Explainer: Nine ‘Tipping Points’ that Could Be Triggered by Climate Change

Climate change is disrupting the Atlantic Ocean’s Meridional Overturning Circulation (AMOC) system.

The AMOC system is part of a global network of water and heat circulation. Warm water flows from the tropics northward to Europe. There the water cools, sinks and moves back toward the equator. Global warming heats the water and dilutes it with freshwater runoff, mainly from Greenland’s ice sheet. The lighter fresh water can no longer sink, so circulation slows down. Research suggests a 15% weakening of the AMOC system since the mid-20th century. 

“We could pretty well minimise this risk by limiting the warming to below 2C. So, if we actually take the Paris Agreement seriously, then I would feel relatively relaxed about the risk of a shutdown. But if we continue on the current path and heading for three or more degrees, then this becomes a really serious concern.” (Stefan Rahmstorf, professor of physics of the oceans, Potsdam University)”

Some projections indicate a complete shutdown within 100 years, triggering catastrophic climate impacts on Earth’s northern hemisphere. Colder temperatures and less rainfall will cause loss of farmlands, and massive sea life die-offs. These effects are almost certain if global temperatures rise 3 to 4°C [5.4 to 7.2°F].

If the West Antarctic Ice Sheet (WAIS) disintegrates, coastlines will experience significant impacts.

The WAIS has constant contact with water, making it particularly vulnerable to warming oceans. Annual ice loss has increased threefold from 1997 to 2017, due to warming temperatures. Its eventual collapse, although not anticipated this century, could cause a sea level rise of more than 3 meters and swamp much of West Antarctica. Researchers continue to monitor a glacier drainage area on the WAIS called the Amundsen sea sector for clues. History suggests the WAIS disappeared from a few hundred thousand years to a few million years ago, when the Earth went through a similar warming trend.

Models indicate that the Amazon rainforest may begin an irreversible transition to treeless grasslands.

The Amazon rainforest’s massive moisture expiration creates the region’s rainfall. A global warming of 3°C plus continued deforestation will severely diminish the rainforest’s ability to produce rain. At 4°C the Amazon’s central, southern and eastern regions are projected to become savannahs.

“The post-deforestation climate will no longer be a very wet climate like the Amazon. It will become drier, it will have a much longer dry season, like the long dry seasons in the savannahs in the tropics in Africa, South America and Asia.” (Professor Carlos Nobre, University of Saõ Paulo’s Institute for Advanced Studies)”

Even without global warming, an excess of 40% of deforestation could prompt the transition. So far about 17% of the Amazon rainforest has been cleared, mostly for cattle ranching and agriculture. In 2019, deforestation rates were 85% higher than the previous year. Some areas have already reached a tipping point, and will probably become grasslands in 15 to 20 years. One recent study found that more drought resistant tree species are beginning to grow in these regions. The damage includes catastrophic loss of wildlife, and significant socioeconomic effects.

Because of its size, loss of the Amazon rainforest would seriously affect global weather patterns. Increased CO₂ release from tree loss would contribute to global warming, requiring even deeper cuts in emissions.

“Dry seasons in Amazonian regions are already hotter and longer. Mortality rates of wet climate species are increased, whereas dry climate species are showing resilience. The increasing frequency of unprecedented droughts in 2005, 2010 and 2015/16 is signaling that the tipping point is at hand.” (Professor Carlos Nobre, University of Saõ Paulo’s Institute for Advanced Studies, and Professor Thomas Lovejoy, George Mason University)”

The dieback of tropical forests worldwide could be reversible over centuries, but would be limited by how quickly new trees can grow.

West African monsoon regions could become wetter or drier as global warming increases.

The annual wet season (June through September) brings monsoon rains to West Africa and the Sahel, a stretch of grasslands between the Sahara desert and southern tropical rainforests. 

The West African monsoon (WAM) can be unreliable, causing droughts in some years. Severe dry seasons are thought to be caused by warmer tropical oceans and a cooler North Atlantic (due to air pollution) rather than by over-farming.

Some researchers think 3°C of global warming could bring more rain and vegetation to this region, as monsoons drive further north. Other studies show that ocean current changes caused by a slower AMOC could lead to warmer weather and weaker monsoon storms. Some predict a “mix of impacts,” with drought years alternating with extremely wet years. Most models indicate overall negative impacts on livestock, crops and human health.

“The question is which influence will win out for West Africa in the future – the direct influence of warming, which wettens, or the AMOC influence, that dries West Africa.” (Professor John Chiang, University of California, Berkeley)

Thousands of years ago, the Sahara desert may have been covered with lush vegetation and lakes. The changes were driven primarily by the sun, not large increases in greenhouse gases. Therefore, it provides limited insight into how the WAM will be affected by climate change.

Thawing permafrost significantly exacerbates global warming.

Permafrost, frozen organic material and ice (often undisturbed for thousands or millions of years), covers large areas of Earth’s northern and southern hemispheres, and shallow ocean regions. Permafrost contains twice as much carbon as Earth’s atmosphere. Because some areas are 2 to 3°C warmer than they were 30 years ago, massive amounts of CO₂ and methane are now being released at an alarming rate.

The 2019 Arctic Report Card from the US National Oceanic and Atmospheric Administration (NOAA) concluded that thawing permafrost may be releasing up to 600 tons of carbon into the atmosphere every year. By 2100, the surface layers may abruptly disappear, releasing hundreds of billions of tons of CO₂ and methane. 

