Costs and global impacts of black carbon abatement strategies

Abatement of particulate matter has traditionally been driven by health concerns rather than its role in global warming. Here we assess future abatement strategies in terms of how much they reduce the climate impact of black carbon (BC) and organic carbon (OC) from contained combustion. We develop global scenarios which take into account regional differences in climate impact, costs of abatement and ability to pay, as well as both the direct and indirect (snow-albedo) climate impact of BC and OC. To represent the climate impact, we estimate consistent region-specific values of direct and indirect global warming potential (GWP) and global temperature potential (GTP). The indirect GWP has been estimated using a physical approach and includes the effect of change in albedo from BC deposited on snow. The indirect GWP is highest in the Middle East followed by Russia, Europe and North America, while the total GWP is highest in the Middle East, Africa and South Asia. We conclude that prioritizing emission reductions in Asia represents the most cost-efficient global abatement strategy for BC because Asia is (1) responsible for a large share of total emissions, (2) has lower abatement costs compared to Europe and North America and (3) has large health co-benefits from reduced PM10 emissions

A continued role of short-lived climate forcers under the Shared Socioeconomic Pathways

Mitigation of non-CO2 emissions plays a key role in meeting the Paris Agreement ambitions and sustainable development goals. Implementation of respective policies addressing these targets mainly occur at sectoral and regional levels, and designing efficient mitigation strategies therefore relies on detailed knowledge about the mix of emissions from individual sources and their subsequent climate impact. Here we present a comprehensive dataset of near- and long-term global temperature responses to emissions of CO2 and individual short-lived climate forcers (SLCFs) from 7 sectors and 13 regions – for both present-day emissions and their continued evolution as projected under the Shared Socioeconomic Pathways (SSPs). We demonstrate the key role of CO2 in driving both near- and long-term warming and highlight the importance of mitigating methane emissions from agriculture, waste management, and energy production as the primary strategy to further limit near-term warming. Due to high current emissions of cooling SLCFs, policies targeting end-of-pipe energy sector emissions may result in net added warming unless accompanied by simultaneous methane and/or CO2 reductions. We find that SLCFs are projected to play a continued role in many regions, particularly those including low- to medium-income countries, under most of the SSPs considered here. East Asia, North America, and Europe will remain the largest contributors to total net warming until 2100, regardless of scenario, while South Asia and Africa south of the Sahara overtake Europe by the end of the century in SSP3-7.0 and SSP5-8.5. Our dataset is made available in an accessible format, aimed also at decision makers, to support further assessment of the implications of policy implementation at the sectoral and regional scales

An AeroCom assessment of black carbon in Arctic snow and sea ice

Though many global aerosols models prognose surface deposition, only a few models have been used to directly simulate the radiative effect from black carbon (BC) deposition to snow and sea ice. Here, we apply aerosol deposition fields from 25 models contributing to two phases of the Aerosol Comparisons between Observations and Models (AeroCom) project to simulate and evaluate within-snow BC concentrations and radiative effect in the Arctic. We accomplish this by driving the offline land and sea ice components of the Community Earth System Model with different deposition fields and meteorological conditions from 2004 to 2009, during which an extensive field campaign of BC measurements in Arctic snow occurred. We find that models generally underestimate BC concentrations in snow in northern Russia and Norway, while overestimating BC amounts elsewhere in the Arctic. Although simulated BC distributions in snow are poorly correlated with measurements, mean values are reasonable. The multi-model mean (range) bias in BC concentrations, sampled over the same grid cells, snow depths, and months of measurements, are −4.4 (−13.2 to +10.7) ng g−1 for an earlier phase of AeroCom models (phase I), and +4.1 (−13.0 to +21.4) ng g−1 for a more recent phase of AeroCom models (phase II), compared to the observational mean of 19.2 ng g−1. Factors determining model BC concentrations in Arctic snow include Arctic BC emissions, transport of extra-Arctic aerosols, precipitation, deposition efficiency of aerosols within the Arctic, and meltwater removal of particles in snow. Sensitivity studies show that the model–measurement evaluation is only weakly affected by meltwater scavenging efficiency because most measurements were conducted in non-melting snow. The Arctic (60–90° N) atmospheric residence time for BC in phase II models ranges from 3.7 to 23.2 days, implying large inter-model variation in local BC deposition efficiency. Combined with the fact that most Arctic BC deposition originates from extra-Arctic emissions, these results suggest that aerosol removal processes are a leading source of variation in model performance. The multi-model mean (full range) of Arctic radiative effect from BC in snow is 0.15 (0.07–0.25) W m−2 and 0.18 (0.06–0.28) W m−2 in phase I and phase II models, respectively. After correcting for model biases relative to observed BC concentrations in different regions of the Arctic, we obtain a multi-model mean Arctic radiative effect of 0.17 W m−2 for the combined AeroCom ensembles. Finally, there is a high correlation between modeled BC concentrations sampled over the observational sites and the Arctic as a whole, indicating that the field campaign provided a reasonable sample of the Arctic

