More Evidence On The Complex Role Of Aerosols In The Climate System

The role of aerosols within the climate system is a complex subject and is  becoming better recognized that it is even more so than previously thought.

Two recent news items illustrate this increasing complexity.

The first is an NCAR/UCAR news release titled

Broken glass yields clues to climate change

It reads in part

“A study appearing this week in Proceedings of the National Academy of Sciences finds that microscopic particles of dust, emitted into the atmosphere when dirt breaks apart, follow similar fragment patterns as broken glass and other brittle objects. The research, by National Center for Atmospheric Research (NCAR) scientist Jasper Kok, suggests there are several times more dust particles in the atmosphere than previously believed, since shattered dirt appears to produce an unexpectedly high number of large dust fragments.

The finding has implications for understanding future climate change because dust plays a significant role in controlling the amount of solar energy in the atmosphere. Depending on their size and other characteristics, some dust particles reflect solar energy and cool the planet, while others trap energy as heat.”

Kok’s research indicates that the ratio of silt particles to clay particles is two to eight times greater than represented in climate models.

Since climate scientists carefully calibrate the models to simulate the actual number of clay particles in the atmosphere, the paper suggests that models most likely err when it comes to the number of silt particles. Most of these larger particles swirl in the atmosphere within about 1,000 miles of desert regions, so adjusting their quantity in computer models should generate better projections of future climate in desert regions, such as the southwestern United States and northern Africa.

Additional research will be needed to determine whether future temperatures in those regions will increase more or less than currently indicated by computer models.

The study results also suggest that marine ecosystems, which draw down carbon dioxide from the atmosphere, may receive substantially more iron from airborne particles than previously estimated. The iron enhances biological activity, benefiting ocean food chains, including plants that take up carbon during photosynthesis.

In addition to influencing the amount of solar heat in the atmosphere, dust particles also get deposited on mountain snowpacks, where they absorb heat and accelerate melt.”

The second news article is in the January 8-14 2011 issue of the Economist and is titled

A fistful of dust – The true effect of windblown material is only now coming to be appreciated

Excerpts from this article read

The importance of this long-distance logistical chain has become apparent only in the past few years, and researchers are still working out its many repercussions—for the more you look at dust, the more effects it seems to have. African dust is thought, for example, to stimulate plant growth in the Amazon by bringing in phosphorus (which is in short supply there). This may put a check on global warming by removing what would otherwise be a long-term constraint on the forest’s ability to suck up carbon dioxide as it grows.

Dust aloft cools the land below, as Europe’s meteorologists found out in May 2008. It does this directly, by reflecting sunlight back into space, and indirectly, by helping clouds to form. The effect is significant. The carbon dioxide which has been added to the atmosphere since the industrial revolution began has a greenhouse effect equivalent to the arrival of about 1.6 watts of extra solar power per square metre of the Earth’s surface. The direct effects of dust are estimated to provide a countervailing cooling of about 0.14 watts per square metre. Add the indirect effect on clouds and this could increase markedly, though there are great uncertainties.

To get a better sense of the net effects brought about by the ups and downs of dust, it would help to have a detailed historical record of the dustiness of the planet. And this is what Natalie Mahowald of Cornell University and 19 colleagues have achieved. They analysed cores from glaciers, lake bottoms and coral reefs and measured how the levels of some telltale chemicals changed with depth, and thus with time. They then used models of global wind circulation to deduce which dust sources have become stronger and which weaker. Their conclusion, published recently in Atmospheric Chemistry and Physics, is that in fits and starts over the past century the air became twice as dusty.

Part of the increase stems from human activities—directly, in the case of construction, or indirectly, when it results from clearing vegetation from marginal land in order to farm it. Another part of the explanation may be global warming itself, shifting the boundaries of deserts and intensifying dust production in some areas.”

The work on aerosols reported in the NCAR/UCAR press release is also reported on in the Economist article.

A bottom line message from these news reports is that, as we learn more about the climate system, including the human role within it,  the more complicated it shows itself to be. This also means simplistic attempts to predict climate decades from now and claim the ability to provide skillful impact information to policyholders are misguided and misleading.

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