Tipping Elements - the Achilles Heels of the Earth System

Tipping elements are components of the Earth System that are sub-continental in scale and could be tipped into qualitatively different states by small external perturbations. After the transgression of a tipping point, self-reinforcing mechanisms drive the process without further external influences, and a rapid and often irreversible transition to a new state takes place. The environmental impacts are profound and could endanger the livelihoods of millions of people..

Figure: Map of the most important tipping elements in the Earth System overlain on global population density. There are three groups of tipping elements: melting ice bodies, changing circulations of the ocean and atmosphere, and threatened large-scale ecosystems. Question marks indicate systems whose status as tipping elements is particularly uncertain.

Click on the bold titles below to access more detailed information on a particular tipping element.

Ice Masses

When ice, which is light in colour, melts, it exposes a generally darker underlying surface, whether the rocky bed of a glacier or the sea. This darker surface absorbs more radiation from the sun, and this in turn accelerates the melting of the remaining ice. This mechanism, known as the ice-albedo feedback, is a classic example of a self-reinforcing process. But there are many other mechanisms (described below) which make the Earth's large ice masses a tipping element.

Melting of the Arctic sea ice

The Arctic sea ice is melting more rapidly than many scientists expected. It is likely that the Arctic will be ice-free in summer by the end of this century. It is not yet certain, however, whether this represents a tipping process or not, since thin ice, at least, recovers quickly in cold years. The loss of the ice the whole year round, however, would very likely be a tipping point.

Loss of the Greenland ice

The amount of ice that Greenland loses as glaciers flow into the sea and as melting occurs in summer has increased considerably in recent years. The ice sheet – three kilometres thick in places – is thus becoming thinner. While its surface until recently reached into high, cold air layers, it now is partially located in lower and warmer layers of air. This accelerates the melting. New studies indicate that the tipping point to a complete loss of ice could be reached if global temperature rises by slightly less than 2°C. The complete collapse of the Greenland ice sheet would bring about sea-level rise of up to seven metres over a timescale of hundreds to thousands of years.

Collapse of the West Antarctic ice sheet

Unlike the East Antarctic ice sheet, which is so far stable, the West Antarctic ice sheet is losing mass. Large parts of the sole of the ice sheet rest on the continental base beneath the surface of the ocean. If these ice masses were undercut by sea water, the entire ice sheet could become unstable. There are many indications that the West Antarctic ice sheet collapsed several times between 3 and 5 million years ago, when the Earth was considerably warmer than it is today. If the West Antarctic ice sheet were to break up, sea levels would rise by about five metres - a process which would take several centuries, however.

Shrinkage of the Tibetan glaciers

On the Tibetan Plateau, the roof of the world, temperatures increased about twice as much in the second half of the 20th century as the global mean. Most of the glaciers have shrunk in surface area and volume. Whether there is a tipping point at which irreversible melting would set in is as yet unknown. In any case, melting of the glaciers which feed the great rivers of southern and eastern Asia could endanger the fresh water supplies of millions of people.

Methane emissions from the ocean

Methane hydrates are lumps of methane and ice buried deep in ocean sediments. These help store an estimated up to two trillion tonnes of carbon in the ocean. Methane hydrates are a slowly acting tipping element. Global increase of temperature of about 3°C could—over millennia—release more than half the methane stored. Methane is a short-lived but very potent greenhouse gas. Most of it would oxidise to carbon dioxide within a decade and would add to warming in the atmosphere over a scale of thousands of years.

Melting of the Yedoma permafrost

The perennially frozen soils of the Siberian and North American Arctic could release huge amounts of carbon dioxide and methane if they thawed. This process is going on continuously in most parts of these regions without a tipping point being transgressed. But the Yedoma permafrost is an exception. Up to 500 billion tonnes of carbon are stored in this frozen carbon-rich loess soil in eastern Siberia. Here, warmth caused by microbial decomposition accelerates thawing and the decomposition of the soil. Under warming of 9°C in this region the thawing process could become unstoppable and three-quarters of the stored carbon could be released in the form of carbon dioxide within a century.

Circulation Systems

The world's main annual or seasonal patterns of atmospheric and ocean circulation are relatively stable, but they can change. In the course of our planet's climatic history there have been many phases of disruption and re-organisation. A brief outline of the possibly abrupt changes in circulation systems which could potentially occur in the future is given here.

Slowing down of the Atlantic thermohaline circulation

The overturning circulation of the Atlantic is like a huge conveyor belt, transporting warm water northwards and cold water southwards. The Gulf Stream, which is responsible for the mild climate of northwestern Europe, is part of this system of currents. One of its main motors is the cold, dense salt water which sinks to the depths near Greenland and the Labrador coast. If the amount of less dense freshwater from melting ice in the northern latitudes increases, this deep water formation could cease and the motor come to a stillstand. This would result in falling temperatures in the North Atlantic region and warming in the Southern Hemisphere, including possible effects on the tropical precipitation belt and regional sea level rise of up to one metre. Experts estimate the tipping point for the Atlantic thermohaline circulation to be above a global rise in temperature of 4-5°C.

