In a biophysical perspective, the Earth is a system that primarily gets its energy from the sun, while the economy is seen as a metabolic organism that develops within the limits of the biosphere. Anthropogenic pressures on the Earth System have reached a scale where abrupt global environmental change can no longer be excluded.
The resilience of the planet has kept it within the range of variation associated with the Holocene state, with key biogeochemical and atmospheric parameters fluctuating within a relatively narrow range. At the same time, marked changes in regional system dynamics have occurred over that period. Despite some natural environmental fluctuations over the past 10 000 years (e.g., rainfall patterns, vegetation distribution, nitrogen cycling), Earth has remained within the Holocene stability domain. Although the imprint of early human activities can sometimes be seen at the regional scale (e.g., altered fire regimes, megafauna extinctions), there is no clear evidence that humans have affected the functioning of the Earth System at the global scale until very recently. However, since the industrial revolution, humans are effectively pushing the planet outside the Holocene range of variability for many key Earth System processes. A continuing trajectory away from the Holocene could lead, with an uncomfortably high probability, to a very different state of the Earth system, one that is likely to be much less hospitable to the development of human societies. Making incremental improvements to these existing systems is not enough to deal with challenges of this magnitude; instead, these disruptions are paving the way for transformational and paradigmatic change.
Earths life-sustaining systems have, for example, an influence on the composition of the atmosphere, the water cycle, the nutrient cycle, the pollination of plants and soil fertility. There is an urgent need to integrate the continued development of human societies and the maintenance of the Earth system in a resilient and accommodating state. Supporting the resilience and health of the whole system is to foster diversity and redundancies at multiple scales, and to facilitate positive emergence through paying attention to the quality of connections and information flows in the system. To participate appropriately in a complex dynamic eco- psycho-social system that is subject to certain biophysical limits, is to pay more attention to systemic relationships and interactions.
To stay within biophysical limits there are the non-negotiable planetary preconditions that humanity needs to respect in order to avoid the risk of deleterious or even catastrophic environmental change at continental to global scales. These thresholds are defined as non-linear transitions in the functioning of coupled human–environmental systems and are intrinsic features of those systems and are often defined by a position along one or more control variables. Trespassing thresholds -tipping points- trigger non-linear dynamics at the lower scales from a desired to an undesired state. These tipping points are thresholds where little change could push a system into a completely new state. For not trespassing thresholds / tipping points boundaries, human-determined values of the control variables, are established to keep a safe distance from collapse. There are nine planetary boundaries identified and cover the global biogeochemical cycles of nitrogen, phosphorus, carbon, and water; the major physical circulation systems of the planet (climate change, stratosphere, ocean systems); biophysical features of Earth that contribute to the underlying resilience of its self-regulatory capacity (marine and terrestrial biodiversity, land systems); and two critical features associated with anthropogenic global change (aerosol loading and chemical pollution).
Many planetary-scale processes primarily produce impacts at a sub-Earth System scale, where such sub-systems show varying degrees of sensitivity to change. The persistent march of a warming climate, for example as one of the boundaries, is seen across a multitude of continuous, incremental changes. Each creep up year after year, while the cumulative impact of these changes could cause fundamental parts of the Earth system to change dramatically and irreversibly. Climate change is associated with several sub-system tipping elements which all show varying degrees of sensitivity to a change in radiative forcing or temperature rise. The Earth’s climate does not respond to forcing in a smooth and gradual way. Rather, it responds in sharp jumps which involve large-scale reorganization of Earth’s system.
SUB-SYSTEM ELEMENTS CLIMATE CHANGE
Ice Masses (cryosphere entities)
- Arctic sea ice has been melting at an unprecedented rate, reducing in thickness and extent. The ice-albedo feedback, together with other processes, contributes to an amplification of the regional warming in high northern latitudes, which is currently twice as fast as the global mean. Some differences in seasonal sea ice extent between the Arctic and Antarctic are due to basic geography and its influence on atmospheric and oceanic circulation. The Arctic is an ocean basin surrounded largely by mountainous continental land masses, and Antarctica is a continent surrounded by ocean. In the Arctic, sea ice extent is limited by the surrounding land masses. In the Southern Ocean winter, sea ice can expand freely into the surrounding ocean, with its southern boundary set by the coastline of Antarctica.
