The critical threshold of the Greenland ice sheet

The climate crisis is one of the biggest threats to the future of human society. The consequences of a warming world are vast and potentially catastrophic. Before 2100, millions of people may be displaced by rising sea levels, which are closely linked to the Greenland ice sheet.

As global temperatures continue to rise, extreme weather events such as heat waves, droughts, and floods are becoming more frequent. Changes in ocean currents, precipitation, and circulation patterns are expected, all while the global sea level steadily increases.

Some of these changes are expected to happen faster than others. For example, the atmosphere usually reacts to changes much faster than the ocean. In some cases, a change can happen abruptly. In this context, the terms “tipping points” and “tipping elements” are frequently mentioned, but what do they actually mean?

A tipping point is a critical threshold in a system, where positive feedbacks create abrupt, nonlinear changes. Close to the tipping point, even small perturbations can lead to big changes. These changes can be partially or fully irreversible, meaning that even if we reverse the change and go below the threshold again, the system may not recover. Such components of the Earth system are known as “tipping elements”.

To date, several potential tipping elements have been identified, for example, the Atlantic Meridional Overturning Circulation, the Amazon rainforest, the West Antarctic ice sheet, and the Greenland ice sheet. It seems more and more likely that we will cross the critical thresholds for many tipping elements by the end of this century. But what happens if we actually cross a tipping point? Is there a time window in which we could prevent the worst-case scenario?

We recently investigated this question for the Greenland ice sheet using computer models. We identified a critical threshold between 1.7 and 2.3°C above pre-industrial global mean temperature. Beyond this threshold, the Greenland ice sheet could be lost in the long term even without any additional warming. However, we also found a window of opportunity to reduce temperatures before the loss of the ice sheet becomes irreversible.

The Greenland ice sheet has already been significantly affected by rising temperatures. Currently, it loses more than 270 billion tonnes of ice annually, equivalent to around 0.8 mm of sea level rise per year. A complete melt would contribute over 7 m to global sea level rise.

Simply put, the health of an ice sheet is determined by its mass balance, the difference between ice accumulation and ablation (loss). Rising temperatures drive increased surface melt, leading to increased ice loss and a smaller mass balance.

In Greenland, various processes interact, shaping the fate of the ice sheet. Ice loss leads to lower ice surfaces and warmer temperatures, intensifying melting in a feedback loop known as the melt-elevation feedback, which is the main reason we expect a tipping of the Greenland ice sheet after we cross a critical threshold. Bedrock uplift due to reduced ice load counteracts this feedback to some extent, but does not prevent major ice loss. Additional factors, like the reflectivity of fresh snow versus old ice, and ice dynamics further impact the stability of the ice sheet.

In our study, we use two state-of-the-art ice-sheet models. Both models yield consistent results in the short- and long-term behavior of the Greenland ice sheet. We use these models to investigate how the ice sheet responds to temperature changes. Therefore, we force both models with increasing regional temperatures relative to the present day.

First, we raise temperatures at varying rates until the year 2100. Then we gradually decrease the temperatures within time windows ranging from 100 to 10,000 years. After the cooling phase, we stabilise at a “convergence temperature” between 0°C and 4.0°C above today’s levels. We run our simulations for at least 100,000 years to ensure that the ice sheet is in equilibrium.

Both models indicate increased melting of the Greenland ice sheet in response to rising temperatures, and that melting continues beyond 2100. The extent of melting depends on the temperature rise. Larger temperature increases lead to significant ice loss in both models, with a consistent pattern of sensitivity—initial retreat in the southwestern part, followed by the northern part. Extreme warming scenarios result in rapid ice sheet loss. Note that in this context, “rapid” means on the order of a few thousand years.

Most importantly, our models show that a temporary overshoot of the critical threshold is possible without committing to a full or irreversible retreat of the ice sheet. That means, if we manage to reverse the temperatures within a reasonable time after an initial exceedance of the critical threshold, large-scale loss of the Greenland ice sheet might be preventable. This is because the ice sheet has a lot of inertia and reacts on a very slow time scale. However, the window of opportunity strongly depends on the overshoot temperature and the subsequent cooling rate. In general, the higher the overshoot, the shorter the time window. It is important to keep in mind that the temporary sea level rise can still exceed several metres.

Our findings support previous research, highlighting the importance of limiting global warming to 1.5–2.5°C above pre-industrial levels to mitigate loss of the Greenland ice sheet and associated sea level rise. Timely action is crucial to prevent extreme warming by the end of this century.

However, long-term climate projections for Greenland beyond 2100 remain uncertain, with potential Arctic amplification reducing the ice sheet’s safety margin. The risk of irreversible loss of the Greenland ice sheet is a significant concern. While rapid cooling can offer a solution, prolonged warmth worsens sea level rise. It is vital to emphasise that even if the Greenland ice sheet does not reach an irreversible tipping point, its contributions to sea level rise can have a lasting impact. This underscores the urgency of proactive measures to protect our climate.