Revised 2 December 2023

A tipping point in our climate system is a threshold which, if exceeded, leads to abrupt and large changes in the state of the system. Some of these changes can be irreversible. Timothy Lenton and colleagues (2008) identified nine tipping points and Will Steffen and colleagues (2018) have identified which tipping elements are most at risk. 

Figure 1: Tipping Elements at Risk (Steffen et al. 2018) 

Because different climate systems are interconnected, one system can have an impact on another. The arrows in Figure 15 show the potential interactions among tipping elements. Each increase in global warming risks a domino like cascade where a series of tipping point thresholds are exceeded. 

The Greenland ice sheet, Arctic summer sea ice, alpine glaciers, coral reefs, and West Antarctic ice sheets are already undergoing change with a 1.0 degree Celsius increase in global warming above preindustrial levels. Sea ice reflects more sunlight into outer space than uncovered water where sea ice used to be. With each melting of sea ice, the oceans absorb more heat which increases the level of global warming. The melting of sea ice involves a positive feedback loop where the melting of sea ice results in an acceleration in the melting of more sea ice. 

Figure 2: Positive Feedback Loop of Melting of Melting of Sea Ice 

An existential life-threatening tipping element is the thawing of permafrost. Tundra is located in a large treeless plain in the Arctic regions where the subsoil is frozen. This frozen subsoil, or permafrost, holds a vast amount of carbon accumulated from dead plants and animals over thousands of years. 

Figure 3: Tundra in Arctic Regions 

The units of carbon stock in Figure 4 are in Petagrams. That is one gram multiplied by ten to the power of fifteen - in other words multiplied by 10 fifteen times. The atmosphere holds about 589 Petagrams and permafrost stores about 1,700 Petagrams. There is much more carbon locked in permafrost than is currently in the Earth’s atmosphere. 


Figure 4 : Location of Carbon in the Biosphere

There is a side issue here. Carbon is also stored in the ground in the form of fossil fuels reserves. These reserves include gas (383 - 1,134 Petagrams), oil (173 - 264 Petagrams), and coal (446 - 541 Petagrams). The carbon locked in these fossil fuels far exceed that of carbon in the atmosphere. A year before the 2015 Paris Agreement, Christophe McGlade and Paul Ekins (2014) cautioned that most of these fossil fuel reserves must stay in the ground. At the 2015 Paris Agreement, nations agreed to restrict their use of fossil fuels and keep within a carbon budget in order to achieve net zero greenhouse emissions by 2050. But many countries have continued to explore for more reserves of fossil fuels. By doing so, these countries contravene the spirit of the 2015 Paris Agreement and use part of the agreed global carbon budget in exploration for more fossil fuels which should be used to transition from fossil fuels to renewable energy and infrastructure. 

As the climate warms, permafrost begins to 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 which is a far more potent greenhouse gas than CO2. 

Figure 5: Thawing of Permafrost 

There is already evidence of permafrost thawing. Large scale thawing of permafrost would result in irreversible change in climate. The result would be a hothouse Earth where no life forms can survive. 

Figure 6: Example of Current Thawing of Permafrost