Avoiding overshoot: why and how

Most existing climate scenarios focus on distant temperature goals at the end of the century. This allows them to temporarily overshoot those temperatures, with CO₂ emissions that break the carbon budget in mid-century, followed by net-negative emissions to repay the deficit.

The ENGAGE project, coordinated by IIASA, has studied the implications of being more ambitious, with scenarios that balance the carbon budget at the time of reaching net-zero emissions. It has also looked at the implications of following existing Nationally Determined Contributions (NDCs) until 2030. These scenarios inform the Intergovernmental Panel on Climate Change (IPCC)’s sixth assessment report (mitigation of climate change) due to be published in March 2022.

• 1.5°C is out of reach with existing NDCs.
• Net-negative scenarios lead to a hazardous level of overshoot. Given a budget of 1000 Gt CO₂, scenarios that rely on net-negative emissions result in mid-century peak temperatures that are up to 0.15°C higher than if the budget is met by the time of net-zero. This would mean substantially higher climate impacts and risks of reaching tipping points.
• Investment in low-carbon power should at least double by 2030 to avoid overshoot (under a 1000 Gt budget). This is mainly in solar, wind, power grids and storage.
• Upfront investment brings long-term economic gains. End-of-century GDP is higher in scenarios that avoid overshoot.
• Carbon removal technologies need urgent development and deployment. Even in scenarios that avoid net-negative emissions, CO₂ removal is needed to accelerate near-term mitigation and to offset emissions from hard-to-abate sectors.

The full text of this policy brief is currently under embargo pending publication in Nature Climate Change. If you would like further information about this research, please contact Keywan Riahi, Program Director, Energy, Climate and Environment Program, IIASA.

Last edited: 27 October 2021