What Is Climate Overshoot?

Climate overshoot is the situation in which global mean surface temperature exceeds a specified limit, most often the 1.5 degree Celsius level named in the Paris Agreement, for a period before falling back below it. The term describes a trajectory rather than a state: it presumes both a failure to stay under the threshold and a subsequent decline driven by net negative carbon dioxide emissions. Scenario studies characterize an overshoot by three quantities, its peak magnitude, its duration, and the rate of the return, because the consequences depend on all three and not on the peak alone.

The concept became central to climate assessment once it was clear that most feasible emissions pathways consistent with 1.5 degrees involve some exceedance. Scenario databases distinguish no or limited overshoot, generally a peak up to about 0.1 degrees above the limit, from high overshoot, where the excursion is larger and longer. The IPCC Special Report on Global Warming of 1.5 degrees C set out the logic in its treatment of pathway feasibility: the larger and longer the overshoot, the greater the volume of carbon dioxide removal required to reverse it, and the greater the exposure to impacts during the excursion.

Overshoot in Emissions Pathways

Overshoot follows from the near-linear relationship between cumulative carbon dioxide emissions and peak warming. If cumulative emissions pass the budget associated with a temperature limit, warming exceeds that limit, and only net removal of carbon from the atmosphere can bring it down again. Integrated assessment models produce overshoot pathways when near-term mitigation is slower than the budget requires, compensating later with large deployment of removal technologies. That compensation is not symmetric in time: emissions occur over decades and are cheap to continue, while removal at the required scale is expensive and constrained by land, energy, and geological storage capacity. Analysis of the carbon dioxide removal range in 1.5 degree compatible and high overshoot pathways shows how cumulative removal requirements rise sharply as near-term ambition falls, spanning hundreds of gigatonnes of carbon dioxide across the scenario set.

Reversibility of the Physical System

The premise of overshoot is that temperature declines when carbon dioxide concentration declines, and for global mean surface temperature the assessed evidence supports that. Other components of the system do not reverse on the same schedule. Thermal expansion of the ocean and mass loss from ice sheets continue for centuries after peak warming, so sea level does not return with temperature. Ocean acidification responds to carbon chemistry with its own lag. Permafrost thaw releases carbon that must then be removed again, adding to the required budget. Ecosystem loss during the excursion, including coral reef mortality and species range collapse, is not recovered when temperature falls. The IPCC Sixth Assessment Synthesis Report assesses these asymmetries and concludes that overshoot carries risks that persist beyond the excursion itself.

Tipping Elements and Risk Under Excursion

The strongest argument against treating overshoot as a manageable detour concerns threshold behavior in large-scale components of the climate system. Sustained high temperature raises the probability of crossing a threshold in the Greenland or West Antarctic ice sheets, the Amazon forest, or the Atlantic overturning circulation, and those transitions can become self-sustaining once initiated. Whether a temporary excursion is enough to trigger them depends on the response time of the element relative to the overshoot duration, which is poorly constrained for most candidates. This has produced a body of work on peak-shaving strategies that prioritize keeping the peak low over promising a rapid return, and it is the main technical reason why near-term emission cuts and later removals are not interchangeable.

Applications

The concept of climate overshoot is used in a range of fields, including:

• Integrated assessment modeling and scenario design for mitigation policy
• Carbon dioxide removal technology assessment, including direct air capture and enhanced weathering
• Carbon accounting and net zero target setting for states and firms
• Long-term infrastructure and coastal planning, where sea level commitment governs design life
• Climate risk analysis in finance and insurance
• Research on solar radiation modification as a temporary peak-shaving measure