The AeroCom evaluation and intercomparison of organic aerosol in global models

This paper evaluates the current status of global modeling of the organic aerosol (OA) in the troposphere and analyzes the differences between models as well as between models and observations. Thirty-one global chemistry transport models (CTMs) and general circulation models (GCMs) have participated in this intercomparison, in the framework of AeroCom phase II. The simulation of OA varies greatly between models in terms of the magnitude of primary emissions, secondary OA (SOA) formation, the number of OA species used (2 to 62), the complexity of OA parameterizations (gas-particle partitioning, chemical aging, multiphase chemistry, aerosol microphysics), and the OA physical, chemical and optical properties. The diversity of the global OA simulation results has increased since earlier AeroCom experiments, mainly due to the increasing complexity of the SOA parameterization in models, and the implementation of new, highly uncertain, OA sources. Diversity of over one order of magnitude exists in the modeled vertical distribution of OA concentrations that deserves a dedicated future study. Furthermore, although the OA / OC ratio depends on OA sources and atmospheric processing, and is important for model evaluation against OA and OC observations, it is resolved only by a few global models. The median global primary OA (POA) source strength is 56 Tg a−1 (range 34–144 Tg a−1) and the median SOA source strength (natural and anthropogenic) is 19 Tg a−1 (range 13–121 Tg a−1). Among the models that take into account the semi-volatile SOA nature, the median source is calculated to be 51 Tg a−1 (range 16–121 Tg a−1), much larger than the median value of the models that calculate SOA in a more simplistic way (19 Tg a−1; range 13–20 Tg a−1, with one model at 37 Tg a−1). The median atmospheric burden of OA is 1.4 Tg (24 models in the range of 0.6–2.0 Tg and 4 between 2.0 and 3.8 Tg), with a median OA lifetime of 5.4 days (range 3.8–9.6 days). In models that reported both OA and sulfate burdens, the median value of the OA/sulfate burden ratio is calculated to be 0.77; 13 models calculate a ratio lower than 1, and 9 models higher than 1. For 26 models that reported OA deposition fluxes, the median wet removal is 70 Tg a−1 (range 28–209 Tg a−1), which is on average 85% of the total OA deposition. Fine aerosol organic carbon (OC) and OA observations from continuous monitoring networks and individual field campaigns have been used for model evaluation. At urban locations, the model–observation comparison indicates missing knowledge on anthropogenic OA sources, both strength and seasonality. The combined model–measurements analysis suggests the existence of increased OA levels during summer due to biogenic SOA formation over large areas of the USA that can be of the same order of magnitude as the POA, even at urban locations, and contribute to the measured urban seasonal pattern. Global models are able to simulate the high secondary character of OA observed in the atmosphere as a result of SOA formation and POA aging, although the amount of OA present in the atmosphere remains largely underestimated, with a mean normalized bias (MNB) equal to −0.62 (−0.51) based on the comparison against OC (OA) urban data of all models at the surface, −0.15 (+0.51) when compared with remote measurements, and −0.30 for marine locations with OC data. The mean temporal correlations across all stations are low when compared with OC (OA) measurements: 0.47 (0.52) for urban stations, 0.39 (0.37) for remote stations, and 0.25 for marine stations with OC data. The combination of high (negative) MNB and higher correlation at urban stations when compared with the low MNB and lower correlation at remote sites suggests that knowledge about the processes that govern aerosol processing, transport and removal, on top of their sources, is important at the remote stations. There is no clear change in model skill with increasing model complexity with regard to OC or OA mass concentration. However, the complexity is needed in models in order to distinguish between anthropogenic and natural OA as needed for climate mitigation, and to calculate the impact of OA on climate accurately

