Abstract
Evaluating multicomponent climate change mitigation strategies requires knowledge of the diverse direct and indirect effects of emissions. Methane, ozone, and aerosols are linked through atmospheric chemistry so that emissions of a single pollutant can affect several species. We calculated atmospheric composition changes, historical radiative forcing, and forcing per unit of emission due to aerosol and tropospheric ozone precursor emissions in a coupled composition-climate model. We found that gas-aerosol interactions substantially alter the relative importance of the various emissions. In particular, methane emissions have a larger impact than that used in current carbon-trading schemes or in the Kyoto Protocol. Thus, assessments of multigas mitigation policies, as well as any separate efforts to mitigate warming from short-lived pollutants, should include gas-aerosol interactions.
Generated Summary
This study investigates the impact of various emissions on climate change by using a coupled composition-climate model (G-PUCCINI). The research focuses on calculating atmospheric composition changes, historical radiative forcing, and forcing per unit of emission for aerosols and tropospheric ozone precursors. The study aims to quantify the relative importance of different emissions and their effects, particularly concerning methane, ozone, and aerosols, which are linked through atmospheric chemistry. The approach involves calculating both abundance-based and emission-based radiative forcing to understand how emissions and composition changes influence radiative forcing. The core methodology includes a detailed analysis of the impacts of emissions from methane, carbon monoxide, volatile organic compounds (VOCs), NOx, SO2, and ammonia. The study’s scope encompasses an evaluation of multicomponent climate change mitigation strategies and the need for accurate quantification of the relative impact of different emissions on climate.
Key Findings & Statistics
- The global warming potential (GWP) is used as a scale related to radiative forcing (RF) to compare emissions, as adopted in the Kyoto Protocol.
- The 100-year GWP for methane when including only the responses of methane, ozone, and stratospheric water vapor is almost identical to the comparable AR4 value.
- The GWP is substantially larger when the direct radiative effects of the aerosol responses are included, however.
- The 100-yr GWP for CO was 1.9 in AR4, with a 1.0 to 3.0 range based on the third IPCC assessment and subsequent results (3, 14).
- Emissions of NOx, CO, and methane have substantial impacts on aerosols.
- Global burdens of hydroxyl and sulfate change by 18% and 13% for increased NOx, by -13% and -9% for CO, and by -26% and -11% for methane.
- The uncertainties in the abundance-based values are 0.16 for CO2, 0.05 for methane, +0.15 to -0.10 for ozone, 0.20 for sulfate, 0.10 for nitrate, and 0.05 for stratospheric water (5).
- For emissions-based values, we estimate uncertainties by adding the forcing uncertainties for each component in quadrature, yielding 0.14 for methane, 0.04 for CO+VOCs, 0.09 for NOx, 0.23 for sulfate, and 0.10 for ammonia.
- The 100-year GWPs for methane, CO, and NOx are given in the AR4 and in this study when including no aerosol response, the direct radiative effect of aerosol responses, and the direct+indirect radiative effects of aerosol responses.
- The AR4 did not report uncertainties for methane or CO and gave no mean estimate for NOx.
- Our calculations for the shorter 20-year GWP, including aerosol responses, yield values of 79 and 105 for methane, 11 and 19 for CO, and -335 and -560 for NOx, including direct and direct+indirect radiative effects of aerosols in each case.
- The 100-yr GWPs for SO2 (per Tg SO₂) and ammonia would be -22 and -19, respectively, including direct aerosol radiative effects only, and -76 and -15 adding indirect aerosol radiative effects.
Other Important Findings
- Gas-aerosol interactions substantially alter the relative importance of various emissions.
- Methane emissions have a larger impact than that used in current carbon-trading schemes or in the Kyoto Protocol.
- The study calculates atmospheric composition changes, historical radiative forcing, and forcing per unit of emission due to aerosol and tropospheric ozone precursor emissions.
- The responses to individual species emissions changes are largely additive.
- Emissions of NOx, CO, and methane have substantial impacts on aerosols.
- The global sulfate response to oxidant changes can be large.
- Including aerosol indirect effects (AIE) on clouds could make SO2 emissions the stronger contributor to negative historical forcing.
- The ratio of the sulfate to hydroxyl burden changes is greater in response to NOx and CO emissions than for methane.
- NOx emissions cause a substantial net cooling at all time scales, while CO emissions cause warming.
- The 100-year GWP for methane is ~10% greater than earlier estimates that neglected interactions between oxidants and aerosols.
Limitations Noted in the Document
- The GWP concept has limitations, including only physical properties and a definition equivalent to an unrealistic economic scenario.
- GWP does not take into account the rate of change, and neglects that the surface temperature response to regionally distributed forcings depends on the location of the RF and that precipitation and circulation responses may be even more sensitive to RF location.
- GWPs are particularly ill-suited to very short-lived species such as NOx, SO2, or ammonia.
- The study notes that additional processes should be included as they become better understood, such as mixing between aerosol types, formation of secondary organic aerosols, and interactions between pollutants and ecosystems.
- Uncertainties related to aerosol indirect effects (AIE) on clouds pose a challenge, and the study acknowledges that these effects are highly uncertain.
Conclusion
The study underscores the importance of accounting for gas-aerosol interactions in climate change mitigation strategies. The findings suggest that methane emissions have a more significant impact than previously understood, particularly in the context of current carbon-trading schemes. The research highlights that assessments of multigas mitigation policies should incorporate these interactions to improve accuracy. The study emphasizes that the GWP concept, while widely used, has limitations, and alternative approaches might be needed for a more comprehensive understanding of climate change impacts. It highlights that the direct and indirect effects of aerosols influence the effects of emissions, as shown through interactions that can significantly alter the relative importance of different emissions, specifically when analyzing the impact of NOx, CO, and methane on aerosols. The study concludes by stating that improving knowledge of aerosol-climate interactions is crucial for better understanding past and future climate change and for accurately evaluating the effects of long-lived greenhouse gas emissions from methane-oxidant-aerosol interactions. Furthermore, it suggests that policies aiming to mitigate climate change should consider the overall impact of emissions, rather than focusing solely on long-lived gases or short-lived species. The study’s results underscore the need for a more nuanced approach to climate change mitigation.