The science of temperature overshoots: impacts, uncertainties and implications for near-term emissions reductions
Climate science uses emission pathways to assess different trajectories towards limiting warming to dedicated warming levels, most commonly to below 1.5°C or 2°C. In recent years, so-called overshoot pathways have also increased in prominence. In overshoot pathways global mean temperatures temporarily exceed a specific target, such as 1.5°C, before bringing temperatures back down below.
The technologies that are currently suggested to support this temperature drawdown are summarised under the term carbon dioxide removal (CDR). While some potential for sustainable CDR deployment exists, reliance on CDR at a large scale comes with its own risks, uncertainties and side-effects. Here we provide an overview of the science on overshoot pathways and its implications:
Global mean temperatures will continue to increase until net zero CO2 emissions are reached. Whether temperatures overshoot the 1.5°C temperature goal or not depends on cumulative emissions until net zero and the ultimate warming response from these emissions. Uncertainties in the temperature outcomes under emission pathways are still substantial.
Pursing the stringent mitigation as outlined in “as likely as not” (33-66% likelihood) 1.5°C pathways would lead to a small or even no overshoot. The IPCC special report on Global Warming of 1.5°C categorises these pathways as ‘low or no overshoot pathways’. Pathways that are “likely” (66-100% likelihood) to exceed 1.5°C are not considered Paris Agreement compatible.
The lowest illustrative pathway included in the IPCC’s Working Group I (WGI) contribution to the Sixth Assessment Report (AR6), the SSP1-1.9 pathway, keeps temperatures below 1.5°C by 2100 with a greater than 50% likelihood, with a best estimate of a low overshoot of 0.1°C (IPCC, 2021). Many more emission reduction pathways consistent with 1.5°C will be assessed in the IPCC’s mitigation Working Group (WGIII) report, out in 2022.
Our best bet to lower the probability of overshooting 1.5°C is to lower emissions as quickly as possible. Even for the most ambitious below 1.5°C scenarios, there is a chance to overshoot 1.5°C if the warming response from the climate system turns out to be on the higher side of current estimates. Overshoots cannot be avoided with certainty anymore. At the same time, warming lower than the current median estimate would also avoid any overshoot altogether, even under 1.5°C low overshoot pathways.
Whether it is feasible or desirable to reduce temperatures after peak warming is reached – at what pace and with what tools – will be a decision that will depend on the magnitude of peak warming and available CDR options. Small overshoots could potentially be reversible with sustainable amounts of CDR. However, depending on the reason for high peak warming outcomes, CDR could be required to merely balance long-term Earth system induced warming and render effective temperature decline infeasible. Increased dependency on CDR comes with increased sustainability concerns and the potential benefits of decreasing temperatures need to be put into perspective with the negative consequences of large-scale CDR.
The potential reversibility of global mean temperature does not mean that the impacts of climate change can be reversed. A range of impacts are generally considered to be reversible with lowering global mean temperature, but other impacts, such as sea level rise, loss of ecosystem functionality, increased risks of species extinction, as well as glacier and permafrost loss are not, on timescales of decades to millennia. Critically, in addition to this, the risk of abrupt changes and tipping points in the Earth’s system increases with higher warming levels and longer overshoot periods.
Our current best estimate for the impacts of climate change lie at peak warming levels – even where impacts are reversible. Any detectable overshoot of the 1.5°C temperature limit will likely last several decades – so human systems will have to adapt to impacts related to overshoot temperatures rather than end of century warming levels. However, long-term declining temperatures may be desirable to reduce long-term impact consequences e.g. of sea level rise.