- Greenland ice loss due to glacier flow into the sea and enhanced melting during summer has considerably increased in recent years. As a consequence, the ice sheet (three kilometers thick in some places) is becoming thinner and thereby loses height. The complete collapse of the Greenland ice sheet would cause a sea-level rise of up to seven meters over a timescale of hundreds to thousands of years. Around half of the melt that the Greenland ice sheet experiences occurs at the surface. The remainder occurs through melting at the ice sheet base and via the breaking off, or calving, of icebergs from its edge. Surface melt is likely to involve several different self-reinforcing feedback loops that can speed up melting. Probably the most important tipping point feedback though are elevation feedbacks – as the ice sheet gets lower via melting, more are areas are at lower and warmer altitudes, leading to further melting. Also important is the snowline – the elevation at which the ice sheet is covered in snow. Bright white snow has a higher albedo than dark, bare ice – which means it reflects back much more of the sun’s energy. So if the snowline migrates to higher elevations as the ice sheet warms, it means the ice will be absorbing more of the incoming solar radiation, causing more melting.
- The West Antarctic ice sheet can be destabilized by certain flow dynamics. As a consequence, the ice sheet is becoming thinner and thereby loses height. As its surface, which today still reaches into high, cold air layers, sinks it will be increasingly exposed to lower and warmer layers of air. This accelerates the melting process. The tipping point to a complete loss of ice could be already reached if global temperature rises by slightly less than 2°C. Although much smaller than its neighbor to the east, the West Arctic Ice sheet still holds enough ice to raise global sea levels by around 3.3 meters. Therefore, even a partial loss of its ice would be enough to change coastlines around the world dramatically. The long-term stability is of particular concern because it is a marine-based ice sheet. This means that it sits upon bedrock that largely lies below sea level and [is] in contact with ocean heat, making [it] vulnerable to rapid and irreversible ice loss.
- While the East Antarctic ice sheet, which stores the biggest part of Earth’s frozen reservoir of freshwater, seems stable today, large basins are also characterized by unstable topographic configurations. The West Antarctic Ice Sheet, whose base is below sea level, has long been considered the most vulnerable to collapse. With an assist from gravity, a deep current of warm water slips beneath the sheet, melting it from below until it becomes a floating shelf at risk of breaking away. In contrast, frigid temperatures and a base mostly above sea level are thought to keep the East Antarctic Ice Sheet relatively safe from warm water intrusion. But as climate change shifts wind patterns around Antarctica, it is suspected that warm water carried by a circular current off the continental shelf will start to invade East Antarctica’s once unassailable ice. If intensifying polar winds are responsible for the intrusion of warm waters beneath East Antarctica, the situation is likely to get worse. The increasing strength of those winds is owed in part to the contrast in temperature between Antarctica and the rest of the world. As greenhouse gases warm much of the planet, this temperature differential is likely to intensify.
- The arctic permafrost, which has been frozen for centuries or even millennia, is located in Siberia and North America. As the climate warms, there is an increasing risk that that permafrost will thaw. This brings microbes in the soil out of hibernation, allowing them to break down the organic carbon in the soil. This process releases CO2 and – to a lesser extent – methane. Thus, large-scale thawing of permafrost has the potential to cause further climate warming. Overall, the evidence indicates that there are several mechanisms for abrupt regional thawing, while for permafrost thaw more generally, it is expected that it will act as more of continuous positive feedback on climate change rather than an abrupt tipping point.
