Controlling Emissions of Non-CO2 Greenhouse Gases and Aerosols: Scientific and Policy Challenges II
Presiding: V Naik, Princeton University; D Mauzerall, Princeton University; J Hansen, NASA Goddard Institute for Space Studies
A52B-01 10:30h
Does Location Matter in the Reduction of Greenhouse Gases?
Today's climate policy is based on the assumption that the location of emissions reductions has no impact on the overall climate effect. However, this may not be the case since reductions of greenhouse gases generally will lead to changes in emissions of short-lived gases and aerosols. Abatement measures may be primarily targeted at reducing CO2, but may also simultaneously reduce emissions of NOx, CO, CH4 and SO2 and aerosols. Emissions of these species may cause significant additional radiative forcing. We have used a global 3-D chemical transport model and a radiative transfer model to study the impact on climate in terms of radiative forcing for a realistic change in location of the emissions from large-scale sources. Based on an assumed 10 percent reduction in CO2 emissions, reductions in the emissions of other species have been estimated. Climate impact for the SRES A1B scenario is compared to two reduction cases, with the main focus on a case with emission reductions between 2010 and 2030, but also a case with sustained emission reductions. The emission reductions are applied to four different regions (Europe, China, South Asia, and South America). In terms of integrated radiative forcing (over 100 years), the total effect (including only the direct effect of aerosols) is always smaller than for CO2 alone. Large variations between the regions are found. Inclusion of the indirect effects of sulphate aerosols reduces the net effect of measures towards zero. The global temperature responses, calculated with a simple climate model, show an initial additional warming of different magnitude between the regions followed by a more uniform reduction in the warming later. A major part of the regional differences can be attributed to differences related to aerosols, while ozone and changes in methane lifetime make relatively small contributions. Emission reductions in a different sector (e.g. transportation instead of large scale sources) might change this conclusion since the NOx to SO2 ratio in the emissions is significantly higher for transportation than for large scale sources. The total climate effect of abatement measures thus depends on (i) which gases and aerosols are affected by the measure, (ii) the lifetime of the measure implemented, (iii) time horizon over which the effects are considered, and (iv) the chemical, physical and meteorological conditions in the region. There are important policy implications of the results: Equal effects of a measure cannot be assumed if the measure is implemented in a different region and if several gases are affected. Thus, the design of emission reduction measures should be considered thoroughly before implementation.
A52B-02 INVITED 10:45h
Design of Metrics for Inclusion of NOx in Global Climate Agreements: Scientific Issues
The Kyoto Protocol seeks to limit the emissions of several greenhouse gases, but excludes emissions of short-lived species and their pre-cursors even though these may also have a significant climate impact. We explore the difficulties that are faced when designing a metric to compare the climate impact of NOx emissions with other emissions. There are two dimensions to this difficulty. The first concerns the definition of a metric that satisfactorily accounts for its climate impact. NOx emissions increase tropospheric ozone, but this increase, and its climate impact, depend strongly on the location of the emissions; low latitude emissions have a larger impact. NOx emissions also decrease methane concentrations causing a climate impact that is, to first-order, equal in size but opposite in sign to the ozone impact, at least for the global mean. We use model simulations which examined the impact of regionally constrained emissions of NOx on concentrations of tropospheric ozone and methane, the resulting radiative forcing, surface temperature response and GWPs. Because of the difficulties in modeling these processes, results from 2 chemical transport models (CTMs), 3 radiative forcing codes and 2 general-circulation models (GCMs) are used in order to provide some indication of uncertainties. We explore the use of indicators that could lead to metrics that, instead of using global-mean inputs, are computed locally and then averaged to the global level. The second dimension concerns the inter-model differences in the values of computed metrics. The local metrics may be less dependent on cancellation in the global mean; the possibilities presented here appear more robust to model uncertainty, although their applicability depends on the poorly-known relationship between local climate change and its impact. We conclude that if it becomes a political imperative to include NOx emissions in future climate agreements, policymakers will be faced with a number of difficult choices in selecting an appropriate metric.
