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Global Dimming, Global Warming, and Bad Reporting

Reports that British researchers are claiming that cutting back the use of fossil fuels will make global warming worse are popping up on the web today, and many strike the headline and text pose of this piece from the otherwise reliable Reuters, which mixes an alarmist lede with long-discredited crap that "scientists differ" about global warming. The implication of the article is that attempts to fight climate disruption are silly and will just make matters worse. Because this story will undoubtedly be racing around the blogosphere (especially on the "it's not happening, we didn't make it happen, and it will hurt us too much if we stop making it happen" circuit), here's what the report really says.

Global Dimming is the observed effect of increases in particulate matter in the atmosphere. There has been a substantial drop in the amount of sunlight hitting the planet over the past half-century -- 22% globally, and up to 30% in some locations. It seems to be caused by pollution from burning coal, oil and wood. Because it cuts the amount of sunlight hitting the ground, it cuts the amount of heat trapped by greenhouse gases, and that has troubling implications.

But perhaps the most alarming aspect of global dimming is that it may have led scientists to underestimate the true power of the greenhouse effect.

They know how much extra energy is being trapped in the Earth's atmosphere by the extra carbon dioxide we have placed there.

What has been surprising is that this extra energy has so far resulted in a temperature rise of just 0.6 degree Celsius.

This has led many scientists to conclude that the present-day climate is less sensitive to the effects of carbon dioxide than it was, say, during the ice age, when a similar rise in CO2 led to a temperature rise of six degrees Celsius

Cutting back on the use of fossil fuels will reduce the amount of particulate matter in the atmosphere faster than it will reduce the amount of carbon dioxide. This will increase the amount of solar radiation getting through and trapped, initially accelerating global warming. But the same can be said about ongoing efforts to put particulate filters on coal plant smokestacks, increased use of "clean" diesel formulations, or even moving from burning wood to the use of other power sources in the developing world. Regardless of CO2 mitigation needs, necessary reductions in particulate pollution (which include sulfur compounds leading to acid rain) will reduce the global dimming effect, increasing the heat reaching the ground. Indeed, recent analysis suggesting that the planet was starting to get warmer faster may reflect the effects of a reduction in global dimming. This reduction in global dimming will likely have short-term negative effects -- but a reduction in global dimming without a reduction in global warming will be far, far worse.

The real implication of this story is not that efforts to reduce fossil fuel use are pointless, but that we need to redouble our efforts to mitigate greenhouse effects. These efforts take three key paths: methane (CH4) emission reduction, to slow shorter-term (3-10 year) greenhouse effects; carbon sequestration to reduce the amount of atmospheric CO2 in the short-to-medium-term; and CO2 emission reduction, to slow and stop medium-term (50-100 year) warming. You'll see plenty of commentary over the next few days arguing that we should abandon efforts to move away from fossil fuels; such self-appointed pundits are wrong. We need to move faster, not slower.

Comments (9)

Further, it must be noted that dimming has many negative effects that may outweigh its attentuation of warming trends. First of note is the effect on rain patterns, which have already been severely altered by extensive logging (forests appear to help produce rain clouds [anyone have a good link to back this up?], and hold in water that does come down). As the BBC explains, "Scientists are now worried that dimming, by shielding the oceans from the full power of the Sun, may be disrupting the pattern of the world's rainfall." Columbia University's Earth Institute News has more thorough coverage of dimming in general and effects on rain in particular.

Other effects that come to mind: how does reduced sunlight impact crop yields? solar power generation? And of course the cause of the dimming is also the cause of acid rain — man-made particulates with high-levels of sulphur or nitrogen — which as we know causes all kinds of problems.


from http://www.mongabay.com/0902.htm

Additionally, the forest adds to local humidity through transpiration (the process by which plants release water through their leaves), and thus adds to local rainfall. For example, 50-80% of the moisture in the central and western Amazon remains in the ecosystem water cycle. In the water cycle, moisture is transpired and evaporated into the atmosphere forming rain clouds before being precipitated as rain back onto the forest. When the forests are cut down, less moisture is evapotranspired into the atmosphere resulting in the formation of fewer rain clouds. Subsequently there is a decline in rainfall, subjecting the area to drought. If rains stop falling, within a few years the area can become arid with the strong tropical sun baking down on the scrub-land.

