Evidence for air pollution affecting climate

I’ve gathered together some material on air pollution and its affects on climate. This shows very conclusively that the link has been known for some time … however, the associated commentary has been moved to a new article as I just come across some important findings that need more space.

Sulfate Aerosol and Global Warming

An aerosol is a suspension of very fine particles of a solid, or of droplets of a liquid, in a gaseous medium. Fog, smoke, and volcanic dust are naturally occurring examples of aerosols. Sulfates are salts that contain a charged group of sulfur and oxygen atoms: SO , the basic constituent of sulfuric acid. The sulfate particles in the atmosphere are about 0.1 to 1.0 micrometer (a millionth of a meter) in diameter and are particularly concentrated over industrial areas. They contribute to acid rain, cause lung irritation, and have been a main culprit in causing the haze that obscures a clear view of the Grand Canyon.

Much of the sulfate aerosol in the atmosphere derives from the oxidation of sulfur dioxide produced in the combustion of fossil fuels. Industrial activities are not the only source: Natural aerosols–mostly dust, sea salt, and other compounds of marine origin–do exist, but they have remained in nearly constant concentrations in the atmosphere for a long time. However, the man-made version has increased dramatically since about 1950.

Aerosols can cool the climate in two basic ways: either directly, under clear sky conditions, by reflecting away some of the incoming solar radiation, or indirectly, by increasing the reflectivity of clouds.

During the late 1960s, in the process of trying to measure and understand the optical clarity of the atmosphere, Charlson realized that existing instrumentation for measuring aerosols was inadequate. The old measurements were done by eye and were only approximate. So he designed and built a new device, patented by the University, to analyze the light-scattering power of atmospheric aerosols. The technology currently forms the basis of the Model 3550/3560 Integrating Nephelometer marketed by a Minnesota-based company called TSI Incorporated. The nephelometer made it possible for the first time to quantitatively assess the amount of sunlight reflected back into space by sulfate aerosols.

Charlson’s original work on the role of aerosols in the global heat balance, published in 1969, did not attract much attention, nor did a subsequent paper on sulfate in 1976. It wasn’t until the global warming debate heated up, so to speak, in the late 1980s and early 90s that the importance of his work began to be fully appreciated.

Charlson and colleagues have shown that the cooling effect of sulfate aerosols does not neatly cancel out the effects of greenhouse warming, but rather, makes the situation more complex. “Aerosol cooling and the greenhouse effect have characteristics that prevent them from neatly offsetting each other,” note Charlson and colleague Tom Wigley, who heads the Office for Interdisciplinary Earth Studies at the University Corporation for Atmospheric Research in Boulder, Colorado.

First, the cooling and warming occur mostly over different parts of the world: the aerosol effect is focused over industrial areas in the Northern Hemisphere, whereas warming effects may be greatest over subtropical oceans and deserts. There are also temporal variations. Aerosol effects are most pronounced during daylight hours during the summer season; the activity of greenhouse gases differs very little over the course of a day, or over a year.

The work of Charlson and colleagues suggests that forcing by sulfate aerosol is not evenly distributed over the globe–it can vary by roughly a factor of five from region to region. As a result, the world might expect to see dramatic changes in regional weather patterns in the future, not just an increase in average global temperature.

Furthermore, Charlson’s work demonstrates how to incorporate particular chemical and physical measurements made on the local scale into models of atmospheric dynamics on a global scale. For that achievement, Charlson was awarded an honorary doctoral degree from Stockholm University in 1993. Sulfate aerosol was named by the journal Science as one of nine runners-up for Molecule of the Year in 1995.

  • “Sulfate Aerosol and Climatic Change,” Robert J. Charlson and Tom M. L. Wigley, Scientific American, 270 (2), 48-57, February 1994.
  • “Climate Forcing by Anthropogenic Aerosols,” R. J. Charlson, S. E. Schwartz, J. M. Hales, R. D. Cess, J. A. Coakley, Jr., J.E. Hansen, D. J. Hofmann, Science, 255, 423 (1992).
  • “Radiative Forcing of Climate Change,” The 1994 Report of the Scientific Assessment Working Group of IPCC, Intergovernmental Panel on Climate Change, World Meterological Organization and United Nations Environment Programme.
  • “Molecule of the Year: A New Form of Matter Unveiled,” Science, 270, 1902 (1995).


From:”The Myth of the 1970s Global Cooling Scientific Consensus”

Section: AEROSOLS.

