Changing Climate, Tipping Earth

Every year we hear more about how the climate is changing, and we learn more about the dynamics of the Earth System. In 2021 and 2022, the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment report was released. It stated more clearly than ever before that anthropogenic actions have and are changing the climate, and that rapid action is needed. With COP27 over, however, it seems that this rapid change is unlikely. It is easy to be lulled into a false sense of security over the seemingly-slow effects of climate change. However, the gradual changes to the climate that we have experienced so far are unlikely to continue forever: there are tipping points in the Earth System, where a small change in drivers can lead to a rapid and significant change in the system’s state.

Timothy Lenton and colleagues provided a formal definition of climate tipping points in their 2008 paper. Tipping points occur when a critical threshold is crossed (such as a certain air temperature), after which the system changes suddenly and sometimes irreversibly (such as ice sheet collapse). Tipping points can occur across the Earth System, from forest dieback to sudden ocean current shifts.

These tipping points do not exist in isolation. The tipping of one element can lead to the tipping of another due to feedback mechanisms in the Earth System, leading to a tipping cascade. For example, dieback of the Amazon rainforest could cause changes in the Earth’s climate system through climate and hydrological feedbacks. Additionally, collapse of the Greenland Ice Sheet could trigger changes in the Atlantic Meridional Overturning Current (AMOC).

In 2022, David Armstrong Mckay and colleagues (including Timothy Lenton) conducted a reassessment of the tipping elements identified in the 2008 paper.They found that even at 1.5ºC-2ºC of global warming (the goal of the Paris Agreement), multiple elements in the Earth System are at a greater risk of tipping, including the collapse of the Greenland and Antarctic ice sheets, and the abrupt thaw of boreal permafrost. These significant risks of rapid environmental change highlight the importance of limiting global warming to 1.5º through rapid action.

One limitation of the 2022 paper is that it only evaluates the temperature thresholds for passing these tipping points. However, there are other factors that could cause Earth System elements to tip. For example, due to a combination of climate change and deforestation, significant evidence suggests that the Amazon rainforest is reaching its tipping point. This has global implications due to the significant role the Amazon plays in regional and global circulation patterns and water recycling. Furthermore, the risk of tipping cascades is not evaluated within their temperature thresholds. For example, although they estimated that AMOC will collapse at a temperature increase of 4ºC, feedback mechanisms involving the Greenland ice sheet (which has a much lower threshold) could lead to an earlier collapse.

Although our knowledge of the Earth System and it’s tipping points has progressed, there are still many uncertainties. While some of these tipping points could be crossed at higher temperature increases than predicted, others could be reached at lower thresholds. Our understanding of the interactions between tipping points is limited, and some tipping points have not been quantified due to significant uncertainties. These uncertainties must not prevent anthropogenic action to mitigate climate change. Indeed, they should encourage urgent action.