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Science Forum Index » Environment Forum » Global dimming
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| Steve Schulin |
 Posted: Thu Dec 18, 2003 12:28 pm |
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The Guardian newspaper has a couple of articles by David Adam today that
some climate aficionados here will surely enjoy:
* David Adam, "Goodbye sunshine", The Guardian (London), December 18,
2003, Science Pages, p. 4 --
http://www.guardian.co.uk/life/feature/story/0,13026,1108853,00.html
* David Adam, "Earth is 20% darker, say experts", The Guardian (London),
December 18, 2003, Home Pages, p. 8 --
http://www.guardian.co.uk/international/story/0,3604,1109125,00.html
One of the references cited is also freely available on the web:
* B Liepert, "Observed Reductions of Surface Solar Radiation at Sites in
the US and Worldwide", Geophysical Research Letters Volume 29 (2002),
pages 1421-1433 -- http://www.ldeo.columbia.edu/~liepert/liepert.PDF
Amongst the highlights:
* "Data from 100 stations around the world show that the amount of black
carbon in the atmosphere is twice as big as we assumed," said Dr Hansen
[Jim Hansen, NASA/GISS].
* It turns out that Ohmura was the first to document a dramatic effect
that scientists are now calling "global dimming". Records show that over
the past 50 years the average amount of sunlight reaching the ground has
gone down by almost 3% a decade. It's too small an effect to see with
the naked eye, but it has implications for everything from climate
change to solar power and even the future sustainability of plant
photosynthesis. In fact, global dimming seems to be so important that
you're probably wondering why you've never heard of it before. Well
don't worry, you're in good company. Many climate experts haven't heard
of it either, the media has not picked up on it, and it doesn't even
appear in the reports of the Intergovernmental Panel on Climate Change
(IPCC). "It's an extraordinary thing that for some reason this hasn't
penetrated even into the thinking of the people looking at global
climate change," says Graham Farquhar, a climate scientist at the
Australian National University in Canberra. "It's actually quite a big
deal and I think you'll see a lot more people referring to it."
* Tiny particles of soot or chemical compounds like sulphates reflect
sunlight and they also promote the formation of bigger, longer lasting
clouds. "The cloudy times are getting darker," says Cohen [Shabtai
Cohen, co-author of "Global Dimming: A Review of the Evidence",
Agricultural and Forest Meteorology 107:255-278 (2001)], at the Volcani
Centre [in Bet Dagan, Israel]. "If it's cloudy then it's darker, but
when it's sunny things haven't changed much."
* ... in some cases photosynthesis could paradoxically increase slightly
with global dimming as the broken, diffuse light that emerges from
clouds can penetrate deep into forest canopies more easily than direct
beams of sunlight from a clear blue sky.
* The other major impact global dimming will have is on the complex
computer simulations climate scientists use to understand what is
happening now and to predict what will happen in the future. For them,
global dimming is a real sticking point. "All of their models, all the
physics and mathematics of solar radiation in the Earth's atmosphere
can't explain what we're measuring at the Earth's surface," Stanhill
says. Farquhar agrees: "This will drive what the modellers have to do
now. They're going to have to account for this."
* It's also possible that global dimming is not entirely down to air
pollution. "I don't think that aerosols by themselves would be able to
produce this amount of global dimming," says Farquhar. Global warming
itself might also be playing a role, he suggests, by perhaps forcing
more water to be evaporated from the oceans and then blown onshore
(although the evidence on land suggests otherwise). "If the greenhouse
effect causes global dimming then that really changes the perspective,"
he says. In other words, while it keeps getting warmer it might keep
getting darker. "I'm not saying it definitely is that, I'm just raising
the question."
* "The IPCC is the group that should investigate this and work out if
people should be scared of it," says Cohen. Whatever their verdict, at
least we are no longer totally in the dark about global dimming. |
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| James Acker |
| Posted: Thu Dec 18, 2003 12:28 pm |
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I guess "my future's so bright, I gotta wear shades"
doesn't apply here.
Jim Acker
*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
Jim Acker
jacker1@gl.umbc.edu
"Since we are assured that an all-wise Creator has observed the
most exact proportions, of number, weight, and measure, in the
make of all things, the most likely way therefore, to get any
insight into the nature of those parts of the creation, which
come within our observation, must in all reason be to number,
weigh, and measure." - Stephen Hales |
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| James Acker |
| Posted: Thu Dec 18, 2003 12:28 pm |
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wmc@bas.ac.uk wrote:
: Steve Schulin <steve.schulin@nuclear.com> wrote:
:>* David Adam, "Goodbye sunshine", The Guardian (London), December 18,
:>2003, Science Pages, p. 4 --
:>http://www.guardian.co.uk/life/feature/story/0,13026,1108853,00.html
:>* B Liepert, "Observed Reductions of Surface Solar Radiation at Sites in
:>the US and Worldwide", Geophysical Research Letters Volume 29 (2002),
:>pages 1421-1433 -- http://www.ldeo.columbia.edu/~liepert/liepert.PDF
: Note that this says "only" 4% 1960-90.
:>* It turns out that Ohmura was the first to document a dramatic effect
:>that scientists are now calling "global dimming". Records show that over
:>the past 50 years the average amount of sunlight reaching the ground has
:>gone down by almost 3% a decade.
: "Ohmura's results suggested that levels of solar radiation striking
: the Earth's surface had declined by more than 10% in three decades."
