A tipping point is a threshold beyond which a climate subsystem shifts to a new state with no possibility of return on human timescales. Most concerning: Greenland ice collapse (+1.5–2 °C), Siberian permafrost thaw, Amazon forest dieback, AMOC weakening.
Some tipping points may already be activating: Greenland ice is losing mass at an accelerating rate and the AMOC has weakened by 15% since the mid-20th century. A cascade effect (one tipping point triggering another) is climatologists' most feared scenario.