- If the 1.5°C warming limit is exceeded during an overshoot, risks become increasingly severe with every increment of global warming. Impacts for humans and ecosystems worsen in relation to the magnitude and duration of overshoot period (i.e. by how much and for how long temperatures exceed 1.5°C). Impacts include loss and damage due to heatwave exposure and other extreme events, especially in tropical countries, as well as accelerated loss of biodiversity.
- Minimising the magnitude and duration of overshoot is crucial. An overshoot of multiple decades holds significant risks of irreversible changes in regional climate. There is also considerable risk that a long period above 1.5°C could trigger self-perpetuating feedbacks, destabilising the Greenland or West Antarctic ice sheets, which would result in their almost complete and irreversible loss over multiple millennia and several metres of sea-level rise.
- Keeping global mean temperature rise within 1.5°C is only possible in the near term with immediate, transformative action that rapidly decarbonises the economy, energy and land-use systems, cutting emissions by 43% by 2030 relative to 2019 levels. Mitigation efforts will also have to be complemented with carefully selected CDR methods at scale (Insight 3).
Recent evidence shows that we are not on track to keep global warming below or at 1.5°C, implying an overshoot of the pathway to comply with the Paris Agreement. Continuing to emit GHGs at the current rate will use up our carbon budget for 1.5°C warming in the next six to seven years. Current policies, if fully implemented, take global temperatures well above 2°C by the end of this century. Even fulfilling all national short- and long-term climate pledges would almost certainly exceed warming of 1.5°C.
The peak level of global warming depends directly on cumulative CO2 emissions, and this relationship determines the remaining carbon budget for the Paris Agreement temperature limits. The extent to which cumulative CO2 emissions exceed the carbon budget consistent with 1.5°C warming determines the magnitude of the overshoot. The duration of the overshoot period is determined by how long it takes to achieve net-negative CO2 emissions and thereby reduce CO2 concentrations in the atmosphere (see In Focus box). Both the magnitude and duration of the overshoot will determine the extent of climate impacts, during the overshoot period and afterwards.
Governments, corporations and other actors must now focus on minimising the magnitude and duration overshoot, while still acting to avoid it. Although the implementation of strong mitigation measures this decade to minimise the overshoot presents near-term challenges, postponing decisive action beyond 2030 raises feasibility concerns due to volatility and uncertainty caused by higher climate impacts (especially in low-income countries). Bringing temperatures down from peak overshoot will require every tonne of CO2 exceeding the carbon budget for 1.5°C to be removed from the atmosphere through CDR technologies, which may be unfeasible due to high costs, or unacceptable at scale due to their social and ecological impacts (see Insight 3).
Widespread and potentially irreversible impacts worsen with higher magnitude and longer duration of overshoot. Exposure to extreme events, such as heatwaves, will exacerbate biodiversity loss and economic damage, particularly in tropical countries. Irreversible impacts are especially likely for marine biodiversity, with species facing the added pressure of prolonged ocean acidification after overshoot. An overshoot period of multiple decades would have a long-lasting legacy on the Earth system. This is because some key Earth system components respond slowly to temperature changes. In some regions, surface air temperature and precipitation may not return to pre-overshoot values for several hundred years. Other irreversible changes (timescales of centuries or more) include permafrost carbon loss, sea-level rise from ocean thermal expansion, melting of ice sheets, and changes in ocean acidity, oxygenation and temperature.
Furthermore, a long period above 2°C of warming carries a considerable risk of triggering self-perpetuating feedbacks leading to instabilities of the Greenland or West Antarctic ice sheets or mountain glaciers, largely irreversible even on timescales of centuries to millennia. The impacts could include several metres of sea-level rise in the long term, causing loss of land, livelihoods and cultural heritage in coastal communities and small island states, and irreversible degradation of mid-latitude coral reef species.
Reaching net-zero CO2 emissions is crucial for containing the peak warming level
The world’s ability to bring temperature down after exceeding 1.5°C warming, relative to the pre-Industrial era, depends on removing more CO2 from the atmosphere than is emitted, or net-negative CO2 emissions. There are significant unresolved questions regarding the feasibility, risks and effectiveness of CDR at scale, so achieving net-negative emissions is far from a given. If achieved, there will still be a delay of several years before the climate cools, due to lags in the carbon cycle and thermal response.
Moreover, there is a risk that during overshoot the release of GHGs from natural carbon sinks could be triggered in ways not yet clearly anticipated. Already now, elevated tree mortality events following droughts and heatwaves are being observed worldwide (see Insight 4). This could prolong the overshoot and increase the risk of irreversible impacts, and their potential magnitude.
Implications & Recommendations
If 1.5°C is exceeded, as is now likely based on past and current actions, decision-makers should continue to strive to limit warming as close to 1.5°C as possible. Arresting further rise is crucial: every fraction of a degree matters.
- Governments should urgently work to close the emission gap consistent with the Paris Agreement climate goals by implementing concrete measures in their existing nationally determined contributions (NDCs) – with sector-specific details for driving down emissions in the near term. But it is important to prepare for the impacts of overshooting 1.5°C, which will be geographically uneven and, in some cases, irreversible.
- Governments should urgently implement policies to catalyse systemic mitigation action, chiefly the phase-out of fossil fuels (see Insight 2), and transformations in other sectors (see Insight 10) to limit the degree of overshoot and its harmful impacts.
- Global collective action towards net zero will determine the world’s peak warming level and the associated costs and climate impacts. High-income countries should take the lead on setting net-negative targets and implementing measures accordingly. This will be essential to bring temperatures down and limit the duration of overshoot and its accumulating risks.