We’re Dimmed if We Do & Fried if We Don’t – Global Dimming

September 9, 2007 at 1:10 am | Posted in Miscellaneous, science | 2 Comments
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Earlier this week I awoke in the wee hours of the morning and could not go back to sleep. I clicked the television on hoping to find something that would bore me back to sleep. Instead what I found was a PBS presentation interesting enough to keep me awake and watching. I tuned in about midway through a show about global dimming, a term I hadn’t heard before.

It seems that air pollution has had a significant dimming effect on sunlight reaching the earth that is actually cooling the planet and slowing the acceleration of global warming.

In the early 21st century, it’s become clear that air pollution can significantly reduce the amount of sunlight reaching Earth, lower temperatures, and mask the warming effects of greenhouse gases. Climate researcher James Hansen estimates that “global dimming” is cooling our planet by more than a degree Celsius (1.8°F) and fears that as we cut back on the pollution that contributes to dimming, global warming may escalate to a point of no return. Regrettably, in terms of possibly taking corrective action, our current understanding of global dimming has been a long time in the coming, considering the first hints of the phenomenon date back to 18th-century observations of volcanic eruptions. In this slide show, follow a series of historic events and scientific milestones that built the case for global dimming. Click on the image at left to begin.—Susan K. Lewis read more (new window)

Something I found particularly interesting was the effect of vapor trails from aircraft.

If conditions are right, newly formed contrails will begin feeding off surrounding water vapor. Like vaporous cancers, they start growing and spreading. In time, they can expand horizontally to such an extent that they become indistinguishable from cirrus clouds, those thin, diaphanous sheets often seen way up high. These artificial cirrus clouds can last for many hours, and the amount of sky they end up covering can be astonishing: one study showed that contrails from just six aircraft expanded to shroud some 7,700 square miles. read more (new window)

Climatologists studying jet contrails realized that the three day period after 9/11, during which all planes over the U.S. were grounded, afforded a rare opportunity, as until this point there was no basis for comparison.

At least that was the case until September 11, 2001. For the first time since the jet age began, virtually all aircraft were grounded over the United States for three days. Even as they tried like the rest of us to absorb the enormity of the terrorist attacks, climatologists realized they had an unprecedented opportunity to scrutinize individual contrails, and several studies were quickly launched.

One study looked at the aforementioned contrails that grew to cover 7,700 square miles. Those condensation trails arose in the wake of six military aircraft flying between Virginia and Pennsylvania on September 12, 2001. From those isolated contrails, unmixed as they were with the usual dozens of others, Patrick Minnis, a senior research scientist at NASA’s Langely Research Center, and his colleagues were able to gain valuable insight into how a single contrail forms. Those once-in-a-lifetime data sets are so useful that Minnis is about to analyze them again in an expanded study.

Another study that took advantage of the grounding gave striking evidence of what contrails can do. David Travis of the University of Wisconsin-Whitewater and two colleagues measured the difference, over those three contrail-free days, between the highest daytime temperature and the lowest nighttime temperature across the continental U.S. They compared those data with the average range in day-night temperatures for the period 1971-2000, again across the contiguous 48 states. Travis’s team discovered that from roughly midday September 11 to midday September 14, the days had become warmer and the nights cooler, with the overall range greater by about two degrees Fahrenheit.

These results suggest that contrails can suppress both daytime highs (by reflecting sunlight back to space) and nighttime lows (by trapping radiated heat). That is, they can be both cooling and warming clouds. But what is the net effect? Do they cool more than they warm, or vice versa? “Well, the assumption is a net warming,” Travis says, “but there is a lot of argument still going on about how much of a warming effect they produce.”

For more complete information visit Dimming the Sun.



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  1. Does CO2 emissions cause any dimming? Or does it just cause a greenhouse effect and warming?

  2. Kenzrw,
    No,the CO2 emmissions are what causes the greenhouse effect and warming. Pollution is the contributor to dimming.

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