A52B-03 11:00h
Assessing global radiative forcing due to regional emissions of tropospheric ozone precursors: a step towards climate credit for ozone reductions
The global distribution of tropospheric ozone (O3) depends on the location of emissions of its precursors in addition to other chemical and dynamical factors. Global O3 forcing is, therefore, a sum of regional forcings arising from emissions of precursors from different countries. The Kyoto Protocol does not include ozone as a greenhouse gas, and emission reductions of ozone precursors made under Kyoto or any similar agreement would presently receive no credit. Unlike gases which are directly emitted, to include O3 in a climate agreement, reductions in its chemical precursors must be given credit and the resulting change in radiative forcing must be determined. In this study, using a global chemical tracer model (MOZART-2) we quantitatively estimate the contribution of emissions of anthropogenic O3 precursors (NOx, CO, and NMHCs) from specific countries and regions of the world to global O3 distributions. We then estimate the global radiative forcing on climate due to changes in O3 and CH4 resulting from the regional reductions of NOx emissions alone and combined reductions of NOx, CO, and NMHCs using a global radiation model from the Geophysical Fluid Dynamics Laboratory. Our results show that O3 production and resulting distributions depend strongly on the geographical location of emissions of its precursors. For reductions of NOx emissions alone, decreases in radiative forcing due to O3 reductions per molecule of NOx reduced are largest for tropical regions (Southeast Asia, South America, and the Indian subcontinent) and smallest for emission reductions from mid- and high latitude regions (Europe, the Former Soviet Union and North America). However, due to increases in CH4 concentrations resulting from the O3 decreases, the net radiative forcing from NOx emission reductions alone is positive for all regions except Southeast Asia and the Indian subcontinent. In contrast, for combined reductions of anthropogenic emissions of NOx, CO, and NMHCs, changes in O3 and CH4 result in a net reduction in radiative forcing for all regions we consider. Our key finding is that in order to reduce climate forcing resulting from emission of tropospheric O3 precursors, it is necessary to consider simultaneous reductions of CO, NMHCs, and NOx; NOx emission reductions alone are not sufficient to guarantee a reduction in climate forcing when the full effect of changes in O3 and CH4 are taken into account.
http://www.wws.princeton.edu/mauzerall
A52B-04 11:15h
Sensitivity of global tropospheric O3 distribution and its radiative forcing to regional biomass burning emissions
Anthropogenic biomass burning is a major source of air pollutants in the atmosphere. Emissions of ozone (O3) precursors - nitrogen oxides (NOx), carbon monoxide (CO), and non-methane hydrocarbons (NMHCs) from biomass burning have been shown to contribute significantly to O3 levels in the troposphere. For some regions (tropics, southern hemisphere) emissions of O3 precursors from biomass burning exceed emissions from industrial sources. Currently, O3, the third most important greenhouse gas, is not included in the Kyoto Protocol or any climate agreement mainly because of the complexity in estimating the contribution of individual countries to the global O3 distribution and the resulting climate forcing. In this study, we evaluate the sensitivity of global O3 distributions and their radiative impact to regional biomass burning emissions. We use the global chemistry transport model, MOZART-2, to simulate the O3 distribution for a 1990 base year and perturbations to this distribution caused by a 10 percent reduction in the base emissions of O3 precursors from biomass burning from Africa, North America, South America, Southeast Asia, and the Former Soviet Union. We then calculate the radiative forcing due to the reduced O3 column resulting from regional reductions in biomass burning emissions. We also evaluate changes in CH4 concentrations and associated radiative forcing resulting from the O3 changes. We will present a quantitative analysis of the contribution of regional biomass burning emissions to global O3 distribution and its radiative forcing and associated changes in CH4 concentrations and associated radiative forcing resulting from the O3 changes.
A52B-05 11:30h
Tropospheric ozone and aerosols in climate agreements: scientific and political challenges
In addition to the six greenhouse gases included in the Kyoto Protocol, the tropospheric ozone precursors CO, NMVOC and NOx and the aerosols/aerosol precursors black carbon, organic carbon and SO2 also play significant roles in climate change. We review some of the main scientific and political challenges associated with incorporating tropospheric ozone and aerosol precursors into climate agreements and discuss how these challenges have a bearing on the design of future climate agreements. We argue that the optimal policy design for a particular substance depends on a combination of scientific and political concerns. We look particularly at the importance of location of emissions, negative forcing, metrics (measuring climate effects against other gases on a common scale; e.g. GWP), political attractiveness, and verification and compliance. We review the existing knowledge on these issues, explore their impact on policy design, and conclude that, with current scientific knowledge, CO and NMVOC could conceivably be included in a global climate agreement, either in a basket with the long-lived greenhouse gases or in a separate basket. However, the complexity and fairness implications of including tropospheric ozone precursors and aerosols might negatively affect the political feasibility of a future agreement.
A52B-06 INVITED 11:45h
Short-lived Trace Constituents and International Climate Change Policy: To Credit or Not to Credit?
Non-CO2 trace gases and aerosol provide a significant fraction of current and projected climate forcing. While the emission allowance trading system embodied in the Kyoto Protocol, which recently came into force, makes provision for national crediting of reductions in emissions of long-lived trace gases (specifically methane, nitrous oxide, fluorocarbons, sulfur hexafluoride, and perfluorocarbons), emissions reductions due to precursors of ozone or black carbon aerosol are not credited. Systems for greenhouse-gas emissions trading are being adopted within and among "Kyoto" countries, including the EU, and emissions trading systems are under development for several states of the US. Successor arrangements to Kyoto for subsequent periods beyond 2012 are also under consideration. Therefore, it is important to examine whether an extended allowance trading system involving a broader suite of trace constituents can be constructed that would meet environmental, political, and economic criteria of efficacy and feasibility. In this paper, we explore the specific characteristics needed to make such a system successful including design of monitoring, modeling, and enforcement. We consider whether such a system might improve upon or dilute the benefits of a system based on long-lived constituents alone.