Overshoot scenarios

By joining the Paris Climate Agreement in 2015, 192 countries committed to keeping global average temperature rise well below 2°C above pre-industrial levels and to make efforts to limit temperature rise to 1.5°C. However, over the past seven years, the efforts of the countries have been extremely insufficient to implement this agreement. Analysing current NDCs, experts estimate the projection of the global average temperature rise as approximately 2.3°C. Unfortunately, the best chance of limiting warming to 1.5°C is over—the IPCC Special Report on Global Warming of 1.5°C does not contain emission reduction scenarios that limit warming to 1.5°C with a probability of 66% or more. Since there is still hope that the NDCs can be revised to make them stronger, experts consider pathways in which the increase in global temperature would temporarily exceed 1.5 °C, but would remain below 2 °C and would decrease by the year 2100 to levels below 1.5 °C. Such scenarios are referred to as “1.5°C overshoot scenarios”.

The global average temperature will stop rising when net zero GHG emissions are reached. The decrease of the global average temperature requires net negative CO2 emissions. The technologies that are currently suggested to enable this temperature drawdown are summarised under the term carbon dioxide removal (CDR).

CDR methods differ significantly in how they capture and store CO2. Approaches such as afforestation and reforestation, soil carbon sequestration, enhanced weathering and biochar increase the amount of CO2 taken up by plants, minerals and nutrients and store it on land or in the ocean. Bioenergy with carbon capture and storage (BECCS) and direct air carbon capture and storage (DACCS) remove CO2 and transport it to geological reservoirs.

The IPCC Special Report on Climate Change and Land (see the briefing on this report here) identified co-benefits of some CDR deployment options, especially for afforestation and reforestation, soil carbon sequestration and biochar. However, CDR deployment of any type has not been tested in practice and is associated with numerous feasibility and sustainability limitations. Potentially wide-ranging side effects of CDR methods could affect their potential for CO2 removal and temperature lowering, as well as the achievement of sustainable development goals, for example in relation to water, food and biodiversity.

The main challenges for CDR are related to the large land, water and financial requirements, as well as limitations on long-term storage of removed CO2. BECCS and DACCS carbon capture and storage technologies currently lack technological readiness. BECCS has additional challenges due to particularly high demands on land and water and the problems related to competition with food crops, damage to biodiversity and heavy use of fertilisers. However, DACCS requires much more energy to remove the same amount of CO2 than BECCS, and these energy must be produced by renewable energy sources only.

Even assuming an annual potential of CDR of 10 GtCO2 or more, the actual rate of temperature reduction they provide would be about 0.05°C per decade. This means that it will take decades for any temperature overshoot to be reversed, and for high overshoots, global average temperatures would rise above 1.5°C for multiple decades. The figure below shows the relationship between the overshoot magnitude and the overshoot duration, assuming a temperature decrease of 0.05°C per decade. For lower, and potentially more sustainable levels of CDR, the decrease in temperature would be even slower.

In overshoot scenarios, various critical thresholds may be temporarily exceeded, leading to potentially irreversible consequences. These include: rising ocean levels, disruptions in some ocean circulation systems, loss of glaciers, ice sheets and permafrost. The risk of abrupt changes and of exceeding critical thresholds increases with higher warming levels and longer overshoot periods.

Global sea level rise will continue for centuries to millennia even after temperatures have peaked. Glacier loss may not be reversible on time scales ranging from decades to centuries. Glaciers are the main source of fresh water for billions of people around the world. After the initial increase in glacial runoff, there will be a significant risk of water shortages in runoff basins.

Shifting habitats and extreme events will lead to the risk of species loss and extinction, which may be irreversible. Ecosystems and species not only need to adapt to rapidly rising temperatures, but also face the challenge of coping with lower levels of warming after peak temperatures. Coral reefs are projected to decline by 70-90% in 1.5°C warming and virtually all (>99%) will disappear in 2°C warming.

Similarly, extreme events such as extensive droughts and associated fire risks can destroy forest ecosystems (e. g. Amazon rainforests or Siberian forests), which are unlikely to recover to the same state later even if temperatures drop to previous levels.

Permafrost and its frozen carbon will be lost during and after the warming phase. Because the release of carbon from melting permafrost is irreversible over hundreds of years, overshoot pathways would have severe permafrost impacts, and the released carbon would further accelerate warming. The problem of permafrost thawing is especially relevant for Russia, where permafrost soils occupy about 2/3 of the territory. In a large part of this area, almost half, the permafrost is predicted to thaw (see figure below, where light gray is for the permafrost areas to be thawed by 2025, and dark gray – by 2050). The area will turn into an impenetrable swamp, and the buildings erected there will literally "fall into the ground."

Human systems will have, at least for the time, to adapt to the impacts related to overshoot temperatures rather than the end of the century levels. However, according to the IPCC, adaptation capacity is limited and may be exceeded in some regions with warming above 1.5°C.

Even with strong and urgent decarbonisation measures, the temperature overshoot cannot be ruled out with certainty because of the uncertainty in the response of the climate system to increases in atmospheric GHGs. Also, there is no confidence that there are scenarios with high overshoots and with subsequent decrease in the global average temperature, since the feasibility and effectiveness of CDR for temperature reduction is questionable. But while 1.5°C temperature overshoots cannot be ruled out due to uncertainties in climate responses, strong mitigation measures in the very near future could reduce the damages and risks associated with the overshoot.

A. Fedorov