Climate and health impacts of Short Lived Atmospheric Components

The aim of SLAC, Climate and health impacts of Short Lived Atmospheric Components, was to increase the knowledge of climate and health effects of short-lived components and to provide a sound scientific basis for development and evaluation of policy strategies. Short-lived climate forcers can warm or cool the climate system, but the uncertainties related to the magnitude of the effects are large. To improve our understanding of aerosol radiative forcing (RF), the leading model groups have run the same experiments within the AeroCom initiative. Myhre et al. presents one of the main results of the second round of AeroCom, an overview of the models' estimates of RF for the direct aerosol effects. Like in the first round soot, sulfate and organic aerosols are studied, but additionally nitrates, aerosols from biomass burning and secondary aerosols are included. The paper shows that there are differences among the models, but that the direct aerosol effects nonetheless are well understood and quantified. It pointed to several areas where further studies are needed, as the effect of aerosol in areas with clouds. In a Nature Climate Change article, Samset et al. argued that the uncertainty in the direct aerosol effect presented in Myhre et al. must be adjusted upwards to take into account the full range of uncertainties in the models. Soot particles, or Black Carbon (BC), warm the climate, both directly by absorbing sunlight and by making snow darker. In addition, BC can alter cloud properties, which may warm or cool climate. A comprehensive article by Bond et al. concluded that BC might be the second largest warming component following CO2, but the uncertainties are large. A series of articles in SLAC have investigated the vertical distribution of BC in the atmosphere and how it affects RF. Compared with flight measurements from south to north in the Pacific, the models overestimate BC in upper troposphere and lower stratosphere. RF is larger per mass at higher altitudes and 40% of the forcing in the models is due to BC above 5 km. If the AeroCom results are adjusted according to the observations from the flight campaigns, the direct forcing is reduced by 25%. With a more realistic vertical profile of BC in a climate model Hodnebrog et al. found a smaller climate impact of BC. In addition to the changed vertical profile, this was caused by the influence of BC on clouds which counteracted the direct warming effect. Samset and Myhre found using a climate model, that this semi-direct effect counteracts the direct warming effect of BC at all altitudes except near the ground. Through the Atmospheric Chemistry & Climate Model Intercomparison Project (ACCMIP) a series of studies on ozone budget and drivers is published. ACCMIP formed an important basis for parts of the IPCCs Fifth Assessment Report. CICERO contributed to this model intercomparison work with the chemistry transport model OsloCTM2. This model has been further developed and upgraded to OsloCTM3, documented by Søvde et al. Short-lived climate forcers have negative health effects. Within SLAC, studies in China of the total exposure of people exposed to both indoors and outdoors sources are done. Urbanization may have reduced the populations total exposure to particles, despite the fact that more people are exposed to polluted urban city air. The reason is cleaner indoor air in cities. One of the aims of SLAC was to strengthen the foundation for development of climate and environmental strategies for aerosols and gases not covered by the Kyoto Protocol. Climate policy that includes more gases than CO2 requires emission metrics to compare the climate effects of various emissions and convert these to a common unit. Tol et al. presents a consistent framework that shows the relationship between the different types of metrics and how these fit into the cost-benefit and cost-effectiveness analyzes. The article presents the types of metrics that are consistent with a stabilization goal on temperature. An overview of the most commonly used emission metrics, and the mathematical formulations behind, is discussed by Aamaas et al. The climate impact of global emissions or emissions divided into countries, sectors or pollutants will depend on metrics used. The importance of non-CO2 components varies greatly with the type of metric and time horizon, but either way it is CO2 emissions which contribute most to the total climate impact in both the short and long term. Radiative forcing or temperature has been used as key parameter in metrics. Shine et al. takes a step further and calculate the effect of different components on precipitation. Short-lived climate components represent a considerable uncertainty in the quantification of anthropogenic climate change. In a Nature Geoscience article, Myhre et al. shows that the uncertainty in climate sensitivity will be reduced in the coming decades if emissions of short-lived components is reduced.

The proposed project aims to narrow the uncertainties associated with physical, chemical, radiative and health impacts of the short-lived atmospheric components (SLAC). We will study the chain from emissions to atmospheric concentrations and deposition an d further effects on health and climate. The importance of emission location and emission sources will be explored. Case studies will be selected for regions and sectors with potentially high benefits from integration of health and climate perspectives. I mproved understanding of the climate impacts are also crucial for narrowing the range of the climate sensitivity and thus for calculations of future warming from CO2 and other climate components. The overall objective of the proposal is to develop a sound scientific basis for controlling the components by improving the quantifications of their climate and health effects.