Black Carbon - Mitigation

Mitigation

In its 2007 report, the IPCC estimated for the first time the direct radiative forcing of black carbon from fossil fuel emissions at + 0.2 W/m2, and the radiative forcing of black carbon through its effect on the surface albedo of snow and ice at an additional + 0.1 W/m2. More recent studies and public testimony by many of the same scientists cited in the IPCC’s report estimate that emissions from black carbon are the second largest contributor to global warming after carbon dioxide emissions, and that reducing these emissions may be the fastest strategy for slowing climate change.

Since 1950, many countries have significantly reduced black carbon emissions especially from fossil fuel sources, primarily to improve public health from improved air quality, and “technology exists for a drastic reduction of fossil fuel related BC” throughout the world.

Given black carbon’s relatively short lifespan, reducing black carbon emissions would reduce warming within weeks. Because black carbon remains in the atmosphere only for a few weeks, reducing black carbon emissions may be the fastest means of slowing climate change in the near-term. Control of black carbon, particularly from fossil-fuel and biofuel sources, is very likely to be the fastest method of slowing global warming in the immediate future, according to Dr. Mark Jacobson of Stanford University, and he believes that major cuts in black carbon emissions could slow the effects of climate change for a decade or two. Reducing black carbon emissions could help keep the climate system from passing the tipping points for abrupt climate changes, including significant sea-level rise from the melting of Greenland and/or Antarctic ice sheets.

“Emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions,” according to Ramanathan and Carmichael. They calculate black carbon’s combined climate forcing at 1.0–1.2 W/m2, which “is as much as 55% of the CO₂ forcing and is larger than the forcing due to the other such as CH₄, CFCs, N₂O or tropospheric ozone.” Other scientists estimate the total magnitude of black carbon’s forcing between + 0.2 to 1.1 W/m with varying ranges due to uncertainties.2 (See Table 1.) This compares with the IPCC’s climate forcing estimates of 1.66 W/m2 for CO₂ and 0.48 W/m2 for CH₄. (See Table 2.) In addition, black carbon forcing is two to three times as effective in raising temperatures in the Northern Hemisphere and the Arctic than equivalent forcing values of CO₂.

Jacobson calculates that reducing fossil fuel and biofuel soot particles would eliminate about 40% of the net observed global warming. (See Figure 1.) In addition to black carbon, fossil fuel and biofuel soot contain aerosols and particulate matter that cool the planet by reflecting the sun’s radiation away from the Earth. When the aerosols and particulate matter are accounted for, fossil fuel and biofuel soot are increasing temperatures by about 0.35 °C.

Black carbon alone is estimated to have a 20-year Global Warming Potential (GWP) of 4,470, and a 100-year GWP of 1,055-2,240. Fossil fuel soot, as a result of mixing with cooling aerosols and particulate matter, has a lower 20-year GWP of 2,530, and a 100-year GWP of 840-1,280.

The Integrated Assessment of Black Carbon and Tropospheric Ozone published in 2011 by the United Nations Environment Programme and World Meteorological Organization calculates that cutting black carbon, along with tropospheric ozone and its precursor, methane, can reduce the rate of global warming by half and the rate of warming in the Arctic by 2/3s, in combination with CO2 cuts. By trimming “peak warming”, such cuts can keep current global temperature rise below 1.5˚C for 30 years and below 2˚C for 60 years, in combination with CO2 cuts. (FN: UNEP-WMO 2011.) See Table 1, on page 9 of the UNEP-WMO report.

The reduction of CO2 as well as SLCFs could keep global temperature rise under 1.5˚C through 2030, and below 2C through 2070, assuming CO2 is also cut. Please see the graph on page 12 of the UNEP-WMO report.