The Thresholds of Climate Tipping Risks

The risk of crossing climate tipping points is an imminent reality, not a distant concern. Within the Paris Agreement warming range of 1.5 °C to 2 °C, several systems are already likely to tip. These include the collapse of major ice sheets, the widespread die-off of coral reefs, and the abrupt thawing of permafrost.

Major climate tipping points include (continued from Map of Climate Tipping Points):

Permafrost thaw and collapse

Permanently frozen soils and sediments in vast boreal regions of Siberia and North America contain more carbon than is currently in the atmosphere – and as permafrost thaws, this carbon can be partially released as CO₂ and methane.

While gradual thawing is already underway, warming beyond 1.5 °C may trigger abrupt permafrost thaw in some regions, nearly doubling emissions compared to gradual melting.

Beyond 4 °C of warming, the abrupt drying of carbon-rich areas could lead to permafrost collapse or a “compost bomb” effect. In this scenario, the rapid decomposition of organic material produces its own heat, which further accelerates decomposition in a self-sustaining cycle. The resulting release of greenhouse gases could raise global temperatures by an additional 0.2–0.4 °C in just 50 years.¹

Atlantic circulation collapse

The Atlantic Meridional Overturning Circulation (AMOC) acts as the planet’s central heating system, carrying massive amounts of heat from the tropics toward the North Atlantic and warming Northwestern Europe. This circulation is self-sustaining: salty tropical water cools and becomes denser in the North Atlantic, causing it to sink and create deep currents flowing back south.

Global warming adds freshwater to the North Atlantic – from both the melting Greenland ice sheet and increased Arctic rainfall – which prevents the water from sinking. Studies suggest the AMOC has already weakened by about 15%. While a total collapse is considered unlikely during the 21st century, it becomes likely if warming reaches approximately 4 °C. Such a collapse would lead to regional cooling in the North Atlantic of 4–10 °C, profoundly altering the climate of Europe.

Closely linked to the AMOC is the Labrador Sea Subpolar Gyre, a smaller rotating current between Greenland, Canada, and Iceland. It is more vulnerable and may reach a tipping point much sooner – at roughly 1.8 °C of warming. Its collapse would cause regional cooling of 2–3 °C and increase weather extremes in Europe.

Amazon rainforest dieback

The Amazon, the largest and most biodiverse rainforest on Earth, was historically a vital carbon sink. However, a combination of deforestation and climate-induced droughts and wildfires (about 18% of the forest has been lost as of 2021)² has caused the Amazon basin as a whole to become a net carbon source.

As warming continues, the trend will only worsen − rainfall will decline and dry seasons will lengthen in southern and eastern areas of the forest. Scientists estimate that at 3.5 °C of warming, a partial dieback affecting roughly 40% of the Amazon becomes likely, shifting the region into a degraded, or savanna-like state. This self-reinforcing drying could be triggered even sooner if deforestation reaches 20–25%. The dieback would further disrupt regional rainfall and raise global temperatures by approximately 0.1 °C over the following century.

Footnotes

1. Temperature thresholds, time horizons, and other details on this page are based on a thorough assessment of the tipping points by McKay, D.A. et al. (2022). Exceeding 1.5°C global warming could trigger multiple climate tipping points. Science 377, eabn7950. DOI: 10.1126/science.abn7950. ↩

2. Science Panel for the Amazon (2021). Amazon Assessment Report 2021. Executive Summary. United Nations Sustainable Development Solutions Network. ↩