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02/02/2005
Darker Denver Skies
When I accompanied my parents on a visit to Denver, Colorado
in 1941 one vivid memory stands out to this day. The memory is
of how crystal clear and blue was the sky. Since that time, I’ve
been to Denver a few times and have been disappointed that the
sky was no different from that here in New Jersey. Somehow,
even on the clearest days, the sky just doesn’t have that crisp
sparkling blue hue. An exception was on 9/11/2001, when the
sky rivaled my memory of 1941 Denver. Is there normally so
much polluting of the air that it’s a cause for wonder when an
exceptional truly clear day comes along?
A few weeks ago, I mentioned Discover magazine’s number 1
science story of 2004 - global warming. There was a very brief
item in Discover on their number 43 story - solar dimming. The
atmosphere has become so polluted that there is substantially less
sunlight reaching the ground than there was back in 1941. You
might say this could be a good thing; if not as much sunlight and
heat reaches the ground it should keep down global warming.
Well, there’s a good chance that you’re right, at least about the
warming. Some scientists have been surprised that our Earth
hasn’t heated up more than it has. Global solar dimming could
be the answer.
If global dimming combats global warming, shouldn’t we work
to cut out more of the sunlight reaching the ground? Well, not
exactly. Pollution of our air isn’t the greatest thing for our
health. Who knows how many cases of cancer, asthma or other
respiratory diseases are due to pollution of our air. In my state of
New Jersey, as well as most other states in the U. S., air quality
does not measure up to clean air standards and there’s a lot of
effort and legislation aimed at reducing air pollution.
How do pollutant particles fit into the picture of global dimming?
Let’s look at clouds. Clouds form when water droplets form
from water vapor in the air. The water vapor typically condenses
on particles in the atmosphere. The oceans are one natural
source of particles, salts carried into the air by wind and wave
action. When pollutant particles are added to the naturally
occurring particles, this means there are more particles in the air.
There’s only so much water vapor in the air and when it
condenses on a lot of particles the size of the droplets in the
clouds becomes smaller. It has been found that clouds consisting
of many smaller droplets reflect more of the sun’s rays back into
space than clouds with fewer, larger droplets. The clouds act like
mirrors, in addition to shading the ground and atmosphere below
the cloud. Thus there is additional cooling.
This again sounds like a good thing but Professor V. Ramanathan
calls our atmosphere an “insidious soup”, with all the soot,
sulphates, and ash, etc. that we put into it. Global dimming may
have actually caused a famine of monumental proportions, the
1984 Ethiopian famine, a result of a 10-year drought in a region
known as the Sahel. This region is very dry most of the year and
depends on a monsoon season every summer to provide water for
growing crops to sustain the inhabitants for the rest of the year.
The monsoon depends on the heating of the oceans north of the
equator to draw the rain belt that forms at the equator northward
to the Sahel.
Leon Rotstayn and his colleagues applied a climate model to the
situation where pollution from Europe and North America was
allowed to affect the clouds in the northern hemisphere and, sure
enough, the northern hemisphere oceans cooled. In the model,
the cooler northern oceans cause the rain belt to move south, not
north to the Sahel. The model, if correct, confirms that pollution
can affect the climate on a global scale and there is concern
about how global dimming will affect other countries, notably
India, where monsoons play a major role.
So, the pollution causing global dimming is not a good thing.
We’ve taken steps to cut down on pollution, California being one
state that has led the way. We’ve cut down emissions from our
cars, we don’t burn our leaves in the fall, restrictions have been
placed on some industrial pollution, etc. We may even have
turned the corner in lowering the number of particles we
introduce into our air. Hopefully, we’ll achieve substantial
reductions in pollution in the years to come. So, we can relax,
right? Not exactly!
Remember Discover’s number 1 story – global warming? If
global dimming has kept down the rate of global warming, what
happens if we succeed in clearing our air and letting more
sunlight hit the ground? You’re right; it’s going to get hotter.
Even if we cut down on emissions of greenhouse gases along
with the soot and other particles, is it too late? Without global
dimming, will global warming accelerate even beyond what the
experts have been predicting? Time will tell.
How was global dimming discovered? It’s an intriguing story in
itself. Back in the 1960s, Gerald Stanhill a biologist was
involved in designing irrigation schemes for Israel and his job
was to measure the intensity of the sun’s rays over Israel. He
used a network of light meters to accomplish the task. In the
1980s, Stanhill decided to repeat his measurements just to check
on their accuracy. He was shocked to find the sunlight had fallen
by 22 percent! That’s a big deal, but publication of his results
met with silence from the scientific community. But another
fellow by the name of Beate Liepert in Germany also found a
dimming of sunlight, in the Bavarian Alps. Both found the same
thing reported elsewhere. Again the scientific community paid
little or no heed. After all, the world was warming not cooling,
so how could these guys be taken seriously?
Fortunately, Down Under, Michael Roderick and Graham
Farquhar of Australian National University were looking at
something quite simple, yet profound. They were looking at how
rapidly water evaporated from a pan of water. Scientifically, the
name of this area of study is called “pan evaporation rate” and,
unbeknownst to most I suspect, agricultural scientists have been
carrying out pan evaporation rate measurements all over the
world for more than a century. All you do is put out a pan of
water in the morning, come back the next morning and measure
how much water you have to add to the pan to bring the water
level back to what it was the morning before. That’s my kind of
experiment! I could spend the rest of the day playing golf.
Apparently, results from all over the world have been published
so there’s a continuing record over a long period of time. Well,
in the 1990s something odd was noticed. The pan evaporation
rate was going down. This was odd because, after all, the world
was getting warmer and anyone knows that if you heat water it’s
going to evaporate faster than if it’s cooler. However, the
Australian researchers made calculations that showed that pan
evaporation was not just a function of the temperature but that
sunlight, wind and humidity played important roles. In fact, they
found that it was those photons from the Sun hitting the surface
and knocking the water molecules into the atmosphere that was
the most important factor in determining how fast the water
evaporated.
Suspecting that dimming of sunlight was responsible for the
decreased pan evaporation, they found other pan data from
Russia, the U. S. and Europe. When the pan rates were
compared with the sunlight data that Beate Liepert and Gerald
Stanhill had collected, the drops in pan rate and in sunlight
matched perfectly. Publication of the Australians’ work last year
clinched the existence of global dimming. Russia, incidentally,
had the biggest drop in sunlight that I saw reported, 30 percent,
down by almost a third!
I can’t help thinking; did I miss evidence of global dimming
when I made those trips to Denver?
Allen F. Bortrum
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