What is Short Lived Climate Forcers

Short-lived climate forcers (SLCFs) include all atmospheric agents that have the potential to directly or indirectly alter Earth’s radiative energy budget, and which also have relatively short residence times in the atmosphere. Depending on how they alter the energy budget, SLCFs can contribute to either warming or cooling the planet.

They encompass gaseous species – such as ozone (O3), methane (CH4), and nitrogen oxides (NOX) – and also aerosols, which can be solid or liquid and include black carbon (BC), sulphate (SO4), and mineral dust. Some SLCFs are emitted directly into the atmosphere, while others – notably O3 and SO4 aerosols – are formed in the atmosphere from precursor emissions. Methane is a precursor for tropospheric ozone formation.

There is not a universally agreed upon lifetime threshold for agents to be considered ‘short-lived’, but all agents included in the SLCF basket are shorter-lived than CO2.

A key feature that distinguishes SLCFs from long-lived substances, such as CO2, is that atmospheric concentrations of SLCFs are governed more by their rates of emission than their cumulative emissions, so cutting emissions of SLCFs leads to rapid reductions in their atmospheric concentrations. 


Properties of different short lived climate forcers

Methane is a powerful greenhouse gas, especially on decadal timeframes. While it is 28-36 times as potent as carbon dioxide over a 100-year timeframe, it has a warming potential 84 times that of carbon dioxide over a 20-year timeframe according to the IPCC Fifth Assessment Report. Methane also affects air quality due to its role in ozone formation. Methane is emitted from a variety of anthropogenic and natural sources. Anthropogenic emission sources include landfills, oil and natural gas systems, agricultural activities, coal mining, stationary and mobile combustion, wastewater treatment, and certain industrial processes.

Ozone is an air pollutant that forms in the lower atmosphere when sunlight interacts with precursor gases: nitrogen oxides, carbon monoxide, volatile organic compounds, and methane. It is also a greenhouse gas and can affect the atmospheric lifetime of methane. It is harmful to human health and vegetation.

Sulfate aerosols are formed from emission of sulfur compounds, such as sulfur dioxide. Sulfate aerosols make up a significant portion of the fine particulate matter in ambient PM2.5, which is harmful to human health and covered by many air quality guidelines. Sulfate aerosols scatter sunlight efficiently and enhance the brightness of clouds. This causes a cooling of the climate, offsetting some of the warming impacts of greenhouse gases and other SLCFs. The climate impacts of clouds are a key uncertainty in climate modelling. Anthropogenic sources of sulfur compounds are from the combustion or burning of fossil fuels, specifically oil and coal. Natural sources are volcanic eruptions. 

Black carbon (often referred to as soot) and organic carbon contribute to levels of ambient particles that degrade air quality and are harmful to human health. Black carbon absorbs sunlight and thereby contributes to climate warming, while organic carbon tends to reflect light. When deposited on snow, black carbon decreases the surface’s ability to reflect sunlight, enhancing climate warming. The climate impact of organic carbon is small.

Climate impacts in the Arctic of reducing SLCFs

Emission of SLCFs have both regional and large-scale impacts on the climate. SLCFs emitted in or transported to the Arctic affect heat transport in the Arctic atmosphere and also lead to decreased albedo when dark particles, such as black carbon, land on snow and ice, which then absorb heat instead of reflecting it. Emissions that occur at high-latitudes have the strongest effect - on a per unit of emissions basis - on Arctic warming. However, because SLCF emissions, and thus concentrations at mid-latitudes, are much greater than concentrations in the Arctic, measures to reduce air pollution in mid-latitudes have the greatest potential to influence Arctic warming. However, per unit of emissions reduced, high-latitude measures still have the greatest effect. Success in reducing emissions of sulfur dioxide, driven by policy to improve air quality and reduce premature deaths caused by sulfate aerosols, has 'unmasked'  some of the warming caused by other SLCFs and GHGs.

Health impacts in the Arctic

There are only a limited number of studies of the impact of air pollution on people living in the Arctic. Those that do exist for Alaska show that exposure to PM2.5 is an important health concern. While estimating the contribution of air pollution to disease among people who live in the Arctic is a challenging task, current understanding of the cause-effect relationships indicates that most efforts to reduce emissions would have health benefits. Local sources are important and measures to reduce emissions from residential heating, waste burning, diesel generators, and surface transport would have local health benefits. Ensuring that increased marine shipping does not lead to local air pollution is also important. Another concern is the risk for more wildland fires in the Arctic and the associated health impacts of increased smoke emissions.