Fire & Ice: Black Carbon vs Sulfate
Black carbon is generated from burning both fossil fuels and biomass. Black carbon aerosols absorb solar radiation and are purported to be a major source of global warming. A recent study claims that the extent of black-carbon-induced warming is dependent on the concentration of sulfate (SO2) and organic aerosols—which reflect solar radiation and cool the surface—as well as the origin of the black carbon. The ratio of fossil-fuel-based black carbon to SO2 emissions has increased by more than a factor of two during the twentieth century, and the portion of black carbon from fossil fuels has increased threefold. This could account for a 30% increase in global warming from black carbon, which may account for a quarter of the warming usually attributed to CO2. Even worse, black carbon may be causing millions of deaths among those who have to breath it. Far from being green, climate science's demonizing of CO2 is damaging the pursuit of sound environmental policy.
In “Warming influenced by the ratio of black carbon to sulphate and the black-carbon source ,” appearing in the August 2010 issue of Nature Geoscience, M. V. Ramana et al. report on the results of the Cheju ABC Plume–Monsoon Experiment (CAPMEX) conducted during the summer, 2008. It provided observations for determining the dependence of the warming (or cooling) effect of atmospheric brown clouds (ABCs) on the sources of black carbon (BC), and on the BC-to-sulfate ratios. The campaign was in part motivated by the desire to reduce pollution in Beijing and surrounding areas during the 2008 Summer Olympics. The work, as explained in the paper:
Modeling studies have estimated that (1) the net radiative effect of black carbon (BC) and organics generated by fossil-fuel combustion and biomass-fuel cooking contribute to a warming, (2) open burning leads to net cooling and (3) the net warming effect of fossil-fuel BC is larger than that of biomass-fuel cooking. Furthermore, BC warming is regulated by the ambient concentration of sulphates resulting from sulphur dioxide (SO2) emissions. Sulphate strongly reflects solar radiation, whereas BC strongly absorbs solar radiation. Thus the net radiative forcing is determined by the relative amounts of BC and sulphate. However, BC is invariably internally mixed with sulphates and solar absorption by BC is amplified when it is internally mixed with sulphates. Such mixtures of absorbing and scattering aerosols (including other particulate matter such as nitrate, potassium and so on) are referred to as ABCs, for atmospheric brown clouds.
The results suggest that the absorption efficiency is larger in fossil-fuel-based plumes, but these measurements are subject to large uncertainties. The fossil fuel contribution to total BC is about 60% over East Asia and only about 30% over South Asia on the basis of emission inventories. This blog has previously reported on “brown clouds” causing Arctic ice to melt and warming in general. This is important because, in climate models, the global-mean warming is determined by the balance of the radiative forcings—warming by greenhouse gases balanced against cooling by aerosols. Since a greater cooling effect has been used in climate models, the result has been to credit CO2 with a larger warming effect than it really deserves.
Air pollution at the 2008 Summer Olympics' site in Beijing.
Some people are confused by that claim—that greater warming by black carbon implies less warming by CO2—but the logic is quite simple. In the IPCC AR4 report, each factor that influences global warming is assigned a radiative forcing value, given in Wm-2. Consider a simple equation for global warming:
This says that the total forcing (FTotal) is made up of the sum of all the individual forcings for CO2, NOx, black carbon and all the rest. Note that we are talking about revisions to previous forcing estimates, not increases due to higher emissions in the future. Supposedly, each forcing should cause a portion of any observed temperature change in proportion to its forcing value. So, another way state this is in terms of temperature change:
Scientists must also find a way to relate measurements they do have, such as the amount of a gas in the atmosphere or the density of particles in the brown clouds, to the forcing values. This, in turn, relates actual physical measurements to portions of the temperature change. These relationships, linking real world empirical measurements to changes in global temperature, are a way to describe how sensitive temperature change is to a particular factor.
The next observation is the crucial one—the amount of temperature change in the past is a fixed value. It has been recorded over time and, though there is always some inaccuracy in measurements, changing any of the values on the right hand side of the temperature equation above cannot alter the total temperature change on the left. If global temperature rose 1°C last century and we suddenly discover that black carbon would have caused 50% more warming than previously though, the only way to balance the equation is to take some ΔT from one or more of the other forcings.
