Short-lived climate pollutants (SLCPs) are agents that have a relatively short lifetime in the atmosphere - a few days to a few decades - and a warming influence on climate. The main short lived climate pollutants are black carbon, methane and tropospheric ozone, which are the most important contributors to the human enhancement of the global greenhouse effect after CO2. These short-lived climate pollutants are also dangerous air pollutants, with various detrimental impacts on human health, agriculture and ecosystems. Other short-lived climate pollutants include some hydrofluorocarbons (HFCs). While HFCs are currently present in small quantity in the atmosphere their contribution to climate forcing is projected to climb to as much as 19% of global CO2 emissions by 2050.

Short-lived climate pollutants

Short-lived climate pollutant science - what do we know today?

Short-Lived Climate Pollutant Science Update - What have we learned since 2011?

Short-Lived Climate Pollutant Science Update - What have we learned since 2011?
Esteemed short-lived climate pollutant experts, Drew Shindell (Duke University & Science Advisory Panel Chair) and Klimont Zbigniew (IIASA), present the newest knowledge on SLCP emissions, sectors, and mitigation opportunities. From the perspective of what we've learned since the publication of the 2011 UN Environment & World Meteorological Organisation "Integrated Assessment of Black Carbon and Tropospheric Ozone", the experts explore trends and opportunities for enhancing mitigation of black carbon, methane, and HFCs.


How are particulate matters and black carbon related?

Particle emissions from combustion sources, including fossil fuels, biofuels, and biomass, are broadly referred to as particulate matter (PM). They are usually classified into two types based on their sizes (diameter): PM2.5 and PM10. PM2.5 has diameter less than 2.5 micrometer, while PM10 has diameter less than 10 micrometer and includes PM2.5. Black carbon, as well as secondary sulfate and nitrate particles formed from SOx or NOx precursors, is the major constituent of PM2.5. It represents the solid fraction of PM2.5 that strongly absorbs light energy and converts it to heat, with a resultant effect of causing temperature change, melting of snow and ice (when deposited on snow or ice), and change in precipitation patterns. Furthermore, because of its small size, PM2.5, which includes black carbon, easily penetrates human cells and blood and therefore has high negative impact on human health. 

Why is it important that SLCPs receive urgent attention?

The urgency for SLCP reductions comes from their ability to reduce near-term warming and improve human well-being. Several scientific evidence have shown that swift and large-scale actions to reduce the various sources of black carbon, tropospheric ozone, HFCs, and methane could help deliver reduced climate warming that can be observed soon after emissions reductions measures are implemented, while also yielding substantial public health, food security and near-term regional climate protection benefits. Results also show that implementation of emission reductions measures may have substantial effect on the Asian monsoon, mitigating disruption of traditional rainfall patterns and could also lead to a considerable reduction in the melting of the Himalayan-Tibetan glaciers, and reduce the disruption of traditional rainfall patterns in Africa. Mitigating SLCPs emissions globally therefore presents a win-win opportunity for climate change mitigation and socio-economic development, hence an opportunity that should be urgently harnessed. 

What are the main measures for reducing SLCP emissions and what criteria was used in selecting them?

There are many cost-effective, readily available options for addressing SLCPs. Recent studies examined a range of measures targeting methane and black carbon and show that their implementation, if rapid and sustained, will bring considerable benefits. The UNEP/WMO assessment of black carbon and tropospheric ozone identified 16 measures targeted at reducing emissions of black carbon and methane to achieve near-term climate benefit as well as health and food security benefits. These measures targets 4 black carbon emitting sectors: transportation, residential, industry and agriculture and 3 methane emitting sectors: extraction and transport of fossil fuel, waste management and agriculture. The measures addresses black carbon emissions (and its co-emitted pollutants) from biomass heating and traditional cooking with solid biomass and coal, the burning of agricultural waste, high-emitting on-road and off-road diesel vehicles, brick kilns and coke ovens as well as methane emissions from coal mining, oil and gas production and transport, landfills and wastewater, livestock and rice paddies.

The selection criteria for the 16 measures from a large subset of possible option was based on the net impact that the implementation of the measures would have on global warming, and would yield air quality benefits by reducing particulate matter and/or O3 concentrations (win-win measures). Hence, measures that provide benefit for air quality but increased warming were omitted. Examples of measures include installing diesel particulate filters to trap black carbon emissions from diesel engines, accelerating transitions to cleaner fuels for household cooking, improved brick kilns that minimize black carbon and co-emissions and harnessing methane from landfills as a source of energy.

Measures to tackle the rapidly growing increase of HFCs emissions include using new technologies to avoid use of high global warming potential (GWP) HFCs in air conditioning, refrigeration, solvent, foam, aerosols and fire retardants. Commercially used examples include fibre insulation materials and architectural designs that avoid the need for air-conditioners, alternatives to high-GWP HFCs such as hydrocarbons and ammonia, and the use of low-GWP HFCs. The rapid development of national action planning can also support SLCP mitigation by enabling countries to identify achievable ‘quick-win’ benefits, and to prepare the ground for large-scale implementation of mitigation measures geared to their unique national circumstances, priorities and particular mix of SLCP sources. 

