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Aerosol Percentages Require Context
July 28, 2006
Rebecca Goldin Ph.D
Recent reporting on aerosols illustrate the importance of comparison

Aerosols are tiny particles that come from deserts and industrial pollution; they can affect cloud formation and, therefore, weather patterns. A recent article in the journal Science discussed the mechanism by which aerosols affect whether clouds form or dissipate, based on satellite data from around the world. The Christian Science Monitor took a “Hard Look at Aerosols” but missed a basic mathematical principle.

The Monitor states that “[the scientists] estimate that the net result [of these particles], world-wide, has been a 5 percent increase in global cloud cover.” But percentages are always a comparison. Is this five percent since a certain date in history? Or is this five percent more in regions with more industrialization, compared to regions with less? Or five percent compared to the world without aerosol?

A cursory look at the article finds that the five percent increase is compared to an estimate of a world in which there is no aerosol except that from dust.

A percentage means nothing if it doesn’t compare two quantities; perhaps it is fair to blame the scientists on this, by making a similar five-percent remark in their abstract. But if journalists don’t ask questions about what they don’t understand, why bother writing about it for the general population?