Controlling Emissions of Non-CO2 Greenhouse Gases and Aerosols: Scientific and Policy Challenges I
Presiding: V Naik, Princeton University; D Mauzerall, Princeton University; J Hansen, NASA Goddard Institute for Space Studies
A51B-01 INVITED 08:30h
Global sources of non-CO2 greenhouse gas emissions: regional trends, uncertainties and options for emission reductions
An overview will be presented of sources and trends of global emissions of direct non-CO2 greenhouse gases CH4, N2O and the fluorinated gases HFCs, PFCs and SF6, which are addressed in the Kyoto protocol. Special attention will be given to regional source trends, estimated uncertainties and most recent global emission trends. In addition, the most significant options for emission reductions will be discussed in view of medium term emission scenarios that were meant to illustrate future trends without the effects climate policy. For estimating the recent global emission trends a special approach was used to compile fast annual updates of the EDGAR global emission inventories, based on the more detailed previous version. We present an overview of the approaches used for this `Fast Track' for the different source sectors. Results are presented for 1995-2002 for various anthropogenic sources at regional scales including an estimate of the accuracies achieved. A similar overview will be provided for the emissions of the ozone precursors NOx, CO and NMVOC and of black carbon. Tropospheric ozone and black carbon are both greenhouse gases, which are not considered in the Kyoto protocol, but in industrialised countries the emissions that cause them are often part of environmental policy on local and regional air quality.
A51B-02 08:45h
Aerosols: Non-CO2 Non-Greenhouse Non-Gas Forcing
Tropospheric aerosols influence Earth's radiation budget and climate by scattering and absorbing solar radiation (direct effects) and by modifying the reflectivity and extent of clouds (indirect effects). While aerosol forcing is arguably less important from a policy perspective than greenhouse gas forcing because of the short residence times of these aerosols (about a week) compared to the lifetimes of the well mixed greenhouse gases (decades to centuries), knowledge of aerosol forcing over the industrial period is essential to empirical inference of Earth's climate sensitivity from temperature change over this period and to evaluation of the performance of climate model simulations over this period. Accuracy in global-average forcing by anthropogenic aerosols required for these purposes is estimated to be ~0.5 w m-2 [Schwartz, J. Air Waste Management Assoc. 54, 1351-1359 (2004)]. For an accumulation-mode (radius 50-1000 nm) scattering aerosol above a dark surface the forcing per optical depth (at 550 nm) during daylight hours for cloud-free sky is 50 to 100 W m-2. Such a forcing intensity implies, taking into account 50% nighttime and ~50% cloud cover, that global average optical depth of anthropogenic aerosol must be known to 0.02 - 0.04, an accuracy achievable by careful ground-based measurements, but which would be difficult to achieve globally because of high spatial and temporal variation. Top-of-atmosphere forcing for a given optical depth is sensitive also to single scattering albedo, size distribution (as manifested in asymmetry parameter, backscatter fraction, or Angstrom exponent) and surface reflectance; these sensitivities are examined here. It is necessary as well to determine the fraction of forcing that is due to anthropogenic aerosol. Similar considerations apply to aerosol indirect forcing, which depends to first approximation on the number concentration of cloud condensation nuclei as a function of applied supersaturation, which depends on the size distribution, composition, and mixing state of the aerosol and which likewise must be ascribed to anthropogenic vs. natural aerosol. Indirect forcing is highly sensitive to aerosol perturbations; an increase cloud drop number concentration by 30% in clouds of intermediate reflectivity (30 - 70%) is calculated to result in a forcing of ~ -6 W m-2 locally and instantaneously, which corresponds to ~ -1 W m-2 globally, taking into account the climatological frequency of such clouds. These considerations place severe requirements on knowledge of aerosol distribution, properties, and attribution to sources.
A51B-03 09:00h
Disparity and partnership: black carbon aerosols in the global climate picture
Field measurements and model results have recently highlighted the large climatic impacts of aerosols. One line of inquiry has suggested that reducing emissinos of climate-warming "soot" or "black carbon" particles can form a viable component of mitigating global climate change. We explore this possibility in the context of current understanding of emission sources and modeling results. We discuss the scientific arguments against considering aerosols and greenhouse gases in a common framework, including uncertainties and the vast differences in climatic impacts. We draw on the language of the United Nations Framework on Climate Change to inquire whether aerosols should be considered at all. Next, we synthesize results from published climate-modeling studies, showing that much of the apparent variability in radiative forcing estimates results from choices of input parameters. We estimate a direct global warming potential for black carbon relative to carbon dioxide, based on the model comparison and on a thorough review of optical properties. This calculation enables a discussion of cost-effectiveness for mitigating the largest sources of black carbon, and we show that many reductions are either expensive or difficult to enact, even compared with greenhouse gases. Finally, we propose a role for black carbon in climate mitigation strategies that may be consistent with the apparently conflicting arguments raised during this discussion.
