Today's Special: Dim Sun

Well, it worked! I took a cursory glance at radar and then watered the reseeded patch on the front lawn just before dinner last night. Sure enough, the first raindrops began within less than 2 hours. Showers were more numerous and intense south of the immediate Washington metro area overnight and this morning. By lunch time, radar was showing the northern edge of precipitation retreating south of the Beltway, although some showers near the mouth of the Potomac were moderate to heavy. At mid afternoon, sunshine was widespread here in MoCo north of the Beltway, but in southern portions of the region, clouds, drizzle and some light rain were persisting.

Surface weather and radar map at 2pm today from HPC/NCEP/NWS

Tonight and Tomorrow

Clouds should decrease throughout the area tonight with lows in the mid 40s in the District, lower 40s in the colder 'burbs. Tomorrow's highs will be in the mid 60s under scattered clouds.

Omnimedia: Something New Under the Sun?

PBS is broadcasting a new program in the NOVA science series, "Dimming the Sun", tomorrow night (8pm on WETA 26 in DC and WMPT 22 in MD, repeated Thursday at 1am, Friday at 2am and 4am on 26, Saturday at 10am on 22). The show is evidently based on a BBC Horizon program which aired in Britain about a year ago.

The theme of the presentation is that a reduction in the amount of solar radiation reaching the ground is actually reducing the observed effects of global warming. The appeal of this theory is that it can be used by both sides in the global warming policy debate. The supporters of action on global warming can say, "See, it's even worse than you think, because if it weren't for dimming, global warming would be even more intense." The opponents can say, "See, it's not a problem at all, because we can generate as much pollution as we want, and the greenhouse gas warming effects will be canceled out by the dimming."

Unfortunately for the ideologues, things in science are seldom as simple as the True Believers would like. At the time the original show was broadcast, the people at RealClimate.org pointed out that too much certainty in the global warming debate can be just as dangerous as the attempt to create doubt by fake science sites. Whenever anyone boldly asserts, as the documentary does, that

Global dimming is a killer. It may have been behind the worst climatic disaster of recent times, responsible for famine and death on a biblical scale. And Global Dimming is poised to strike again.

it should raise a red flag.

Studies have shown that aerosols in the atmosphere, especially from volcanoes, can have a dramatic cooling effect on the atmosphere. Benjamin Franklin noted the largest lava eruption in historic time, in Iceland in 1783, as a likely cause of unusually cold weather in Europe. In 1815, Tambora volcano exploded in Indonesia. This was followed the next year by the notorious "year without a summer", in which New England observed frost in every month. RealClimate points out, however, that the claims for the current effects of global dimming simply do not match the observed data.

At a time when less than 20% of the public can be considered scientifically literate, it is important to beware of scientific arguments presented in excessively dramatic terms. By all means watch the program, but use your brain. RealClimate has posted two other analyses of the subject: "Global Dimming II" and "Global Dimming may have a brighter future"