Black carbon from fossil fuels heats the planet, soot from biomass cools it

Black carbon contained in soot from the combustion of fossil fuels may be responsible for around 16% of the gross warming of the planet. According to testimony provided by five scientists before the US House Committee on Oversight and Government Reform, it may be the second-most significant global warming pollutant after carbon dioxide and ahead of methane.

The black carbon in soot performs its warming by absorbing sunlight, converting it into infrared (heat) radiation, and emitting that heat radiation to the air around it. Soot on the surface of snow and sea ice contribute to both the melting of those surfaces as well as the warming of the air (earlier post).

But according to the scientists, particles from burning biomass are less oily and contain a much lower black carbon fraction than fossil fuel soot particles. Biomass-burning particles thus tend to cool climate on a global scale. This results in the so-called 'global dimming' effect.

Testifying before the committee were:

• Dr. Mark Z. Jacobson, Prof. of Civil and Environmental Engineering, Atmosphere/Energy Program, Stanford University
• Dr. Tami C. Bond, Asst. Prof. of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign
• Dr. V. Ramanathan, Prof. of Climate and Atmospheric Sciences, Scripps Institute of Oceanography, University of San Diego
• Dr. Charles Zender, Assoc. Prof. of Earth System Science, University of California at Irvine.
• Dr. Joel Schwartz, Professor of Environmental Epidemiology, Harvard University

Because of the relatively short lifetime of soot in the atmosphere compared to greenhouse gases, control of soot may be the fastest method of slowing warming for a specific period, according to Dr. Jacobson.

Black carbon, noted Dr. Bond, adds 2-3 order of magnitude more energy to the climate system than an equivalent mass of CO2 because black carbon is an extremely good absorber of visible light. While carbon dioxide stays in the atmosphere for decades, it absorbs just a small amount of infrared radiation:

The findings have implications for the use of diesel fuel. Because of their increased fuel efficiency relative to gasoline vehicles, diesels are seen as an improvement over gasoline with respect to global warming issues. However, once soot warming is factored in, the difference between the two platforms is greatly reduced, as diesel emits more soot than gasoline:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: fossil fuels :: soot :: black carbon :: global dimming :: global warming ::

Methods proposed to control fuel soot include improving engines; switching fuels; adding particle traps; and changing vehicle technologies.

In sum, there is not an advantage and a potential disadvantage of diesel versus gasoline in terms of climate and air pollution impact. However, neither type of vehicle is satisfactory or useful for solving climate and health problems as the emissions from both are very high. Even modest improvements in mileage standards for all vehicles are beneficial, but will only delay the eventual increase in emissions due to a larger population. — Dr. Jacobson

The scientists therefor advise a conversion of vehicles from fossil fuels to electric, plug-in hybrid or hydrogen fuel cell vehicles, where the electricity or hydrogen is produced by a renewable energy sources.

The good new is that, given the fact that biomass particles may help cool the planet, their use in dedicated power plants would become a viable strategy to fight global warming. Moreover, unlike any of the conventional renewables (wind, solar, geothermal, etc...) biomass can be used in carbon-negative energy systems.

The overall effect of biomass used in so-called 'bio-energy with carbon storage' (BECS) systems on reducing global warming, can thus become even larger than first predicted.

Image: Map showing the annual mean temperature change due to dirty snow in degrees Celsius.

References:
U.S. House, Committee on Oversight and Government Reform: Hearing Examines Black Carbon and Global Warming - October 18, 2007

GreenCarCongress: Black Carbon May be Second-Most Significant Global Warming Pollutant After Carbon Dioxide; Alters Picture of Diesel Engine Benefits - November 5, 2007.

Biopact: Dirty snow may warm Arctic as much as greenhouse gases - cleaner fuels needed - June 06, 2007