A51B-04 09:15h
The Climate Impact of the Household Sector in China
If it ever enters into force the impact of the Kyoto Protocol on climate change is likely to be small. The USA and Australia have not ratified the Protocol and the initial emission reduction target was only 5.2 per cent. There is an increasing call for post-Kyoto climate treaties, whether they be global or regional, to widen the scope to take into account the impacts that air pollutants as tropospheric ozone and aerosols may have on climate. There are two main reasons for this. First and foremost, there is increasing evidence that these air pollutants play an important role in the climate system. Secondly, it is suggested that including radiative forcing components that also have adverse impacts on human health and environment may increase participation, which will be a prerequisite for future treaties to be effective. China's approval of the Kyoto Protocol in 2002 suggests that it is considering a more active role in the global effort to mitigate global warming. Given its many other priorities, however, China needs to understand what national policies would reduce its contribution to global warming in the most cost-efficient way and at the same time contribute the most to economic and social development in the country. The objective of the present study is to contribute knowledge that is helpful to Chinese policy makers dealing with this question. We do this by addressing emissions that according to the World Health Organisation are among the leading health risks to people in the developing world, China included, i.e. smoke from solid fuels burned in peoples' homes. In China, about 72 per cent of the population lives in rural or peri-urban areas where use of simple, low-efficiency household stoves for coal or biomass is common. Even though the residential sector stands for no more than 11 per cent of the primary energy consumption (biomass included), the sector contributes to, e.g., more than 70 per cent of Chinese emissions of black carbon, about a third of its methane emissions, and more than 40 per cent of the nmVOC emissions (which contributes to global warming through tropospheric ozone production). Thus, policies addressing these sources may be important in the context of global warming in addition to substantially improving living conditions for many people. The question we ask in the present paper is how important are they? Two global models are applied to estimate the climate impact on a global scale of emissions from the Chinese residential sector. To estimate the impact on the development of the global climate in terms of radiative forcing and global mean temperature of a possible reduction in these emissions we use a simple climate model. A global, three-dimensional photochemical tracer/transport model of the troposphere is used to model the changes in concentration of air pollutants that have a radiative forcing. Estimates for Chinese household sector emissions are taken from previous work on emission inventories in Asia.
A51B-05 09:30h
Potential Effects of Methane and Nitrous Oxide on the Recovery of Stratospheric Ozone
Stratospheric ozone concentrations have been significantly reduced in recent decades as a result of human activities. The international agreement to protect stratospheric ozone, the Montreal Protocol, has effectively reduced the human-related emissions of halocarbons containing chlorine and bromine. Since the implementation of the international controls on ozone depleting chemicals, an important focus in studies of stratospheric ozone has been on the detection of a turnaround in the downward trend and determination of when a recovery will occur, where a recovery is defined as a return to levels of ozone in the 1970s before the existence of the Antarctica ozone "hole". If halocarbons remained the only relevant human-related factor affecting ozone, the ozone layer would be expected to recover by roughly 2040-2045. However, there are a number of other factors, including non-CO2 greenhouse gas emissions, affecting the future recovery of ozone. In this study, we considered a range of scenarios for future trace gases emissions developed by IPCC (2001) using the UIUC two-dimensional Chemical-Transport Model (UIUC 2D CTM). We found that the future recovery depended greatly on future emissions of two major greenhouse gases, methane (CH4) and nitrous oxide (N2O). Evaluation of the effects of scenarios developed by the IPCC (Intergovernmental Panel on Climate Change, 2001) for future emissions of methane, nitrous oxide, and other gases suggests that these gases could greatly affect ozone recovery, including the possibility of ozone not recovering in this century. In addition, under all cases, the ozone distribution is always greatly different than that in the pre-1980 atmosphere.
A51B-06 09:45h
Global Health Benefits from Reductions in Background Tropospheric Ozone due to Methane Emission Controls
Increases in background ozone throughout the troposphere are partially attributed to rising anthropogenic methane concentrations, which are projected to continue to increase in the future. Because methane is long-lived and affects background ozone, controls on methane emissions would reduce surface ozone concentrations fairly uniformly around the globe. Epidemiological research indicates that exposure to ozone increases incidence of respiratory ailments and premature mortality. In addition, exposure to ozone reduces agricultural yields and damages natural ecosystems. We use the MOZART-2 global atmospheric chemistry and transport model to estimate the effects on global surface ozone of perturbations in methane emissions. We consider a baseline scenario for 2000 and the 2030 A2 scenario (emissions from the IPCC AR-4 2030 atmospheric chemistry experiments), and examine the impact on ozone of decreasing anthropogenic methane emissions relative to this baseline by 20%. Using the simulated spatially-distributed decreases in surface ozone concentrations resulting from these reductions in methane emissions, we estimate the global benefits to human health in the methane emission reduction scenario. We focus on human mortality, and consider the sensitivity of our estimates to different assumptions of health effect thresholds at low ozone concentrations.