The Intergovernmental Panel on Climate Change (IPCC) says it has “high confidence” that irreversible and widespread disappearance of permafrost will occur this century. Researchers at Stockholm Resilience Centre say that even if global warming ceased immediately, a “compost bomb” could be released in some regions, representing a tipping point. Such rapid thawing could be accelerated by fire or sudden drying events.

“If we stop warming, then we should see the emissions from permafrost stop, but in terms of that carbon going back into permafrost, pragmatically that’s not possible.” (Dr. Andy Wiltshire, terrestrial carbon cycle manager at Met Office Hadley Centre)”

In the Arctic Ocean, “methane bombs” loom on the horizon. Methane hydrates, icy bubbles of water and methane, reside under sea floors and continental shelves. Some theories suggest that ocean warming could melt these crystals, releasing massive amounts of methane. Whether methane bombs could precipitate a tipping point is hotly debated.

Coral reefs, highly vulnerable to global warming, are rapidly disappearing.

Coral ecosystems are collapsing from the stress of rising temperatures, over-fishing, polluted land runoff, storms, ocean circulation shifts and increasing ocean acidity. A 2007 study concluded that CO₂ levels above 500 parts per million (ppm) are dangerous for reefs. The levels are currently above 410 ppm and may exceed 500 ppm by 2100. Mass bleachings are already common worldwide. When corals die, toxic algae quickly take over the reef, making recovery nearly impossible.

A 2016 study showed that (conservatively) 90% of coral reefs will be at risk of bleaching by 2050, if global warming reaches 1.5°C. At 2°C, the risk increases to 99%. Reefs provide home to more than a quarter of fish species, and the consequences of their habitat destruction have not been fully explored. Loss of reefs will also devastate human populations, as they support more than 500 million people worldwide.

“We are already seeing severe bleaching around the world and the recent 2014-17 global coral bleaching event has been devastating for many reefs around the world. For example, the Great Barrier Reef lost half of its corals in just two years.” (Dr. Mark Eakin, coordinator of the US National Oceanic and Atmospheric Administration’s Coral Reef Watch)”

Many experts believe the tipping point for coral reef survival has already been reached.

Indian monsoons may increase in length and intensity due to climate change.

India’s summer monsoon season is critical to farming, which helps feed its population of 1.3 billion. According to recent studies, global warming and air pollution may reduce these rains. Sudden changes in the monsoons are relative to past climate changes, which occurred over thousands or even millions of years.

The IPCC concludes that if global warming is limited to under 2°C, major changes should not occur in India’s monsoon seasons, but increased rain and storms are likely with warming of 3°C, and the annual seasons will likely increase in length.

If the Greenland ice sheet melts, global sea levels would rise by 7.2 meters, devastating coastlines around the world.

Recent measurements indicate that the world’s second largest ice mass is deteriorating rapidly, causing an annual sea level rise of about 0.7 mm. About half of Greenland’s ice sheet melts at surface level, the rest as a result of calving icebergs. A tipping point will not happen quickly, but irreversible collapse is possible. 

“Probably the most important ‘tipping point’ feedback though are elevation feedbacks – as the ice sheet gets lower via melting, more are areas at lower and warmer altitudes, leading to further melting.” (Dr. Ruth Mottram, Danish Meteorological Institute)”

The IPCC reports that complete melting of Greenland’s ice sheet is unlikely in the 21st century. But a 2018 paper released by Nature Climate Change suggests that over thousands of years, 1.8°C could trigger an irreversible decline depending on the ratio of surface melt to annual snowfall.   

A 2019 Science article indicates that by the year 3000, Greenland’s ice sheet could melt from 8 to 100%, depending on the level of global warming. Even a 1.5°C warming represents a moderate risk of an ice sheet tipping point which would be “irreversible for millennia.” Even if the sheet stabilizes, it will not return to its previous size until the world’s next ice age, which could be tens of thousands of years from now.

As the boreal zone warms, tree die-offs are accelerated by fire, disease, drought and pests.

Boreal forests, located just south of the Arctic tundra, account for 30% of the world’s forests and are Earth’s largest ecosystem. Their cold-tolerant spruce, larch and pine hold more than a third of the world’s carbon. But as temperatures increase, they become more vulnerable to disease and less able to reproduce. 

As these trees die, large open grasslands favor more warming and more frequent fires, creating a feedback mechanism. More temperate species such as maples and oaks fill in warmer areas, while northern regions revert to tundra. At 1.5°C, woody shrubs encroach where tree growth is unlikely to recover. A tipping point for the boreal biome has been estimated at 3 to 4°C. Fire frequency could be a significant driver of vegetation change in the next 2-3 decades. The boreal forest will expand northward with a warming climate, along with tundra shrubs. Beneath the new forest growth, permafrost thaw will increase, contributing further to greenhouse emissions.

More tipping points could be alpine glacier loss, the Arctic ozone layer, a decline in Arctic sea ice, El Nino events and ocean anoxia.

Even if a tipping point is not reached, an ecosystem can be wiped out in a cascade effect that becomes an existential threat to humans. On the positive side, tipping points can be applied to positive human transformations, such as using renewable energy or driving electric cars. New policies could set humanity on the path to a more sustainable future. Social tipping points could lead the way to a carbon-neutral society that builds cities less dependent on fossil fuels and engenders climate education. These points must be reached quickly to avoid crossing the dangerous ones, which could irreparably damage the planet.