In June 1991, when Mt. Pinatubo in the Philippines spewed tons of volcanic ash and gases into the atmosphere, it just so happened that halfway around the world scientists were beginning to obtain good data from carbon dioxide monitors high above the tree canopy in Harvard Forest, outside Boston, Mass. Now, more than a decade later, the measurements taken during the years following the eruption are providing new insight into how atmospheric aerosols affect photosynthesis. The findings, published today in the journal Science, are forcing scientists to rethink the factors that influence the cycling of carbon through the environment, particularly carbon dioxide, a major player in global warming.

Within three weeks of the Mt. Pinatubo eruption, the largest volcanic blast of the century, a band of sulfur aerosol had encircled the globe. By early 1992, the volcanic gases and aerosols had diffused through the stratosphere, veiling the earth. During that time, global carbon dioxide levels fell more sharply than any other decline on record. Some scientists suggested that global cooling caused ecosystem respiration to drop, lowering the amount of carbon dioxide emitted into the atmosphere. But Lianhong Gu of Oak Ridge National Laboratory, lead author of the Science report, didn't think that could be the only explanation.

Gu knew that crop scientists had discovered that plants grow best in diffuse light. When sunlight is too intense, some leaves fall into shadow, unable to photosynthesize, while others bask in the direct beams but will reach a photosynthetic saturation point. Moderate cloud cover and aerosols block direct beams, but allow light to bounce back and forth off water vapor and other molecules, creating a "softer" light that reaches leaves that would otherwise be shaded. As a result, the plants photosynthesize more, using up carbon dioxide in the process. Gu and his collaborators determined that the same principles apply to forest canopies. The Harvard Forest data show that carbon dioxide levels dropped for two years following the eruption at Mt. Pinatubo¿findings that the scientists suggest represent a worldwide phenomenon given that the eruption had a global atmospheric effect. "Up until now we hadn't linked aerosols and clouds with carbon studies," Gu says. "In order to understand atmospheric carbon dioxide concentrations, which affect climate, we have to look at how aerosols and clouds affect the global carbon cycle."

3 Comments

1. 1. pgtruspace 02:18 AM 5/7/09

   A cooling hydrosphere will scrub a lot more CO2 out of the atmosphere and result in lowering of the CO2 detected during this period.
   Plant life on this planet is an almost steady state carbon cycle and can not cause the changes observed.
   Only a planet with huge, dirty (with salts) oceans can have a oxygen enriched atmosphere. CO2 is very easy to scrub with sea water as the CO2 goes into solution with water easily and then combines with light metal ions to make carbonates that percipitate out. Solar radiation in the upper atmosphere breaks down H2O into hydrogen that leaks into space and oxygen that sinks toward the surface.
   Global cooling causes lowering of CO2 in the atmosphere.
   Global warming causes raising of CO2 in the atmosphere.
   CO2 has almost nothing to do with the temperature in the atmosphere, it is an effect not the cause. CO2 is a very poor green house gas and comprises only 1/30 of 1 percent of the atmosphere, and carbon is the most important element for life on this planet. The amount of carbon in circulation is the limiting nutritant in all life on this planet
   IT IS THE SUN.
   The total output of the sun causes the goldylocks zone to move in or out and the planet to cool or warm depending on the amount of energy it encounters, and the amount of CO2 to lower or raise with the temperature changes.
   

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2. 2. LeaderofMen 10:34 AM 7/27/09

   A paid lobbyist for a major oil company (who has trouble spelling) has once again scoured this website for articles on global climate change, written up something first (before anyone else has a chance), salted it with bogus 'facts', and told us *directly* that nearly all scientists who study this matter are incorrect. How clever.
   
   This pattern is quite evident at Scientific American. It's a prominent website with lobbyists writing completely ridiculous comments on it.

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3. 3. pgtruspace 11:31 PM 8/27/09

   Maybe if I was a payed lobbyist for a rich major oil company I could afford a decent spell checker. "LeaderofMen" should go back to his computer gaming.

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