Circulation patterns
- The overturning circulation of the Atlantic is like a huge conveyor belt, transporting warm surface water northwards and, after cooling and sinking in high latitudes, cold deep water southwards. One of its main motors is the cold, dense (and therefore heavy) salt water which sinks near Greenland and the Labrador coast. If the amount of freshwater from melting ice in the northern latitudes increases, this deep-water formation could cease, slowing down the circulation motor. The Atlantic Meridional Overturning Circulation (AMOC) is a system of currents in the Atlantic Ocean that brings warm water up to Europe from the tropics and beyond. The AMOC forms part of a wider network of global ocean circulation patterns that transports heat all around the world. Climate change affects this process by diluting the salty sea water with freshwater and by warming it up. The dilution happens through increased rainfall and also melting of continental ice in the vicinity of mainly the Greenland ice sheet. And that makes the water lighter and, therefore, unable to sink – or at least less able to sink – which, basically, slows down that whole engine of the global overturning circulation. Depending on the exact nature of the stability of the circulation, it could be shutdown basically indefinitely for thousands of years into a new stable shutdown state. Or it could eventually recover.
- Normally, trade winds cause an upwelling of cold water in the Pacific near South America. Warm surface water then flows, driven by wind, from South America to South-East Asia. During the El Niño, a natural periodic warming in the Pacific Ocean weather phenomenon, the trade winds are weakened, and the surface water flows in the opposite direction, warming the southeastern Pacific in the region of South America. Strong El Niños can cause severe drought in dry climates such as Australia and India, intense flooding in wetter climates such as the Pacific Northwest and Peru, and causes more hurricanes to form in the Pacific and fewer in the Atlantic. If global temperatures keep rising, El Niños could continue to intensify, with major impacts on societies around the world with induce profound socioeconomic consequences.
- Indian monsoon seasons becomes more chaotic. This shift is driven by the movements of the sun through the seasons. In the northern hemisphere winter, the focus of the sun’s energy is over the southern hemisphere. This causes a prevailing wind over India from the north-east, bringing dry air from across the Asian landmass. The transition to the wet season arrives as the sun moves north of the equator in spring and early summer. Here, northern India, the Tibetan Plateau and surrounding regions heat up rapidly. Even with modest warming projected under the low-emission trajectories, the monsoons are likely to intensify. Extreme precipitation events are on the rise in large parts of India, especially multi-day deluges that lead to large-scale floods. Warmer temperatures are also speeding up glacier melt in the Himalayas, which is projected to increase flow rates in the Ganges and Brahmaputra Rivers. As importantly, destruction of mountains and hills, as well as development on floodplains and marshes, are exacerbating risk.
- West African monsoon shift refers to the seasonal reversal of winds and its accompanying rainfall to West Africa and the Sahel – a band of semi-arid grassland sandwiched between the Sahara Desert to the north and tropical rainforests to the south. The West African monsoon is notoriously unreliable, a very sensitive system. The warm ocean temperatures reduced the temperature contrast between the continent in the hot summer and the cooler surrounding waters. This saw the monsoon rains shift southwards away from the Sahel, causing drought. The effect was reinforced by the climate-vegetation feedback, where drier conditions saw less vegetation growth, a reduction in evapotranspiration and even less rainfall. A warming climate could actually bring more rainfall to the Sahel. As land heats up faster than the water, rising global temperatures could strengthen the land-sea contrast that helps drive the monsoon northwards each year. This could bring more rain to the Sahel and, perhaps, see vegetation return to some southern parts of the Sahara. A wetting that might be different from the past in that it seems to be made up of more extreme events – so more extreme rains and maybe longer periods of dry spells interspersed.
- The Jet Stream, a fast-flowing zonal air current meandering above the mid-latitudes of the Northern Hemisphere at a latitude of about 7 to 12 kilometers, separates cold Arctic air masses from the warmer air of the temperate south. Its air waves migrate eastwards and control synoptic-scale weather systems (i.e. formation of high pressure and low pressure systems) in the mid-latitudes. The air mass movement due to the Jet Stream seems to be slowing down.
Ecosystems (biosphere components)
- Amazon rainforest dieback. Over the past century, the average temperature in the forest has risen by 1–1.5 °C. In some parts, the dry season has expanded during the past 50 years, from four months to almost five. That all is driving a shift in vegetation. A rainforest is sustained by very wet conditions. But the forest itself plays a critical role in the local climate. As the forest is saturated with heavy rains, much of this moisture is returned to the atmosphere through evaporation. In addition, transpiration of moisture from plant leaves transfers water from the soil into the atmosphere. These two processes combined are called evapotranspiration. These processes keep the atmosphere moist, but also help drive convection – strong upward motion of the air – which, ultimately, creates clouds and more rainfall. A large part of the rainfall in the Amazon basin originates from water evaporating over the rainforest. A warmer global climate with declining regional precipitation in combination with deforestation and forest fire could push the rainforest towards a critical threshold.
