1427. The Need to Combat Overshoot

In 2015, 195 countries committed to limiting global warming to "well below 2°C" and "pursuing efforts to limit it to 1.5°C" in order to reduce human impacts on the climate system. While the United States has reportedly officially withdrawn from the Paris Agreement, a Nature commentary/opinion piece discusses the need for the world to address "overshoot," or the loss and damage that would result if global warming were to exceed the 1.5°C limit, and the need for adaptation.

In 2024, global average temperatures exceeded 1.5°C for the first time. The Paris Agreement threshold itself is defined as an average over at least 20 years to account for annual variability. However, conditions indicate that the world could exceed this threshold within 10 years.

In 2018, several modeled pathways suggested that limiting temperature rise to below 1.5°C was still possible. The no-overshoot or limited-overshoot pathways assessed by the Intergovernmental Panel on Climate Change (IPCC) in 2022 required significant reductions in global emissions by 2025. Today, we must recognize that these reductions have not materialized, and cumulative emissions since then indicate that the lowest achievable peak temperature increase is close to 1.7°C.

In an "overshoot" world, countries must commit not only to reducing carbon dioxide emissions to net-zero but also to achieving and sustaining net-negative emissions by removing billions of tons of carbon dioxide from the atmosphere and permanently storing them. However, accepting a pathway with peak temperature rises significantly exceeding 1.5°C creates incentives to delay action. Countries would justify inaction by pointing to this implicit mitigation target, while peak temperature increases would continue to gradually increase, jeopardizing the possibility of returning to levels below the 1.5°C limit.

In a world where overshoot is expected, limiting peak warming to the lowest possible level is a top priority. Continuing to explore feasible, forward-looking pathways to achieve this requires benchmarking maximum emission reduction targets, clarifying the responsibilities of countries and other actors for warming above 1.5°C, and understanding how these responsibilities change depending on the level of future emissions reduction efforts. The concept of "net-zero carbon debt" is one such approach. The first step in calculating it is to determine what fair share each country has already consumed of the global carbon budget. Countries that emit more than their fair share are classified as "carbon debtors," and each unit of subsequent emissions accrues one unit of carbon debt. Using each country's emission projections, we can assess how much carbon debt they may incur in the future.

The scientific community needs to inform the debate over exceeding the 1.5°C limit. Analysis of mitigation pathways in a world expected to overshoot should focus on strategies that limit peak warming and achieve long-term, sustainable reductions in global warming. A coherent, cross-disciplinary approach is needed to facilitate discussions on remedies, costs, and trade-offs. This includes carbon removal options and requirements, adaptation needs, and the potential for and risks to communities associated with climate change, as well as the interactions between them. To provide evidence of how the world arrived at this situation, we need to develop approaches to model the "path not taken." To achieve this, we need to systematically identify global and regional tipping points and adapt modeling approaches to diverge from historical starting points.

The outcome of the COP30 Conference of the Parties to the United Nations Framework Convention on Climate Change (UNFCCC) in Belém, Brazil, recognized this point, with countries reaffirming their commitment to limiting global warming to 1.5°C and "limiting both the scale and duration of temperature overshoot." However, action is needed in addition to words. To overcome overshoot, an essential first step is to establish an accountability framework to address the failure to prevent dangerous human interventions.

Establishing accountability for corrective measures to address a 1.5°C overshoot is likely to emerge as a central theme in the second Global Stocktake, which will measure overall progress toward the Paris Agreement goals, scheduled for completion by 2028.

Contributor: IIYAMA Miyuki, Information Program