Avoiding overshoot: why and how

Overshooting global temperature goals is risky. New research from the ENGAGE project shows the long-term economic benefits of scenarios that avoid overshoot, and points out the investments needed to make it happen.

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Most existing climate scenarios focus on distant temperature goals of 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 the International Institute for Applied Systems Analysis, has now 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 NDCs until 2030. These scenarios inform the 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 emission from hard-to-abate sectors.

Meeting the Paris goals will require a transformation in energy systems. To explore how this might be done, integrated assessment models (IAMs) calculate scenarios that limit global warming, through a portfolio of emissions cuts and land use change, all optimized for low cost.

In most such studies, temperature goals must be met only in 2100, allowing scenarios the freedom to temporarily overshoot those goals. This is the lowest-cost approach across most scenarios, notably those in the IPCC’s Fifth Assessment Report and Special Report on Global Warming of 1.5°C.

These scenarios rely heavily on net-negative emissions later in the century, to reverse the overshoot. CO2 removal on such a large scale puts a burden on later generations, and may turn out to be unsustainable or unfeasible (see IIASA policy brief No. 29).

Capped peaks

To explore the implications of being more ambitious, the ENGAGE project has now compared two approaches: traditional emissions pathways that focus only on end-of-century temperature goals, versus pathways that explicitly put a limit on peak temperature.

Peak temperature depends mainly on the total cumulative CO2 emitted when the world reaches net-zero emissions. So each pathway in the second set requires that the carbon budget is met at the time of net zero, with little or no overshoot. (A small overshoot can occur due to other greenhouse gases.)

Nine modelling teams then fed these pathways into different IAMs to generate mitigation scenarios, revealing the consequences for climate and economics.

Figure 1. Emissions in all scenarios where warming is 2°C or below. Red lines show are scenarios with overshoot and net negative emissions; blue lines show net-zero-budget scenarios with little or no overshoot.

Climate risks  

Research results indicate that scenarios with a carbon budget of 1000 Gt CO2 show end-of-century warming of 1.7-2.1°C. Among these, scenarios relying on net-negative emissions have peak temperature up to 0.15°C higher than those without (Riahi et al. 2021).

This is a considerable climate risk, liable to expose many millions more people to climate extremes, and increasing the risk of climate tipping points and other changes that cannot be readily reversed. According to the Special Report on Global Warming of 1.5°C: “Overshooting poses large risks for natural and human systems, especially if the temperature at peak warming is high, because some risks may be long-lasting and irreversible, such as the loss of some ecosystems.”

Researchers also looked at variants of all these pathways where emissions follow current NDCs until 2030. In every case, this puts a 1.5-degree world is out of reach. The late start means it is simply impossible to scale up decarbonization quickly enough. The lowest attainable peak temperature is 1.6 to 1.9°C.

Doubled investments

Another study (Bertram et al.) took a detailed look at the investments needed to transform the energy system. Scenarios with a 1000 Gt budget  require annual investment in low-carbon energy from 2025 to 2030 to be as at least twice as high as 2020 levels. Most of this is in solar, wind, power grids and storage. In all scenarios that limit peak warming to below 2°C, coal is phased out quickly, with considerable reductions in oil and gas.

As well as decarbonizing the power system, this allows other sectors to be cleaned up. Where possible, that will be through direct electrification. In hard-to-abate sectors such as heavy industry it will mean using renewable power indirectly, requiring considerable investment in green hydrogen.

 Find out more on ENGAGE at www.engage-climate.org  

Keywan Riahi

Program Director and Principal Research Scholar at Energy, Climate, and Environment Program, International Institute for Applied Systems Analysis