Tipping points in the climate system

In short: A tipping point is a critical threshold beyond which part of the climate system shifts into a new state, often one that reinforces itself. Even if warming later falls again, the change can then be reversed only with difficulty or not at all.

In more detail: Scientists call such parts tipping elements, for example large ice sheets, ocean currents or ecosystems. The Intergovernmental Panel on Climate Change (IPCC) assesses the risk of such abrupt and irreversible changes as higher the more the Earth warms.

In detail

• The basic principle: Much like a chair tipped too far back, a system stays stable up to a threshold and then falls into a different state.
• Feedbacks: Many tipping elements reinforce themselves; when ice melts, for instance, less sunlight is reflected, the area warms more, and even more ice melts.
• Ice sheets: With continued warming, the Greenland and West Antarctic ice sheets could melt on a large scale over centuries to millennia and raise sea levels by several metres.
• Permafrost: When permanently frozen ground in the Arctic thaws, it can release carbon dioxide and methane, which drives warming further.
• Amazon rainforest: Deforestation and drought could turn parts of the rainforest into a drier, savannah-like landscape.
• Atlantic overturning circulation: This current system, which includes the Gulf Stream, carries heat northwards; researchers are investigating whether and when meltwater could weaken it severely.
• Coral reefs: Tropical warm-water corals are particularly sensitive to rising sea temperatures and are considered one of the most threatened systems.
• Uncertainty: Exactly where the thresholds lie is hard to determine; some studies suggest that certain tipping points could already be reached at 1.5 to 2 degrees of warming.
• Why it matters: Precisely because the thresholds are uncertain, every tenth of a degree of warming avoided is seen as helping to reduce the risk (klimawandel-grundlagen, pariser-klimaabkommen, moore-als-klimaschuetzer).