Ferdinand Engelbeen:

That more sunlight is reflected, due to (sulphate) aerosols is proven false.

As you may know, we have some satellites flowing around out of the atmosphere, which measure reflected sunlight (SW reflection) and heat (LW emission) from below.

For the (sub)tropics, in the period 1985-2001 the amount of sunlight reflected by clouds reduced with ~2 W/m2. (see: http://www.atmos.ucla.edu/csrl/publications/pub_exchange/Wielicki_et_al_2002.pdf , confirmed for the 30N-30S (sub)tropics in http://pubs.giss.nasa.gov/docs/2002/2002_ChenCarlsonD.pdf )

In the same period, there was a loss of cloud cover, both in the tropics and sub tropics (and even up to 60N-60S).

If there is global dimming at the surface, the only explanation possible is that more sunlight is retained in the atmosphere. Which is (only) possible with (dark brown and black) soot particulate.

If soot particulate is to blame, then a reduction of them would have a cooling effect, not a warming effect!

See also the amount of reflected sunlight from earth on the moon ("eartshine"), which parallels the "global dimming" trend, while it should have opposite trends, at: http://www.bbso.njit.edu/science_may28.html


I think global dimming in consistant with global warming. The measure of global dimming is really simple. You simply measure how quickly radiant heat from the sun heats up a black disk. The extra Co2, water vapour and other greenhouse gasses captures some of that radiant energy high in the athmosphere so it never gets to the disk as radiant energy in the first place!. (However it does heat the earth eventually by convection, etc.So, the air might be a half degree warmer round that black disk but this parameter is not measured. I bet it is simply one of the byproducts of global warming.
Brian White

Whatever the precise mechanism that is in play here may be, the scale of the observed phenomena and the implications for climate change when compared to the risks to us as a species if we do nothing, would suggest that urgent action is prudent. But where to start? Yes we can ratchet up support for sustainable development in all its forms and reduce the net CO2 input from fossil fules to the atmosphere; we can capture methane emissions through eliminating landfill and using anaerobic digesters etc. to process biodegradeable wastes. But what about carbon fixing? what can we do about agriculture and forestry etc? There are enormous potential social and economic benefits in supporting more traditional land management techniques that have a high level of community involvement through social enterprises, co-operatives etc. but what can we grow enough of and fast enough that will start to lock enough Carbon away into a longer cycle? Structural use of timber products instead of plastics or fired ceramics/bricks/concrete would be more sustainable. Does anyone have any references to work in this area? I'd be keen to know.

Colin Keyse. Grant Manager Environment; Wales Council for Voluntary Action.

Dave Mckendrick:

hi, i think this website is the best website in the world wide web. i think it is a very important issue, globabl dimming rules!!! This is better than msn, i know what I am doing tonight instead of watching Eastenders!!!
All the way!!!!!!!!!!!!


With respect to global warming and global dimming changing our natural environment, where would the safest place to live be in the future?

James Grant:

After reading about globale dimming via the BBC website, I watched the related program which was quite astounding.

The simplest experiment to prove whether or not global dimming is a reality, was given by an American scientist. He was studying the effects aircraft contrails have on the enviroment. But since the airspace he was monitoring always contained contrails, he experiment was limited. However after 911 when aircraft were grounded for a number of days, he recorded a 1 degree Celcius increase during the daytime, and a 0.5 degree Celcius decrease during the nighttime, during the period when the aircraft were grounded. When they resumed flying, the temperatures went back to the pre 911 values.

Scary stuff...


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