Generated Summary
This research, published in npj Climate and Atmospheric Science, highlights a straightforward method to improve the transparency of assessments of progress towards global temperature goals. The study advocates for the separate reporting of greenhouse gas contributions, specifically focusing on long-lived gases like CO2 and nitrous oxide, alongside short-lived climate forcers (SLCFs), particularly methane and some hydrofluorocarbons. The aim is to facilitate a clearer understanding of how emission targets translate into global temperature outcomes. This approach aligns with existing frameworks and decisions made under the UNFCCC, emphasizing the scientific consensus on how various greenhouse gases influence global temperatures. The core methodology involves analyzing the impact of different greenhouse gases on global warming, with a specific focus on the implications of aggregate emission targets, which often lack the granularity to provide a clear picture of their effects. The authors utilize climate models and established metrics to demonstrate how separate specifications of different gases can significantly reduce the ambiguity in temperature projections.
Key Findings & Statistics
- The IPCC Special Report on 1.5 °C (SR1.5) stated that reaching and sustaining net-zero global anthropogenic CO2 emissions and declining net non-CO2 radiative forcing would halt anthropogenic global warming on multi-decadal timescales (high confidence).
- The IPCC 6th Assessment Report (AR6) confirmed that limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions.
- Parties to the Paris Agreement agreed in Katowice in 2018 (Decision 18/CMA.1) to report past emissions of individual gases separately and use 100-year Global Warming Potentials (GWP100) when aggregating them to CO2-equivalent (CO2-e100 emissions).
- The climate system responds similarly over a broad range of timescales to equal emissions expressed in tonnes of CO2-e100 of all LLCFs, including CO2.
- Likewise, the net radiative forcing due to SLCFs on multi-decadal timescales is similar to the aggregated rate of SLCF emissions expressed in tonnes of CO2-e100 per year multiplied by the 100-year Absolute Global Warming Potential (AGWP100) of CO2.
- Human-induced warming over any multi-decade time-interval is approximately the sum of (i) aggregate CO2-e100 emissions of LLCFs, including CO2, multiplied by a constant parameter, the Transient Climate Response to cumulative CO2 Emissions, or TCRE; (ii) any change in decadal-average radiative forcing due to SLCFs multiplied by another constant parameter, the Transient Climate Response to Forcing, or TCRF; and (iii) a parameter related to the rate of forcing.
- For representative coefficient values, p ≤ 0.3% per year, making this third term usually small.
- Warming due to an increase in SLCF emissions is 4-5 times greater than would be expected from the same increase in tCO2-e100 emissions of an LLCF over the 20 years following the increase.
- Methane emissions as CO2 equivalent emissions using GWP100 overstates the effect of constant methane emissions on global surface temperature by a factor of 3-4 … while understating the effect of any new methane emission source by a factor of 4-5 over the 20 years following the introduction of the new source.
Other Important Findings
- The separate specification of individual gases minimizes ambiguity in determining the climate impact of past emissions.
- Specifying the contributions of all gases individually in future targets as well as the reporting of past emissions would resolve the ambiguity in global temperature outcomes.
- Separate specification would also facilitate the use of alternate or flexible emission metrics, which may be useful for achieving a cost-effective emission trajectory over time or addressing specific policy goals.
- Indicative contributions from LLCF and SLCF abatement would not preclude trade-offs between them but would clarify the need to monitor the temperature impacts of any such trade-offs over a range of timescales.
- Some countries (but very few companies) already specify the contribution of LLCFs and/or SLCFs to total CO2-e100 emissions in NDCs, LT-LEDSs and science-based targets.
- Quantifying the aggregated implications of these targets for future global temperature simply requires a much wider uptake of this practice, representing a simple and achievable innovation that would enhance the transparency of any stocktake of progress towards any global temperature outcome.
Limitations Noted in the Document
- The evaluation of emission targets at the national or corporate level cannot be undertaken from a physical science perspective alone, but also depends on economic, social, equity, and political considerations, including responsibility for past warming, capacity for and costs of abatement, and non-climate impacts.
- The framework does not address how any additional information might be used, and the interpretation of the Paris Agreement.
- The SR1.5 and AR6 statements are based on the understanding of how different greenhouse gases affect global temperature.
- Separate specification of the contribution from CO2 helps, but ambiguity in global temperature outcomes remains if targets for non-CO2 gases comprise a mixture of long-lived climate forcers (LLCFs), such as nitrous oxide, with atmospheric lifetimes around 100 years or longer, and SLCFs, such as methane, most of which have lifetimes shorter than 20 years.
Conclusion
The authors emphasize the need for greater transparency in emission targets to accurately assess progress towards global temperature goals. By advocating for the separate indication of contributions from long-lived and short-lived greenhouse gases, they aim to provide a clearer understanding of the implications of emission targets on global temperature. The approach supports the existing frameworks and decisions under the UNFCCC. The separate specification of individual gases minimizes ambiguity, while aggregate CO2-e100 emissions often lack the granularity to provide a clear picture of their effects. The study notes that while assessing emission targets involves various considerations, including economic, social, and political factors, the high level of agreement on the underlying science of greenhouse gas impacts makes the proposed approach a valuable innovation. They emphasize that separate specification would also facilitate the use of alternate or flexible emission metrics. This innovation would enhance the transparency of any stocktake of progress towards any global temperature outcome, aligning with the goals of the Paris Agreement and other international climate efforts. The authors conclude that separate specification of emissions could be communicated by countries as additional information, without affecting any existing or planned NDCs or long-term net zero strategies. The key takeaway is that by providing more detailed information on the contributions of different greenhouse gases, the world can achieve better transparency and thus, track the progress toward the goals that it has committed to.