Destabilisation of the Indian monsoon

Up to 90% of rainfall in India comes from the regular summer monsoon. The monsoon occurs due to an internal feedback mechanism which is responsible for constant transport of humid air from the sea to the land. The ensuing rainfall causes a temperature difference between land and sea which in turn supports the circulation of the monsoon system. Both carbon dioxide and aerosols play a key role in this very sensitive system. Air pollution, land-use changes and greenhouse-gas emissions could cause the system to swing between weaker and stronger monsoon events. This would result in alternating severe droughts and catastrophic flooding in Southern Asia.

Shift of the West African monsoon with impacts on the Sahara

Warming of the Atlantic ocean could trigger a "sudden" shift of the West African monsoon system. This could bring increased rainfall or increased drought to the people of West Africa, according to whether the rainfall belt shifts southwards to the Gulf of Guinea or northwards into the Sahel zone. In the latter case rainfall in the Sahel could increase and favour a re-greening of the Sahara – on condition that the region is not overgrazed. But this greening could also have negative effects. It could cut off the supply of desert dust that storms carry westwards across the Atlantic, supplying coral reefs in the Caribbean and even the Amazon rainforest with nutrients.

Disruption of the South Pacific climate oscillation and strengthening of the El Niño phenomenon

Although it is still very uncertain, some climate models predict increasing intensity of El Niño conditions in the South Pacific. In normal circumstances the trade winds bring about an upwelling of cold water in the Pacific near South America. Warm surface waters then flow from South America to South-East Asia. During the El Niño weather phenomenon the trade winds are weakened and the current flows in the opposite direction, warming the southeastern Pacific in the region of South America. The impacts of such a change of oceanic-atmospheric circulation patterns would be felt around the globe, for example, in the form of droughts in Australia and South-East Asia or increased rainfall on the west coast of America. It is even being debated whether there is a connection between El Niño and unusually cold winters in Europe.

Drying out of the South-Western United States

The southwestern part of North America is already experiencing declining rainfall due to the northward expansion of the subtropical dry zone. The oceanic and atmospheric circulation patterns which bring rain to the region show great similarities to a monsoon system. This could point to the existence of a tipping point, which if passed could suddenly confront the South West of the United States with still greater dryness.

Ecosystems

Climatic changes might cause an area to become too warm or too dry for particular types of plants or animals living in it, so that their ecological niches close and they can no longer exist there. Some species are able to migrate, for example further towards the poles or to a greater altitude. For species already living in polar or mountain environments there is no further space to migrate to. Suitable habitats are in any case rare in a world so thoroughly exploited by humans. Climate change could alter whole regions as ecosystems with their typical climate and adapted plant and animal communities disappear.

Retreat of the northern boreal forests

The coniferous forests of the nordic regions represent almost a third of global forest area. Climate change increases the stress on forests caused by pests, fires and storm damage, while at the same time lack of water, increased evaporation and human exploitation inhibit their capacity to regenerate. Once a critical threshold has been crossed, forests can be pushed back by scrub or grassland ecosystems. Loss of the forests would not only mean a loss of habitats for animals and plants, but also massive release of carbon dioxide, which in turn contributes to accelerated global warming.

Transformation of the Amazon rainforest

Much of the rainfall in the Amazon basin originates in water evaporating from the rainforest. A warmer global climate with declining precipitation in combination with deforestation could bring the rainforest to a critical threshold. This threshold could be crossed unnoticed and the tangible effects might only be experienced some decades later. The transformation of the Amazon rainforest into a seasonal forest adapted to drier conditions or to a grass landscape would have fundamental impacts on global climate, since the forest’s function as a sink for carbon dioxide would be lost. The transformation would also mean a massive loss of biodiversity.

Weakening of the marine carbon pump

The world's oceans currently absorb around 2 billion tonnes of carbon. Much of this is used by algae for growth, and sinks with them to the ocean floor when they die. The function of this so-called marine biological carbon pump could be impeded by warming and acidification of the water and the more frequently occurring oxygen depletion. If the ocean's capacity to absorb carbon dioxide is impaired, the concentration of this greenhouse gas in the atmosphere would further increase and both acidification of the ocean and warming of the atmosphere would accelerate in turn.

Destruction of coral reefs

Coral reefs are extremely sensitive habitats which are damaged by slight changes in temperature and especially by ocean acidification. Warmer water is the most common cause of “coral bleaching” increasingly observed in recent years. Bleaching means that the corals expel the algal organisms living within them and then often die themselves. Cold-water coral reefs which grow down to a depth of around 3000 metres are particularly vulnerable to ocean acidification. Once bathed in acidified so-called corrosive water, the calcium skeletons and shells begin to dissolve and the reef may collapse. If a collapse occurs it takes several thousand years for the reef to regrow.

Selected Publications on tipping elements with PIK contribution