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Aerosols (pollutants, like soot) as a driver of climate change

Larry Kummer, Editor

It’s common for advocates to focus on a narrative that they believe will convince the public of the urgency of their cause.  Unfortunately this can blind them to research indicating that the problem is more complex then the simple story they push — and hence that their well-intended solutions might be ineffective, or even counter-productive.

I believe that’s become true today about climate science.  This is another in a series of articles about research showing the complexity and uncertainty in the current models of climate change.  Previous articles have focused on solar influences.  Here are a few discussing the role of atmospheric particles.  Brief excerpts appear below for these two articles.

(1)  “Impure as the Driven Snow“, Scientific American, 8 June 2007 — “Soot is a bigger problem than greenhouse gases in polar meltdown.”  This discusses “Present-day climate forcing and response from black carbon in snow”, Mark G. Flanner, Charles S. Zender, James T. Randerson, and Philip J. Rasch, Journal of Geophysical Research, June 2007 — Abstract.

(2)  “Climate response to regional radiative forcing during the twentieth century”, Drew Shindell and Greg Faluvegi, Nature Geoscience, March 2009 — Abstract, NASA’s summary.

There is a large body of research about effect of particles on Earth’s climate.  Here are a few other articles, with no excerpts given:

Of course, it’s important not to let the proles know about such research (aerosols possible role as both warming and cooling agents).  Hence Obama’s science advisor, John Holdren, was careful not to mention this during his interview with Associate Press in which he discussed using aerosols to geoengineer the Earth’s climate.  Don’t spoil the simple narrative!

Excerpts

(1)  “Impure as the Driven Snow“, Scientific American, 8 June 2007 — This discusses “Present-day climate forcing and response from black carbon in snow”, Mark G. Flanner, Charles S. Zender, James T. Randerson, and Philip J. Rasch, Journal of Geophysical Research, June 2007 — Abstract.  Excerpt:

Belching from smokestacks, tailpipes and even forest fires, soot—or black carbon—can quickly sully any snow on which it happens to land. In the atmosphere, such aerosols can significantly cool the planet by scattering incoming radiation or helping form clouds that deflect incoming light. But on snow—even at concentrations below five parts per billion—such dark carbon triggers melting, and may be responsible for as much as 94 percent of Arctic warming.

… Zender, physicist Mark Flanner and other colleagues built a model to examine how soot impacts temperature in the Arctic and Antarctic regions. Temperatures in the northern polar region have already risen by 1.6 degrees Celsius (2.88 degrees Fahrenheit) since the dawn of the Industrial Revolution.

… Whereas forest fires contribute to the problem—the effect noticeably worsens in years with widespread boreal wildfires—roughly 80 percent of polar soot can be traced to human burning, adding as much as 0.054 watt of energy per square meter of Arctic land, according to the research published this week in the Journal of Geophysical Research. When the snow melts, it exposes dark land below it, further accelerating regional warming. “Black carbon in snow causes about three times the temperature change as carbon dioxide in the atmosphere,” Zender says. “The climate is more responsive to this than [to] anything else we know.”

(2)  “Climate response to regional radiative forcing during the twentieth century”, Drew Shindell and Greg Faluvegi, Nature Geoscience, March 2009 — Abstract, NASA’s summary

Though greenhouse gases are invariably at the center of discussions about global climate change, new NASA research suggests that much of the atmospheric warming observed in the Arctic since 1976 may be due to changes in tiny airborne particles called aerosols.  Emitted by natural and human sources, aerosols can directly influence climate by reflecting or absorbing the sun’s radiation. The small particles also affect climate indirectly by seeding clouds and changing cloud properties, such as reflectivity.

A new study, led by climate scientist Drew Shindell of the NASA Goddard Institute for Space Studies, New York, used a coupled ocean-atmosphere model to investigate how sensitive different regional climates are to changes in levels of carbon dioxide, ozone, and aerosols. The researchers found that the mid and high latitudes are especially responsive to changes in the level of aerosols. Indeed, the model suggests aerosols likely account for 45 percent or more of the warming that has occurred in the Arctic during the last three decades. The results were published in the April issue of Nature Geoscience.

Though there are several varieties of aerosols, previous research has shown that two types — sulfates and black carbon — play an especially critical role in regulating climate change. Both are products of human activity.

Sulfates, which come primarily from the burning of coal and oil, scatter incoming solar radiation and have a net cooling effect on climate. Over the past three decades, the United States and European countries have passed a series of laws that have reduced sulfate emissions by 50 percent. While improving air quality and aiding public health, the result has been less atmospheric cooling from sulfates.

… At the same time, black carbon emissions have steadily risen, largely because of increasing emissions from Asia. Black carbon — small, soot-like particles produced by industrial processes and the combustion of diesel and biofuels — absorb incoming solar radiation and have a strong warming influence on the atmosphere

… The regions of Earth that showed the strongest responses to aerosols in the model are the same regions that have witnessed the greatest real-world temperature increases since 1976. The Arctic region has seen its surface air temperatures increase by 1.5 C (2.7 F) since the mid-1970s. In the Antarctic, where aerosols play less of a role, the surface air temperature has increased about 0.35 C (0.6 F).

… “There’s a tendency to think of aerosols as small players, but they’re not,” said Shindell. “Right now, in the mid-latitudes of the Northern Hemisphere and in the Arctic, the impact of aerosols is just as strong as that of the greenhouse gases.”

The growing recognition that aerosols may play a larger climate role can have implications for policymakers.  “We will have very little leverage over climate in the next couple of decades if we’re just looking at carbon dioxide,” Shindell said. “If we want to try to stop the Arctic summer sea ice from melting completely over the next few decades, we’re much better off looking at aerosols and ozone.”

For more information

To read other articles about these things, see the FM reference page on the right side menu bar.  Of esp relevance to this topic:

Posts on the FM site about the sun’s effect on Earth’s climate:

  1. Worrying about the Sun and climate change: cycle 24 is late, 10 July 2008
  2. Update: is Solar Cycle 24 late (a cooling cycle, with famines, etc)?, 15 July 2008
  3. My “wish list” for the climate sciences in 2009, 2 January 2009
  4. About the recent conference ”Solar Activity during the onset of Solar Cycle 24″, 3 January 2009
  5. A brief look at the Sun’s influence on Earth’s climate, 4 May 2009
  6. An important puzzle from the National Weather Service’s Space Weather Prediction Center, 10 May 2009
  7. Big news from NASA about the causes of climate change!, 5 June 2009
  8. Peer reviewed science: breakthoughs about the sun’s impact on Earth’s climate, 4 September 2009
  9. Another climate wild card: solar cycle 24, perhaps causing food riots during the next decade, 1 February 2011
  10. Update about the weather – on the Sun. Perhaps coming soon to Earth., 9 February 2011
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