Where are Earth’s tipping points?

The IPCC Sixth Assessment Report states that it is unequivocal that human influence has warmed the atmosphere, ocean and land, and quantifies the warming attributable to greenhouse gas emissions. Scientists in the nineteenth century identified the main natural influences on climate and proposed that carbon dioxide from burning fossil fuels could warm it, and satellite and in situ measurements now track the energy the planet is gaining.

Arctic sea ice extent has declined substantially, with the largest losses in summer minimum extent. The IPCC finds that summer sea ice shrinks roughly in step with global temperature, with no tipping point, so this loss is potentially reversible. West Antarctica accounts for most of Antarctica’s ice loss, and the loss rate from it and the Antarctic Peninsula has increased since about 2000. Warm-water coral reefs have experienced repeated mass bleaching events, four of them global, the most recent of which NOAA judges likely ended in mid-2025.

Past climate records, from cave formations to ocean sediments, show large shifts that took place within a few decades. Whatever mechanisms produced them, the record establishes that the system is capable of rapid reorganisation.

A 2022 assessment identified a set of candidate tipping elements, among them the Greenland and West Antarctic ice sheets, warm-water coral reefs, permafrost, and the Atlantic overturning circulation, and estimated central thresholds, several within or near the range of warming already plausible this century.

The authors give each threshold a minimum, a central estimate and a maximum, and call crossing one possible beyond its minimum and likely beyond its central estimate. Several of the estimates, the Atlantic circulation’s among them, they call highly uncertain.

The Atlantic Meridional Overturning Circulation carries warm surface water northward. Whether it weakened over the twentieth century is disputed: several proxy reconstructions say it did, a 2025 study that infers it from air-sea heat fluxes finds no decline at 26.5 degrees north between 1963 and 2017, and the IPCC gives the twentieth-century change low confidence. Models disagree on how close it is to a threshold. The IPCC has medium confidence that it will not collapse abruptly before 2100.

Thawing permafrost releases carbon dioxide and methane, which cause further warming. The magnitude, timing and the balance between the two gases are poorly constrained, and the Arctic is warming up to four times faster than the global average.

Because these systems respond over decades to millennia, commitment to a change can precede the change itself by a long interval. It is possible to have passed a threshold before anyone can tell. The 2022 assessment could not rule out that the Greenland and West Antarctic ice sheets have already passed theirs, and the 2025 Global Tipping Points Report, by 160 authors, says warm-water coral reefs are already crossing their thermal tipping point.

Tipping elements are not independent: ice loss affects circulation, circulation affects monsoons, warming affects carbon release. Whether these couplings cascade or damp each other is unresolved. The 2022 assessment calls the strength, and in some cases the sign, of the interactions uncertain; a 2021 model study of four elements found that on balance they make tipping more likely.

Ecosystem responses are hard to project: uncertainty about each species carries through to the communities they form, and the spread of the answers is where that shows. The 2022 assessment of impacts puts the median share of species at very high risk of extinction at 9 per cent for 1.5 degrees of warming and 15 per cent for 5 degrees, and the likely range around those medians reaches 14 per cent and 48 per cent. At 5 degrees the upper end is three times the median.

If the 2022 assessment’s central estimates are broadly right, a small number of tipping elements (ice sheets and reefs among them) have thresholds close to warming levels already in prospect. This is a serious possibility drawn from a published expert synthesis, not a prediction.

A review of ten proposed tipping elements concludes that most of them do not have the potential for abrupt change within years, and that some may not tip at all, responding instead in proportion to how hard they are pushed. It does not offer that as reassurance: the same review says the uncertainties around many of the elements remain, which is a statement about how little is known rather than about how safe they are.

Systems approaching a threshold can show statistical signatures: slower recovery from perturbation, rising variance. Such signals have been reported in records that stand in for the Atlantic circulation, parts of the Greenland ice sheet and the Amazon rainforest. Turning them into a date is contested: a 2023 study estimated an Atlantic collapse around the middle of this century, and a 2024 analysis argues that the uncertainties are too large to predict any tipping time from historical data.