The main short-lived climate pollutants (SLCPs) are black carbon (or ‘soot’) particles, methane, tropospheric ozone and some hydrofluorocarbons (HFCs). Controlling emissions of these SLCPs or their precursors could roughly halve projected warming over the next few decades while saving millions of lives and increasing crop yields by tens of millions of tons annually via improved air quality. These benefits would be obtained by reducing emissions of some SLCPs, such as black carbon and methane, that are at historically high levels, whereas emissions of others, such as HFCs, would have to prevented from growing from their current small levels. Additional short-lived compounds affect climate and degrade air quality, but do not clearly lead to warming and so are not typically included as SLCPs.
Black carbon is an important component of particulate matter air pollution. Airborne particulate matter is the major environmental cause of premature death globally and contributes substantially to many other adverse health impacts as well. Methane is a precursor to the formation of ozone in the lower atmosphere which, at ground-level, harms human health, crops and the climate. The ozone precursors carbon monoxide, and volatile organic compounds are also considered SLCPs.
In addition, methane, black carbon and ozone are powerful warming agents. Black carbon and ozone also disturb rainfall and regional circulation patterns and black carbon darkens snow and ice, increasing absorption of sunlight and exacerbating melting.
Though HFCs represent a small fraction of the current total greenhouse gases (less than one percent), their warming impact per molecule is particularly strong and, if left unchecked, growth in HFC emissions could lead to substantial additional warming. HFCs themselves do not have direct impacts as air pollutants, but there are studies that suggest some of their products may have environmental impacts when broken down.
Short-lived pollutants, ones that live in the atmosphere for roughly 10 years or less, will be cleansed from the atmosphere fairly quickly once their emissions cease. Their influence will also go away fairly rapidly after the cessation of their emissions. This is unlike CO2 which lives in the atmosphere for a very long time and whose effect on global warming is almost permanent on the human life timescale.
Swift action to reduce the multiple sources of black carbon, ozone, HFCs, and methane can, apart from leading to much less warming beginning soon after the emissions reductions, deliver large benefits in terms of public health, food security and near-term regional climate protection. The emission reductions may have a 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 as well as the disruption of traditional rainfall patterns in Africa. Assessments, including those coordinated by the UNEP and the World Meteorological Organization, indicate that in some extremely vulnerable areas such as those with mountain glaciers the climate benefits may be especially large.
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. These measures include addressing 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. Examples of such 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.
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 2C 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.
Global climate benefits from methane reductions are certain, with the magnitude of the response known approximately as well as the magnitude of the response to CO2 emissions changes. That there would be health and agricultural yield benefits from methane reductions is also certain, though the health benefits are much smaller than those associated with the BC and other particulate matter-related emission reductions and both the health and crop benefits have substantial uncertainties in magnitude. Methane reductions will also enable many regions of the world to better meet their air quality standards. Health and crop benefits would be felt globally in response to methane emission reductions, and are felt locally and regionally from the implementation of the measures targeting black carbon, that also reduce other emissions, including precursors of tropospheric ozone (such as NOx and CO).
Global climate benefits from reductions of BC and co-emitted pollutants vary between measures depending on the mix of emissions. For some, like 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, the benefits are likely. The recent findings of the warming effects of brown carbon from biomass burning increase confidence in the net warming effects of emissions from cookstoves. Even if global mean climate benefits turn 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.
These Frequently Asked Questions have been developed by the Scientific Advisory Panel of the Climate and Clean Air Coalition to Reduce Short-Lived Climate Pollutants (CCAC).
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