast between the continent and the ocean, resulting in the monsoon rains shifting southwards away from the Sahel. Warming of the oceans in response to climate changing and cooling in the North Atlantic due to air pollution from northern hemisphere countries has resulting in drying of the Sahel.
Although the rainfall in the Sahel has shown partial recovery in recent years, this highlights the sensitivity of this system to anthropogenic influences.
Some studies suggest that a warming climate could bring more rainfall to the Sahel, as the land heats up faster than the water, increasing global temperatures could strengthen this contrast and drive the monsoonal rains north. However, this rainfall is likely to occur in more extreme events, with extreme events interspersed with long dry spells. This is represented by climate models, showing both drier and wetter futures.
6. Permafrost Thaw
Permafrost is soil or rock that contains ice or frozen organic material that has been at or below 0oC for at least two years. Permafrost covers a quarter of all non-glaciated land in the northern hemisphere and can be up to a kilometre thick.
This frozen ground holds carbon accumulated from dead plants over thousands of years. Permafrost holds twice as much carbon as the atmosphere. Increasing temperatures risk this permafrost thawing, causing microbes to break down the organic carbon in the soil, which is then released as CO2 and methane into the atmosphere, further exacerbating climate warming. Methane is a greenhouse gas that is 30 times more potent than CO2 over a 100-year period.
The IPCC’s SROCC states that there is ‘very high confidence’ that record high temperatures at 10-20 m depth in permafrost have been documented, with temperatures 2-3oC higher than 30 years ago in the northern hemisphere. This thawing permafrost in the Arctic could release 300 – 600 million tonnes of net carbon per year to the atmosphere. By 2100, near surface permafrost will decrease by 2-66% under RCP2.6. This will release up to 240 GtC of permafrost carbon into the atmosphere.
Melting Permafrost Visible from Space
This system is warned to be ‘irreversible on timescales relevant to human society and ecosystems’ if tipping points are involved. Models suggest that decomposition may release warmth that triggers a compost bomb, where internal heat generation becomes the predominant driver for further thawing even if global warming declined.
7. West Antarctic Ice Sheet Disintegration
The West Antarctic ice sheet has the potential to raise global sea levels by 3.3 metres threatening to alter the worlds coastline with even a fractional loss of ice. This ice sheet is particularly vulnerable due to it being ‘marine based’, meaning it lies below sea level and therefore is in contact with ocean temperatures.
The force of gravity causes the ice sheet to flow from its interior to the Southern Ocean where ice touching the ocean melts and is replaced with snowfall at its interior. Warming temperatures are causing the ice sheet to lose more ice to the ocean than it gains in snow.
The rate of ice loss has tripled from 53bn tonnes a year in 1992 to 159bn tonnes a year in 2017.
The collapse of ice shelves does not directly cause sea level rise because they float on water. But thinning can trigger positive feedback loops that causes rapid and irreversible loss of ice to the ocean which adds to sea levels- known as ‘marine sea ice instability’.
Research has shown that the Amundsen Sea embayment of West Antarctica may have already passed a tipping point, where the ‘grounding line’, where ice, ocean and bedrock meet, is retreating irreversibly.
It is likely that we will experience sea levels continuing to rise, although the rate of melting is dependent on the magnitude of warming. At 1.5oC, it is suggested to take 10,000 years to unfold, compared with 1,000 years above 2oC. Researchers have not yet established whether the ice sheets are reaching a tipping point, although there is great uncertainty about their stability. The temperature threshold refers to regional warming in Antarctica rather than a global figure, as the poles warm more quickly, therefore these temperatures may be reached quicker than expected.
8. Greenland Ice Sheet Disintegration
The Greenland ice sheet is the second largest mass of ice on Earth and holds enough water to raise global sea levels by 7.2 meters. Melting of this ice sheet is currently accelerating and raising global sea levels by 0.77 mm each year. Although the tipping point for Greenland is unlikely to be abrupt, there is a threshold beyond which collapse is irreversible.