In December 1968, a group of scientists convened in Dallas, Texas, for a “Symposium on Global Effects of Environmental Pollution” (Singer 1970). Reid Bryson showed the panel a remarkable graph illustrating the correlation between rising levels of dust in the Caucasus and the rising output of the Russian economy over the previous three decades. It was t he foundation for a n arg ument leading from human activities to dust to changing climate. Atmospheric pollution caused by humans was sufficient, Bryson argued, to explain the decline in global temperatures identified earlier in the decade by J. Murray Mitchell (Bryson and Wendland 1970).

Also on the symposiu m pa nel was Mitchel l himself, and he disagreed. Mitchell’s calculations suggested that particulates added to the atmosphere were insufficient to explain the cooling seen in his temperature records. However, he raised the possibility that, over time, cooling caused by particulates could overtake warming caused by what he called the “the CO2 effect” (Mitchell 1970).

In 1971, S. Ichtiaque Rasool and Stephen Schneider wrote what may be the most misinterpreted and mis-used paper in the story of global cooling (Rasool and Schneider 1971). It was the first foray into climate science for Schneider, who would become famous for his work on climate change. Rasool and Schneider were trying to extend the newly developed tool of climate modelling to include the effects of aerosols, in an attempt to sort out two potentially conflicting trends—the warming brought about by increasing carbon dioxide and the cooling potential of aerosols emitted into the Earth’s atmosphere by industrial activity.

The answer proposed by Rasool and Schneider to the questions posed by Bryson and Mitchell’s disagreement was stark. An increase by a factor of 4 in global aerosol concentrations, “which cannot be ruled out as a possibility,” could be enough to trigger an ice age (Rasool and Schneider 1971). Critics quickly pointed out f laws in Rasool and Schneider’s work, including some they acknowledged themselves (Charlson et al. 1972; Rasool and Schneider 1972). Refinements, using data on aerosols from volcanic eruptions, showed that while cooling could result, the original Rasool and Schneider paper had overestimated cooling while underestimating the greenhouse warming contributed by carbon dioxide (Schneider and Mass 1975; Weart 2003). Adding to the confusion at the time, other researchers concluded that aerosols would lead to warming rather than cooling (Reck 1975; Idso and Brazel 1977).

It was James Hansen and his colleagues who found what seemed to be the right balance between the two competing forces by modelling the aerosols from Mount Agung, a volcano that erupted in Bali in 1963. Hansen and his colleagues fed data from the Agung eruption into their model, which got the size and timing of the resulting pulse of global cooling correct. By 1978, the question of the relative role of aerosol cooling and greenhouse warming had been sorted out. Greenhouse warming, the researchers concluded, had become the dominant forcing [LOL] (Hansen et al. 1978; Weart 2003).

  • Bryson and W. M. Wendland, 1970: Climatic effects of atmospheric pollution. Global Effects of Environmental Pollution, S. F. Singer, Ed., Springer-Verlag/D. Reidel, 130–138
  • Charlson, R. J., H. Harrison, and G. Witt, 1972: Aerosol concentrations: Effects on planetary temperatures. Science,175, 95–96.
  • Hansen, J. E., W.-C. Wang, and A. A. Lacis, 1978: Mount Agung eruption provides test of a global climatic perturbation. Science,199, 1065–1068, doi:10.1126/science.199.4333.1065.
  • Idso, S. B., and A. J. Brazel, 1977: Planetary radiation balance as a function of atmospheric dust: Climatological consequences. Science,198, 731–733
  • Mitchell, 1970: A preliminary evaluation of atmospheric pollution as a cause of the global temperature fluctuation of the past century. Global Ef fects of Environmental Pollution, S. F. Singer, Ed., Springer-Verlag/D. Reidel, 139–155
  • Rasool, S. I., and S. H. Schneider, 1971: Atmospheric carbon d iox ide a nd aerosols: Effects of large increases on global climate. Science,173, 138–141.
  • Rasool, S. I., and S. H. Schneider, 1972: Aerosol concentrations: Effect on planetary temperatures. Science,175, 96
  • Reck, R. A., 1975: Aerosols and polar temperature change. Science,188, 728–730.
  • Schneider, S. H., 1975: On the carbon dioxide–climate confusion. J. Atmos. Sci.,32, 2060–2066.
  • Singer, S. F., Ed., 1970: Global Effects of Environmental Pollution. Springer-Verlag/D. Reidel, 218 pp.
  • Weart, S., cited 2003: The Discovery of Global Warming. [Available online at www.aip.org/history/climate.]

