Climate change is often described as global warming, with the implication of gradual changes caused by a steady increase in temperatures; from heatwaves to melting glaciers.

But we know from multidisciplinary scientific evidence – from geology, anthropology and archaeology – that climate change is not incremental. Even in pre-human times, it is episodic, when it isn’t forced by a human-induced acceleration of greenhouse gas emissions and warming.

There are parts of our planet’s carbon cycle, the ways that the earth and the biosphere store and release carbon, that could trigger suddenly in response to gradual warming. These are tipping points that once passed could fundamentally disrupt the planet and produce abrupt, non-linear change in the climate.

A game of Jenga

Think of it as a game of Jenga and the planet’s climate system as the tower. For generations, we have been slowly removing blocks. But at some point, we will remove a pivotal block, such as the collapse of one of the major global ocean circulation systems, for example the Atlantic Meridional Overturning Circulation (AMOC), that will cause all or part of the global climate system to fall into a planetary emergency.

But worse still, it could cause runaway damage: Where the tipping points form a domino-like cascade, where breaching one triggers breaches of others, creating an unstoppable shift to a radically and swiftly changing climate.

One of the most concerning tipping points is mass methane release. Methane can be found in deep freeze storage within permafrost and at the bottom of the deepest oceans in the form of methane hydrates. But rising sea and air temperatures are beginning to thaw these stores of methane.

This would release a powerful greenhouse gas into the atmosphere, 30-times more potent than carbon dioxide as a global warming agent. This would drastically increase temperatures and rush us towards the breach of other tipping points.

This could include the acceleration of ice thaw on all three of the globe’s large, land-based ice sheets – Greenland, West Antarctica and the Wilkes Basin in East Antarctica. The potential collapse of the West Antarctic ice sheet is seen as a key tipping point, as its loss could eventually raise global sea levels by 3.3 metres with important regional variations.

More than that, we would be on the irreversible path to full land-ice melt, causing sea levels to rise by up to 30 metres, roughly at the rate of two metres per century, or maybe faster. Just look at the raised beaches around the world, at the last high stand of global sea level, at the end of the Pleistocene period around 120,0000 years ago, to see the evidence of such a warm world, which was just 2°C warmer than the present day.

Cutting off circulation

As well as devastating low-lying and coastal areas around the world, melting polar ice could set off another tipping point: a disablement to the AMOC.

This circulation system drives a northward flow of warm, salty water on the upper layers of the ocean from the tropics to the northeast Atlantic region, and a southward flow of cold water deep in the ocean.

The ocean conveyor belt has a major effect on the climate, seasonal cycles and temperature in western and northern Europe. It means the region is warmer than other areas of similar latitude.

But melting ice from the Greenland ice sheet could threaten the AMOC system. It would dilute the salty sea water in the north Atlantic, making the water lighter and less able or unable to sink. This would slow the engine that drives this ocean circulation.

Recent research suggests the AMOC has already weakened by around 15% since the middle of the 20th century. If this continues, it could have a major impact on the climate of the northern hemisphere, but particularly Europe. It may even lead to the cessation of arable farming in the UK, for instance.

It may also reduce rainfall over the Amazon basin, impact the monsoon systems in Asia and, by bringing warm waters into the Southern Ocean, further destabilize ice in Antarctica and accelerate global sea level rise.

The Atlantic Meridional Overturning Circulation has a major effect on the climate. Image: Praetorius (2018)