In other words, if some forcings are found to cause more warming or less cooling (some factors are negative), then the impact of the remaining forcings must be diminished. When new factors are uncovered it makes sense to reduce the change attributed to CO2, since most of the warming is attributed to it. This not only reduces carbon dioxide's current contribution, it reduces the sensitivity of the climate to future increases in CO2 in climate model calculations.
Changing the size of the slices doesn't change the size of the pie.
Why not take the increase from some other forcing? The IPCC wished to make CO2 the villain in the global warming horror show. For years, CO2 has been the catch-all for any warming that could not be attributed to other forcings. CO2 has had its importance inflated by low but uncertain values being attributed to other forcings. So, when better values are identified for factors like black carbon, CO2 must give up some of its influence—it's a zero sum game.
Over the past decade, a huge number of new forcing values have been identified, including values for insolation, NOx, methane, stratospheric ozone and, as in the Ramana et al. report, black carbon and other aerosols. Yet many climate modelers cling to the fiction that CO2 is the primary and overwhelming driver of climate change. In fact, if you add up all of the temperature change attributed to other forcings there isn't any temperature change for carbon dioxide to cause! This is not because CO2 has no effect—it does. CO2 is a greenhouse gas, but at current levels its effect is not large compared with other factors.
Of course, it is really not that simple or scientists would have a better understanding of the whole climate system out by now. The climate system is nonlinear, meaning the response you get from an input change this time may not be proportionally the same as the last time or the next time. On top of its inherent nonlinearity, the climate system also contains many linkages and dependencies among factors, making it impossible to deal with any single forcing value in isolation. An example would be the linkage between black carbon and sulfate in the Ramana et al. study. Still the general principle holds, you cannot revise or introduce new causes for global warming without diminishing the importance of the existing ones, especially CO2
Now, consider that all existing climate models are validated against historical data, a processing called backcasting. The models are tweaked until they reproduce historical climate variations that have been previously recorded. Remember, science's “discovery” of a new factor does not mean that it has suddenly come into being—the new forcing values, either revised or newly uncovered, were present in the past. Introduce a new forcing value and all models are now tuned to produce results they should not have. Which, of course, also means that their predictions for the future are just so much dreck.
This latest study is not the first report that black carbon has a more important impact on global temperature than it is usually credited with. A study led by Drew Shindell of NASA's Goddard Institute for Space Studies reports that, in the high latitudes the impact of aerosols (sulfates and black carbon) may account for 45% or more of the observed warming which has occurred in at the poles over the past three decades. Over that time Arctic temperatures have risen by 1.7°C, while Antarctic regions have witnessed 0.35°C temperature increases. Others credit black carbon with accelerating the melting of Himalayan glaciers.
Black carbon may affect Himalayan glaciers.
Ramana et al. came to the conclusion that black carbon and SO2 are closely interrelated when it comes to atmospheric warming. Reducing BC alone is not going to produce the results that might otherwise be expected. They summarized their findings this way:
Because of its short lifetime, BC offers the greatest potential for slowing down climate change in the coming decades. Worldwide there are efforts to decrease SO2 emissions, and the data presented in this study strongly suggest that such reductions should be accompanied by larger percentage reductions in BC, such that the BC-to-SO2 emission ratio is also decreased. Such a mitigation step will also have significant co-benefits to human health, because air pollution leads to over two million deaths annually.
As stated, there are other, more compelling reasons to reduce emissions. Emissions from internal combustion engines are particularly bad for the environment and human health. Rather than pursue a fools errand—trying to reduce CO2 emissions at all costs—we should reduce the use of dirty fossil fuels instead. Certainly, reduced dependence on coal for electricity generation and rapid development of hybrid and electric vehicles are good places to start.
Far from preserving nature and helping humanity, climate change activism has had the opposite effect. In their bullheaded insistence on demonizing CO2, mainstream climate science has sorely damage the pursuit of reasonable and rational environment policy. Lying about global warming is no way to save the world.
Be safe, enjoy the interglacial and stay skeptical.