Specific climate impact of passenger and freight transport

Emissions of short-lived species contribute significantly to the climate impact of transportation. Th magnitude of the effects varies over time for each transport mode. This paper compares first the absolute climate impacts of current passenger and freight transportation. Second, the impacts are normalized with the transport work performed and modes are compared. Calculations are performed for the integrated radiative forcing and mean temperature change, for different time horizons and various measures of transport work. An unambiguous ranking of the specific climate impact can be established for freight transportation, with shipping and rail having lowest and light trucks and air transport having highest specific impact for all cases calculated. Passenger travel with rail, coach or two- and three-wheelers has on average the lowest specific climate impact also on short time horizons. Air travel has the highest specific impact on short-term warming, while on long-term warming car travel has an equal or higher impact per passenger-kilometer

Mode, load, and specific climate impact from passenger trips

The climate impact from a long-distance trip can easily vary by a factor of 10 per passenger depending on mode choice, vehicle efficiency, and occupancy. In this paper we compare the "specific climate impact" of long-distance car travel with coach, train, or air trips. We account for both, CO2 emissions and short-lived climate forcers. This particularly affects the ranking of aircraft's climate impact relative to other modes. We calculate the specific impact for the Global Warming Potential and the Global Temperature Change Potential, considering time horizons between 20 and 100 years, and compare with results accounting only for CO2 emissions. The car's fuel efficiency and occupancy are central whether the impact from a trip is as high as from air travel or as low as from train travel. These results can be used for carbon-offsetting schemes, mode choice and transportation planning for climate mitigation

Global and regional climate impacts of black carbon and co-emitted species from the on-road diesel sector

Diesel vehicles are a significant source of black carbon (BC) and ozone precursors, which are important contributors to climate warming, degrade air quality and harm human health. Reducing diesel emissions could mitigate near-term climate change with significant co-benefits. This study quantifies the global and regional climate impacts of BC and co-emitted short-lived climate forcers (SLCFs) from present-day on-road diesel vehicles, as well as future impacts following a current legislation emission scenario. Atmospheric concentrations are calculated by the chemical transport model OsloCTM2. The following radiative forcing (RF) and equilibrium surface temperature responses are estimated. For year 2010 on-road diesel emissions we estimate a global-mean direct RF from BC of 44mW/m2 and an equilibrium surface temperature response of 59mK, including the impact of BC deposition on snow. Accounting for cooling and warming impacts of co-emitted SLCFs results in a net global-mean RF and warming of 28mW/m2 and 48mK, respectively. Using the concept of Regional Temperature change Potential (RTP), we find significant geographical differences in the responses to regional emissions. Accounting for the vertical sensitivities of the forcing/response relation amplifies these differences. In terms of individual source regions, emissions in Europe give the largest regional contribution to equilibrium warming caused by year 2010 on-road diesel BC, while Russia is most important for Arctic warming per unit emission. The largest contribution to warming caused by the year 2050 on-road diesel sector is from emissions in South Asia, followed by East Asia and the Middle East. Hence, in regions where current legislation is not sufficient to outweigh the expected growth in activity, accelerated policy implementation is important for further future mitigation

Assessing Metrics of Climate Change. Current Methods and Future Possibilities.

With the principle of comprehensiveness embedded in the UN Framework Convention on Climate Change (Art. 3), a multi-gas abatement strategy with emphasis also on non-CO2 greenhouse gases as targets for reduction and control measures has been adopted in the international climate regime. In the Kyoto Protocol, the comprehensive approach is made operative as the “aggregate anthropogenic carbon dioxide equivalent emissions” of six specified greenhouse gases or groups of gases (Art. 3). With this operationalisation, the emissions of a set of greenhouse gases with very different atmospheric lifetimes and radiative properties are transformed into one common unit – “CO2 equivalents”. This transformation is based on the Global Warming Potential (GWP) index, which in turn is based on the concept of radiative forcing. The GWP metric and its application in policy making has been debated, and several other alternative concepts have been suggested. In this paper, we review existing and alternative metrics of climate change, with particular emphasis on radiative forcing and GWPs, in terms of their scientific performance. This assessment focuses on questions such as the climate impact (end point) against which gases are weighted; the extent to which and how temporality is included, both with regard to emission control and with regard to climate impact; how cost issues are dealt with; and the sensitivity of the metrics to various assumptions. It is concluded that the radiative forcing concept is a robust and useful metric of the potential climatic impact of various agents and that there are prospects for improvement by weighing different forcings according to their effectiveness. We also find that although the GWP concept is associated with serious shortcomings, it retains advantages over any of the proposed alternatives in terms of political feasibility. Alternative metrics, however, make a significant contribution to addressing important issues, and this contribution should be taken into account in the further development of refined metrics of climate change