Climate Overshoot

Climate overshoot occurs when global mean temperature temporarily rises above a specified warming threshold, such as 1.5°C above the pre-industrial level, and later falls below it. It describes a temperature pathway, not a permanent revision of the threshold, and is measured by both the excess warming and the time spent above the threshold.

How overshoot occurs

Overshoot occurs when emissions reductions are too slow to prevent a threshold from being crossed, but stronger mitigation later reverses part of the warming. Returning below the threshold generally requires sustained net-negative global carbon dioxide emissions, achieved when human removal of carbon dioxide exceeds remaining emissions.

• Carbon dioxide removal may include biological, geochemical or technological methods that remove atmospheric carbon dioxide and store it durably.
• Strong reductions in methane and other non-carbon-dioxide greenhouse gases remain necessary alongside carbon dioxide removal.

Measuring an overshoot

Its severity depends principally on overshoot magnitude, the maximum temperature above the threshold, and overshoot duration, the period for which the threshold is exceeded.

• A low and brief overshoot is distinct from a high or prolonged overshoot, even if both eventually return to the same temperature level.
• Overshoot pathways create greater dependence on future carbon dioxide removal, whose scale, feasibility and sustainability are constrained.

Climate implications

Crossing a threshold temporarily does not make the resulting impacts temporary. Greater or longer overshoot increases risks of severe impacts, adaptation limits and irreversible changes, including ecosystem losses and continued sea-level rise.

• Returning temperature below the threshold cannot fully reverse all damage experienced during the overshoot period.
• The Paris Agreement, through Article 2, seeks to hold warming well below 2°C and pursue efforts to limit it to 1.5°C; overshoot describes a pathway relative to such temperature goals rather than replacing them.