Half of this melt occurs at the surface (the rest occurs at the base by the calving of icebergs from its edge) involving numerous feedback loops that can result in the speeding up of melting. Positive feedback at the surface, as the surface lowers elevation from additional melting, creates more areas at lower and warmer altitudes, resulting in exacerbated melting.
Another important aspect is the albedo of the ice. Bright white snow has a higher albedo than dark, bare ice, therefore white snow reflects more of the sun’s energy. If the snowline (the elevation at which the ice sheet is covered in snow) migrated to higher elevations as the ice sheet warms, the ice will be absorbing more of the incoming solar radiation resulting in more melting.
Research suggests that snowline migration, the term given to this phenomenon, accounts for more than half of the year-to year variations in how much solar radiation is available for melting. This meltwater fills pores in the snow, giving off heat when it refreezes. This means it is harder for the snowpack to hold further meltwater, so any additional meltwater runs off into the sea. A tipping point is therefore related to how much of each of these processes occur.
The IPCC’s AR5 report states that it is exceptionally unlikely that Greenland ice sheet will suffer near-complete disintegration in the 21st century. However, studies show it is likely on longer timescales. A 1.8oC warming above pre-industrial could trigger feedback loops of decline in parts of the ice sheet. This could trigger a pattern of irreversible melting that continues for thousands of years. A 2012 study suggests an estimate of 1.6oC for this threshold.
Even following the relatively low emissions pathway (RCP2.6) with a warming of 1.5-2oC, the IPCC 1.5oC report states that it may trigger irreversible loss of the Greenland ice sheet. While it may stabilise at certain points during collapse, it will not regain ice mass until the next ice age which could be tens of thousands of years away.
9. Boreal Forest Shift
Boreal forests are found in the cold climates of the northern hemisphere. Boreal forests are made up of species that can cope with year-round freezing and low precipitation rate. These forests are the largest biome or ecosystem and account for 30% of the world’s forests. The literature suggests that they store more than one third of all terrestrial carbon. As well as this, a third of the boreal biome is underlain by permafrost.
Rapid warming in these regions has resulted in dieback, due to large-scale insect disturbances and an increase in fires, turning some regions from a carbon sink into a carbon source. Warming is causing changes in precipitation patterns, resulting in a shift into a warmer and drier regime. Such forests are an example of tipping points where a rapid shift into an alternative state in response to the climate occurs.
An extreme fire event or repeated severe events results in the ecosystem shifting to a sparsely wooded or grassland ecosystem. Increased tree mortality can result in a positive feedback mechanism of further regional warming and fire frequencies. This change is being seen in Alaska, where a shift from coniferous to deciduous vegetation began in 1990.
The IPCC concludes that a tipping point may exist between 3 and 4oC warming (low confidence) but the complex interacting mechanisms and feedback processes involved make this an uncertain estimate.
Shifts in vegetation cover also affect the albedo of the surface, altering the surface heat absorption and resulting in more rapid permafrost thaw.
These shifts however are stated to be more of a gradual transition, with any tipping points remaining unknown. However, thresholds remain for the conditions of tree species to survive, therefore the geographic location of the threshold is shifting due to climate change.
Conclusion
The tipping points discussed here are not an exhaustive list, with other parts of Earth’s system showing the potential to display tipping points. The evidence presented here highlights the uncertainty around when tipping points may be crossed.
Another concern is the potential for one trigger to cause a ripple effect and cause the risk of crossing thresholds in others. With examples of this already being seen. Recent research suggests that there are 30 different socio-ecological ‘regime shifts’ that could interact and cause a domino effect with each other.
Although evidence is lacking for when tipping points in the earths system may be crossed, the consideration of a tipping points helps strength the call for urgent action. Researchers need to improve their understanding of the observed changes in major ecosystems, as well as where future tipping points may lie.
This is an existential threat to civilisation and highlights the need to change our approach to the climate problem and implement drastic action now to ensure we do not reach tipping point.