WUWT: New paper: man-made aerosols have had a net cooling effect since beginning of industrial revolution

From Smith et al. 2004 “Historical Sulfur Dioxide Emissions 1850-2000: Methods and Results” -PNNL

Historical Sulphur Dioxide Emissions from 1850

More aerosols, means more clouds, which means cooler temperatures. Now that we are cleaning up aerosols worldwide, this may explain why the Earth is getting slightly warmer – more sunlight reaches the surface

A paper published today in Science claims the transition from “pristine” to “slightly polluted” atmosphere at the beginning of the industrial revolution in the 18th century had a “dramatic aerosol effect [of increasing] clouds” over the oceans. According to the authors,

“transition from pristine to slightly polluted atmosphere yields estimated negative forcing of ~15 watts per square meter (cooling), suggesting that a substantial part of this anthropogenic forcing over the oceans occurred at the beginning of the industrial era, when the marine atmosphere experienced such transformation.”

By way of comparison, the IPCC alleged change in radiative forcing from CO2 [plus alleged positive water vapor feedback] since the beginning of the industrial era is +1.8 watts per square meter*, or 8.3 times less. According to an accompanying editorial to the paper, the authors “show that even small additions of aerosol particles to clouds in the cleanest regions of Earth’s atmosphere will have a large effect on those clouds and their contribution to climate forcing.”

*Per the IPCC formula: 5.35*ln(395/280) = 1.8 W/m2 at the top of the atmosphere [or only about 1.8* (1/3.7) = 0.5 W/m2 at the surface]

h/t to The Hockey Schtick

Smith et al. in 2004 finds that sulfur based aerosols, the kind that also get emitted from volcanoes, have been increasing since 1850, but have recently leveled off since about 1975…about the time that the US Clean Air Act really started kicking in (from updates in 1970) and other industrialized countries followed suit.

From Smith et al. 2004 “Historical Sulfur Dioxide Emissions 1850-2000: Methods and Results” -PNNL

From Smith et al. 2004 “Historical Sulfur Dioxide Emissions
1850-2000: Methods and Results” -PNNL

From “Just Add Aerosols“: Science 6 June 2014: Vol. 344 no. 6188 p. 1089 DOI: 10.1126/science.1255398

The more carbon dioxide and other greenhouse gases in the atmosphere, the stronger the climate warming that results. Likewise, the more aerosol particles suspended in the atmosphere, the greater the ability of these particles either to scatter sunlight back to space and cool the planet or to absorb sunlight in the atmosphere, thereby warming the atmosphere while cooling Earth’s surface. However, not all such climate forcing processes depend linearly on the concentrations of their forcing agent. The climatic effects of aerosols are complicated by their interactions with clouds (1). On page 1143 of this issue, Koren et al. (2) show that even small additions of aerosol particles to clouds in the cleanest regions of Earth’s atmosphere will have a large effect on those clouds and their contribution to climate forcing.

The paper:

From aerosol-limited to invigoration of warm convective clouds


Among all cloud-aerosol interactions, the invigoration effect is the most elusive. Most of the studies that do suggest this effect link it to deep convective clouds with a warm base and cold top. Here, we provide evidence from observations and numerical modeling of a dramatic aerosol effect on warm clouds. We propose that convective-cloud invigoration by aerosols can be viewed as an extension of the concept of aerosol-limited clouds, where cloud development is limited by the availability of cloud-condensation nuclei. A transition from pristine to slightly polluted atmosphere yields estimated negative forcing of ~15 watts per square meter (cooling), suggesting that a substantial part of this anthropogenic forcing over the oceans occurred at the beginning of the industrial era, when the marine atmosphere experienced such transformation.

Editors summary: Invigorating convection in warm clouds

Atmospheric aerosols—tiny airborne particles—affect the way clouds form and how they affect climate. Koren et al. investigated how the formation of warm clouds, such as those that form over the oceans, depends on pollution levels (see the Perspective by Remer). Aerosols affect cloud formation in cleaner air disproportionately more than in more polluted air. Before the widespread air pollution of the industrial era, it seems, warm convective clouds may have covered much less of the oceans than they do today.

Source: http://www.sciencemag.org/content/344/6188/1143


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