Earth’s climate systems are characterized by thresholds, levels that once crossed herald a new climate state. For example, we often talk about the 1.5 or 2oC thresholds across which the impacts of climate change become dangerous, for example including long heatwaves, devastating droughts and more common extreme weather events. Thresholds can become tipping points if, once crossed, there is no going back from the new climate state, at least temporarily. One of the best examples of a tipping point is the cessation in Atlantic meridional overturning circulation (AMOC) which we will learn about in Module 6 and which is a key driver of heat transport around the globe. This circulation drives the formation of ocean deep waters which in return feeds the Gulf Stream which warms northern latitudes including Western Europe making them more habitable. It also fuels monsoon rains in places like India. In the past, AMOC has turned off, driving Northern Europe into an ice age. The system is highly complex and difficult to predict, but there have been warning signs that it has been edging closer to that potentially devastating tipping point in recent decades. The system becomes more variable as a tipping point is reached, and that variability is currently showing signs of increasing. Tipping points may involve positive feedbacks. For example, melting and disintegration of the West Antarctic Ice sheet will lower planetary albedo resulting in further melting, and at some stage the feedback will make the system irreversible, at least temporarily. Another example is the melting of permafrost in the Arctic region. Tipping points can lead to cascading changes if they impact one another, for example, significant melting of Antarctic ice can cause enough warming to exacerbate permafrost melting. More examples of tipping points are showing in the figure below.
Tipping points represent one of the most dire threats of climate change due to their irreversible nature. Because of this, they are very much a key driver of the warming thresholds and emissions targets.