In 2008, the climate scientist Tim Lenton and colleagues — writing in the Proceedings of the National Academy of Sciences — published Tipping Elements in the Earth's Climate System, a paper that gave climate-risk vocabulary one of its most-used terms. They identified nine planetary subsystems where positive feedback could lock in a qualitatively different state once warming crossed some threshold: the Greenland and West Antarctic ice sheets, the Atlantic Meridional Overturning Circulation, the Amazon rainforest, the boreal forests, Arctic summer sea ice, the El Niño–Southern Oscillation, the West African monsoon, and the Indian summer monsoon. Most threshold estimates in the original paper were 3–5°C of warming. Fourteen years later, in 2022, Armstrong McKay et al. (Science) argued that several thresholds may already be at high risk between 1.1 and 1.5°C — i.e., now. The paper was contested in detail; the headline framing — from future risk to active monitoring of subsystems possibly already triggered — has been one of the more consequential reframings in climate science of the last decade.
The framework's central claim is that climate risk is not linear in warming. Each additional 0.1°C near a threshold is not equivalent to each previous 0.1°C far from one; the planet contains subsystems whose responses change qualitatively once a tipping point is crossed, and the policy-relevant question becomes not 'how much warming?' but 'how close to which thresholds?'. Some elements have clearly settled cores. The Greenland ice sheet has lost mass at accelerating rates, from ~50 Gt/yr in the 1990s to ~250–300 Gt/yr in the 2010s and 2020s. The Atlantic Meridional Overturning Circulation has weakened roughly 15% since the mid-twentieth century. Coral reefs have lost about half their cover since 1950 and now face bleaching events at unprecedented frequency. Arctic summer sea ice has declined ~13% per decade. The direction of these changes is not in serious dispute. What is contested is the threshold at which each element shifts to a qualitatively different state, and how reversible the shift is once it has happened.
The most-recent literature emphasizes interactions between tipping elements. Greenland melt freshens the North Atlantic, which further slows the AMOC; AMOC slowdown disrupts the Intertropical Convergence Zone, affecting the Amazon and African monsoons; Amazon dieback releases carbon, accelerating warming, threatening other elements. Wunderling et al. (2023) modelled cascading interactions across the cryospheric and ocean elements and found cascade probability rising sharply with warming. The 2023 Global Tipping Points report — coordinated by Lenton's group with ~200 scientists — concluded that five elements are at risk of triggering at current warming. The conclusion was contested by climate scientists who argued the language overstated thresholds, and defended by others who argued it was overdue. IPCC AR6 (2021) reaches substantially the same conclusions in more cautious language: high-confidence risks at 1.5°C, very high at 2°C. What survives the controversy is the vocabulary: the field is now organized around tipping elements as the unit of climate risk.
Climate-tipping-element monitoring is now a funded scientific activity. The International Thwaites Glacier Collaboration (US-UK) tracks the most-vulnerable Antarctic outlet glacier in real time; the RAPID array has been measuring AMOC strength continuously since 2004; the EU's PROVIDE project and the TipMip model intercomparison are running coordinated multi-model assessments. The policy implication is operational: even if global mean temperature is the headline number, what matters for downstream consequences is what tipping elements do under that warming, and that answer is more sensitive than the headline to actual emissions trajectory. The framework was developed in the last twenty years and may be the single most consequential conceptual addition to climate science in that period.