What are the uncertainties associated with the benefits from SLCP emissions reduction?

Methane emissions reduction: there is high confidence in the climate benefits associated with the reduction of methane emissions because of the high degree of confidence in the estimate of the warming effect of methane as a greenhouse house gas. Furthermore, the health and agricultural yield benefits associated with methane reductions is also certain. It must be noted however that the health benefits from methane emission reduction are much smaller than those associated with black carbon and other particulate matter-related emission reductions.

Black carbon emissions reduction: the global climate benefits associated with black carbon emissions reduction is associated with large uncertainty. This is because measures targeted at reducing black carbon also leads to the reduction of other co-emitted substances including organic carbon (OC) and precursors of tropospheric ozone such as carbon monoxide (CO) and NOx, some of which have a climate cooling effect. Hence, the warming effect of BC and O3 and the compensating cooling effect of OC, introduces large uncertainty into the net effect of some BC measures on global warming. This uncertainty is dependent on the black carbon emission source. For some measures, such as those targeted at diesel emissions controls and transitioning to cleaner fuels for household cooking (i.e. away from solid biomass), the benefits are virtually certain, while for others, such as some types of clean-burning biomass cook stoves and open burning of biomass, the benefits are associated with large uncertainties. However, the recent findings of the warming effects of brown carbon from biomass burning increase confidence in the net warming effects of emissions from cook stoves. It must be noted however that even if global mean climate benefits from biomass burning turns out to be not as large as those of diesel emission controls, regional climate benefits are likely to be large. In addition, the health and crop benefits of these measures are certain, and the regional human health benefits in particular are very large.

How does black carbon impact the climate?

Black carbon affects the climate in a number of ways. Its ability to absorb light energy and to darken surfaces link it to a range of climate-related impacts, including temperature rise, ice and snow melt and alteration of precipitation patterns.

Black carbon aerosols contribute to global warming in two ways. First, it captures energy by directly absorbing both incoming and outgoing radiation resulting in the heating up of the atmosphere. Secondly, black carbon, when deposited on snow and ice, darkens the bright surface, thereby weakening its reflective ability and increasing light absorbing ability. Furthermore, black carbon aerosols can modify the microphysical properties of droplets and hence increase the reflectivity and lifetime of clouds.

Apart from global climate impacts, black carbon also has spatial, temporal, and regional climate characteristics. The direct radiative forcing is usually highest in regions where the emission of black carbon is high and vice versa. For example, some studies have shown that China, being one of the world hotspots of black carbon emissions, has the highest direct radiative forcing in East Asia.  Furthermore, the direct warming from black carbon is further enhanced by the snow/ice albedo effect thereby amplifying the impacts of black carbon’s radiative forcing. 

What is the relationship between the climate benefits of SLCP mitigation and those of CO2 reductions?

The short lifetime of SLCPs in the atmosphere means that reducing their emissions will reduce their atmospheric concentrations in a matter of weeks to decades, with a noticeable effect on global temperature during the following decades. Thus large reductions in SLCP emissions over the next 1-2 decades can have a substantial impact on climate during the next few decades relative to waiting to reduce SLCPs until mid-century. Hence, the urgency for SLCP reductions comes from their ability to reduce near-term warming and improve human well-being. To address long-term warming the concentrations of longer-lived greenhouse gases and SLCPs both have to be reduced, and hence SLCP reductions do not replace CO2 mitigation but rather are complementary. Reductions of both SLCPs and CO2 are required as part of any plausible strategy to keep temperatures from exceeding 2°C warming in the 21st century. 

In contrast to SLCPs, plausible emission scenarios for CO2 reductions, even those that are implemented in the near-term, lead to climate benefits that primarily occur after 2050 due to the long lifetime of CO2 and the reduction of SO2 emissions that accompany some of the CO2 emission reduction measures. Given that long lifetime, it is urgent to begin reducing CO2 immediately to avoid the worst impacts of long-term climate change, but such reductions will have comparatively little effect on near-term climate. Hence SLCPs and CO2 affect climate on very different timescales and thus it is very difficult to compare long-lived and short lived warming agents, and it is best to avoid using metrics that combine them.

Mitigation of SLCPs and CO2 is also achieved via different strategies in some cases, especially in the near-term, for example in the case of regulating particulate emissions from vehicles. Slowing the rate of near-term climate change gives us the opportunity to achieve multiple benefits, including reducing impacts from climate change on those alive today, reducing biodiversity loss, providing greater time for adaptation to climate change, and reducing the risk of crossing thresholds activating climate feedbacks (e.g. from emissions associated with melting permafrost). Importantly, there are also immediate visible health and other benefits from reduced air pollution exposure. Additionally, reducing SLCPs is likely to have enhanced benefits in mitigating warming in the Arctic and other elevated snow and ice covered areas such as in the Himalayan/Tibetan region and the Andes. The reductions are also important in reducing regional disruption of traditional rainfall patterns. While fast action to mitigate SLCPs could help slow the rate of climate change, long-term climate protection will only be possible if deep and persistent cuts in carbon dioxide emissions are realized in the near future. SLCPs cannot, by themselves, be used to protect the world from long-term climate change.

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