- The coniferous forests of the Nordic regions represent almost a third of the global forest area. Climate change increases the stress on forests caused by spreading pests, increasing fires and storm damage, while at the same time a lack of water, enhanced evaporation and human exploitation inhibit their capacity to regenerate. Boreal forests are the largest biome, or ecosystem, anywhere on the Earth’s land surface and account for 30% of the world’s forests. They are a very important store of carbon. While there is a lot of uncertainty around the precise amount of carbon they hold, estimates suggest it is more than one third of all terrestrial carbon. The boreal zone, along with the tundra, is warming rapidly – approximately twice as quickly as the global average. Continued temperature rise could generate rapid changes in boreal forests, including dieback. Increasingly warm summers becoming too hot for the currently dominant tree species, increased vulnerability to disease, decreased reproduction rates and more frequent fires causing significantly higher mortality, all contribute. Lower tree species diversity could put boreal regions at particular risk of natural forest disturbances by factors such as drought, fire, pests and disease. Rather than showing gradual responses, boreal ecosystems will tend to shift relatively sharply between alternative states in response to climate change.
- Release of methane from seas and soils in permafrost regions of the Arctic, methane release is exacerbated by global warming. While it is a long-term natural process, it results in a positive feedback cycle, as methane is itself a powerful greenhouse gas. Global warming accelerates its release, due to both release of methane from existing stores, and from methanogenesis in rotting biomass. Obviously great permafrost degradation means a lot more carbon emitted to the atmosphere, which would have the effect of increasing the cycle of warming and even more degradation, with global implications. Palaeoclimate literature indicates that northward forest and tall shrub migration is a major Earth system feedback, mostly because of the darkened surface. This is particularly important in the late winter and spring due to the combination of high snow cover and moderate incoming solar radiation. Carbon emissions from permafrost thaw and dieback of forests at the southern edge of the boreal biome would accelerating feed back mechanisms.
- Coral reefs are extremely sensitive ecosystems which are damaged by slight changes in water temperature and acidity. Warmer water is the most common cause of coral bleaching which has been increasingly observed in recent years. Even with warming limited to 2°C, most of the current coral systems are expected to disappear. Under continued heat stress, corals expel the tiny colorful algae living in their tissues – known as zooxanthellae – leaving behind a white skeleton. The algae provide the corals with energy through photosynthesis. Without them, the corals can slowly starve. Although corals can reacquire their zooxanthellae if conditions turn more favourable, persistent thermal stress can kill off the coral communities of entire reefs. Once a coral system has collapsed, it takes several thousand years for the reef to regrow.
- The world’s oceans absorb large amounts of carbon. Much of this is used by algae for growth, and the carbon sinks with them to the ocean floor after they die. The function of this marine biological carbon pump could be impeded by the warming and acidification of sea water as well as by the more frequently occurring oxygen depletion.
The tipping points described above do not constitute an exhaustive list – indeed there are a number of other parts of the Earth system that have the potential to display tipping point behavior. Some examples include; shutdown of Antarctic bottom water formation; loss of alpine glaciers; a climate change-induced hole in the ozone layer above the Arctic; ocean anoxia (where areas of the ocean see a dramatic decline in oxygen).
Another concern around tipping points is the potential for one to trigger a cascade effect on others. Cascading effects might be common in the Earth system, this would be an existential threat to civilization. About 30 different potential social-ecological regime shifts could interact with one another through domino effects or hidden feedbacks. Domino effects occur when the feedback processes of one regime shifts affect the drivers of another, creating a one-way dependency. Hidden feedbacks rise when two regime shifts combined generate new (not previously identified) feedbacks; and if strong enough, they could amplify or dampen the coupled dynamics.