Black Carbon Mitigation and climate

Several studies have demonstrated that carefully selected measures to prevent the release of microscopic BC particulate products arising from the incomplete combustion of fossil fuels and biomass can reduce near-term warming and improve human health. BC is not the only substance emitted from incomplete combustion, and the climate impacts depend on the full range of co-pollutants emitted from a particular source.

This article has highlights of the 113 page, BLACK CARBON MITIGATION AND THE ROLE OF THE GLOBAL ENVIRONMENT FACILITY: A STAP Advisory Document

There is also a Journal of Geophysical research – Bounding the role of black carbon in the climate system: A scientific assessment

BC is the most strongly absorbing component of fine particulate matter (PM2.5) and contributes to regional and global climate change in the near-term (over months to a few decades). Reducing BC emissions can help slow the rate of climate change, reduce local air pollution, improve human health and security of food and water supplies, and support achieving the Sustainable Development Goals (SDGs).

Moderating the pace and magnitude of global climate change will require aggressive efforts to reduce CO2 emissions (mainly through lowering energy demand and decarbonizing energy systems) which will affect the climate system over centuries, as well as reduce BC and other SLCPs, which would have a more immediate climate effect.

The scientific knowledge and understanding of BC emissions and atmospheric concentrations, their measurement, impacts and mitigation options, continue to advance rapidly. Emissions from solid-fuelled cook-stoves and from the combustion of diesel and other transport fuels, as well as from the flaring of natural gas, burning of crop residues and forests, and heating of brick kilns, can be reduced by appropriate interventions

In addition to long-lived greenhouse gases (GHGs), some pollutants that are short-lived in the atmosphere also contribute a substantial portion of the additional radiative forcing that is causing anthropogenic climate change. These pollutants, including black carbon (BC) and tropospheric ozone, affect global and regional temperatures; are associated with a range of deleterious health effects, including premature death

• BC mainly absorbs solar radiation, while GHGs mainly absorb infrared radiation.
• The radiative (warming) influence of BC after it is emitted lasts only days to weeks, whereas for long-lived GHGs it can last for centuries or longer.
• The climate impacts of BC can differ depending on where it is emitted (as opposed to long-lived GHGs that are well-mixed in the atmosphere).
• A mixture of warming and cooling pollutants are co-emitted with BC.
• BC is a component of fine particulate matter, which directly affects human health.

Although BC emissions within this region are low, the Arctic is particularly sensitive to its climate effects. BC has a short atmospheric lifetime and concentrations are highest near emission sources. However, studies have shown that BC can also travel long distances and impact on more remote regions (Shindell et al., 2008). Emissions from Northern Europe, North America and Asia have been shown to contribute the greatest absolute impact of BC on the Arctic

UNEP/WMO (2011) found that fully implementing 16 specific measures to mitigate BC and methane emissions by 2030 could halve the potential increase in global temperature projected for 2050 compared to a reference scenario. The measures were also estimated to avoid 0.6 to 4.4 and 0.04 to 0.52 million annual premature deaths globally in 2030 from reduced surface concentrations of PM2.5 and ozone, respectively (Anenberg et al., 2012). Around 80% of these health benefits would occur in Asia, where large populations are co-located with high BC emissions. The BC mitigation measures alone were estimated to contribute 98% of the total deaths avoided if methane mitigation measures were also implemented.

A recent study by the World Bank and ClimateWorks Foundation (2014) focused on cleaner cook-stoves in rural China and found that the deployment of 72 million cleaner stoves between the years 2014 – 33 would require a public investment of $400 million in the near term but could lead to the following benefits:
• 87,900 avoided instances of premature mortality from outdoor air pollution globally, of which 85,700 would be within China;
• reduced energy use of 450 GJ per year in 2030;

Black Carbon Climate Warming Effect

The best estimate of industrial-era climate forcing of black carbon through all forcing mechanisms, including clouds and cryosphere forcing, is +1.1 W m−2 with 90% uncertainty bounds of +0.17 to +2.1 W m−2. Thus, there is a very high probability that black carbon emissions, independent of co-emitted species, have a positive forcing and warm the climate. We estimate that black carbon, with a total climate forcing of +1.1 W m−2, is the second most important human emission in terms of its climate forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing. Sources that emit black carbon also emit other short-lived species that may either cool or warm climate. Climate forcings from co-emitted species are estimated and used in the framework described herein. When the principal effects of short-lived co-emissions, including cooling agents such as sulfur dioxide, are included in net forcing, energy-related sources (fossil fuel and biofuel) have an industrial-era climate forcing of +0.22 (−0.50 to +1.08) W m−2 during the first year after emission. For a few of these sources, such as diesel engines and possibly residential biofuels, warming is strong enough that eliminating all short-lived emissions from these sources would reduce net climate forcing (i.e., produce cooling). When open burning emissions, which emit high levels of organic matter, are included in the total, the best estimate of net industrial-era climate forcing by all short-lived species from black-carbon-rich sources becomes slightly negative (−0.06 W m−2 with 90% uncertainty bounds of −1.45 to +1.29 W m−2)

Black carbon has a unique and important role in the Earth’s climate system because it absorbs solar radiation, influences cloud processes, and alters the melting of snow and ice cover.