Parts of the climate system (ice sheets, ocean circulation, permafrost, rainforest) may have thresholds beyond which change becomes self-sustaining. Armstrong McKay and colleagues (2022) assessed 16 such elements; five may already be at risk at today's warming, and more become likely at 1.5-2 °C.
The tier rates the quality of the evidence, not how likely the event is.
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| Source | Statement | Kind |
|---|---|---|
| WMO (2025) ↗ | 2024 was about 1.55 ± 0.13 °C above 1850-1900; the long-term warming level is estimated at 1.34-1.41 °C | OBSERVED |
| IPCC AR6 ↗ | Warming by 2081-2100 of about 1.4 °C (very low emissions) to 4.4 °C (very high emissions) | PROJECTION |
Expert estimates are personal judgements, not measurements, and experts disagree. This site never adds its own probabilities.
Caveat
Thresholds and timescales are uncertain; each element has a wide range. Passing a threshold does not mean the change is instant.
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● core element (global) · ○ regional impact element · dotted bar = uncertainty range, marker = best estimate.
Reading it: the bars that already overlap the cyan "now" band (Greenland, West Antarctica, abrupt permafrost thaw, Labrador Sea convection, coral reefs) are the five Armstrong McKay and colleagues say may already be at risk. A threshold being crossed does not mean the change is instant: the table below shows how long each takes once triggered, from a decade to ten thousand years. The 1.34-1.41 °C band is the WMO's estimate of long-term warming; 2024 alone reached about 1.55 °C.
| Element | Threshold °C | Range | Years to play out |
|---|---|---|---|
| Greenland ice sheet (core) | 1.5 | 0.8–3.0 | 10,000 |
| West Antarctic ice sheet (core) | 1.5 | 1.0–3.0 | 2,000 |
| Low-latitude coral reefs (regional) | 1.5 | 1.0–2.0 | 10 |
| Boreal permafrost (abrupt thaw) (regional) | 1.5 | 1.0–2.3 | 200 |
| Barents Sea ice (regional) | 1.6 | 1.5–1.7 | 25 |
| Labrador-Irminger Seas convection (core) | 1.8 | 1.1–3.8 | 10 |
| Mountain glaciers (regional) | 2.0 | 1.5–3.0 | 200 |
| Sahel / West African monsoon (regional) | 2.8 | 2.0–3.5 | 50 |
| East Antarctic subglacial basins (core) | 3.0 | 2.0–6.0 | 2,000 |
| Amazon rainforest (core) | 3.5 | 2.0–6.0 | 100 |
| Boreal permafrost (collapse) (core) | 4.0 | 3.0–6.0 | 50 |
| Atlantic overturning (AMOC) (core) | 4.0 | 1.4–8.0 | 50 |
| Southern boreal forest (regional) | 4.0 | 1.4–5.0 | 100 |
| Northern boreal forest (regional) | 4.0 | 1.5–7.2 | 100 |
| Arctic winter sea ice (core) | 6.3 | 4.5–8.7 | 20 |
| East Antarctic ice sheet (core) | 7.5 | 5.0–10.0 | varies |
Source: Armstrong McKay et al., Science 2022 ↗; projections from IPCC AR6 WGI SPM ↗; observed from WMO ↗.