: The problem, of course, is how to make sense of it. As the article points
: out, solar radiation has been near constant over the period, and it hints
: that UV hasn't been much affected. And a trend of 3% per decade in
: radiation would make such a huge difference to climate that it would
: be easily detected.
: The article doesn't offer much in the way of real suggestions for
: whats up. One obvious possibility is pollution affecting measurements
: near cities and most of the globe being little affected.
I did a Google search on the phrase "global dimming" after
reading and responding. One of the links to it that came up was
to the study of the "Asian brown cloud" -- atmospheric aerosols/black
soot stemming from various sources of burning on the Indian
subcontinent. The brown cloud covers a large area of land in the
region and a significant area of adjacent ocean.
Another area where there's a lot of aerosol is the U.S.
East Coast. Besides pollution, smoke can cover a surprisingly
large area. A couple of years ago, people in the D.C./Baltimore
region called the news stations with mild alarm about smoke from a
large fire drifting through the area.
It turned out to be smoke from a large fire -- in Quebec.
So I tend to think that this observation may be due to a
significant recent increase in near-surface aerosols, soot, and haze.
Exhibit 1:
http://seawifs.gsfc.nasa.gov/SEAWIFS/IMAGES/NEW/China/S2000002043056.L1A_HKUS.labelled.jpg
Exhibit 2:
http://visibleearth.nasa.gov/cgi-bin/viewrecord?26154
Exhibit 3 (of the event I was describing above):
http://visibleearth.nasa.gov/cgi-bin/viewrecord?17558
In this last image, also note the contrail patterns over central
and eastern Virginia, as well as offshore the mouth of the
Chesapeake Bay.
Jim Acker
*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
Jim Acker
jacker1@gl.umbc.edu
"Since we are assured that an all-wise Creator has observed the
most exact proportions, of number, weight, and measure, in the
make of all things, the most likely way therefore, to get any
insight into the nature of those parts of the creation, which
come within our observation, must in all reason be to number,
weigh, and measure." - Stephen Hales |
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| Guest |
| Posted: Thu Dec 18, 2003 3:35 pm |
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Steve Schulin <steve.schulin@nuclear.com> wrote:
Quote: * David Adam, "Goodbye sunshine", The Guardian (London), December 18,
2003, Science Pages, p. 4 --
http://www.guardian.co.uk/life/feature/story/0,13026,1108853,00.html
* B Liepert, "Observed Reductions of Surface Solar Radiation at Sites in
the US and Worldwide", Geophysical Research Letters Volume 29 (2002),
pages 1421-1433 -- http://www.ldeo.columbia.edu/~liepert/liepert.PDF
Note that this says "only" 4% 1960-90.
Quote: * It turns out that Ohmura was the first to document a dramatic effect
that scientists are now calling "global dimming". Records show that over
the past 50 years the average amount of sunlight reaching the ground has
gone down by almost 3% a decade.
"Ohmura's results suggested that levels of solar radiation striking
the Earth's surface had declined by more than 10% in three decades."
The problem, of course, is how to make sense of it. As the article points
out, solar radiation has been near constant over the period, and it hints
that UV hasn't been much affected. And a trend of 3% per decade in
radiation would make such a huge difference to climate that it would
be easily detected.
The article doesn't offer much in the way of real suggestions for
whats up. One obvious possibility is pollution affecting measurements
near cities and most of the globe being little affected.
-W.
--
William M Connolley | wmc@bas.ac.uk | http://www.antarctica.ac.uk/met/wmc/
Climate Modeller, British Antarctic Survey | Disclaimer: I speak for myself
I'm a .signature virus! copy me into your .signature file & help me spread! |
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| Guest |
| Posted: Thu Dec 18, 2003 5:55 pm |
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James Acker <jacker1@linux3.gl.umbc.edu> wrote:
Quote: wmc@bas.ac.uk wrote:
: Steve Schulin <steve.schulin@nuclear.com> wrote:
:>* B Liepert, "Observed Reductions of Surface Solar Radiation at Sites in
:>the US and Worldwide", Geophysical Research Letters Volume 29 (2002),
:>pages 1421-1433 -- http://www.ldeo.columbia.edu/~liepert/liepert.PDF
: The article doesn't offer much in the way of real suggestions for
: whats up. One obvious possibility is pollution affecting measurements
: near cities and most of the globe being little affected.
So I tend to think that this observation may be due to a
significant recent increase in near-surface aerosols, soot, and haze.
<snip>
The Liepert pdf is worth a look. It includes a map of trends at sites
globally. Well at least you can see how sparse they are.
--
William M Connolley | wmc@bas.ac.uk | http://www.antarctica.ac.uk/met/wmc/
Climate Modeller, British Antarctic Survey | Disclaimer: I speak for myself
I'm a .signature virus! copy me into your .signature file & help me spread! |
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| Vendicar Decarian |
| Posted: Sun Dec 21, 2003 11:57 am |
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Guest
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"James Acker" <jacker1@linux3.gl.umbc.edu> wrote in message
news:brt9cb$f76$1@news.umbc.edu...
Quote: I did a Google search on the phrase "global dimming" after
reading and responding. One of the links to it that came up was
to the study of the "Asian brown cloud" -- atmospheric aerosols/black
soot stemming from various sources of burning on the Indian
subcontinent. The brown cloud covers a large area of land in the
region and a significant area of adjacent ocean.
Global Dimming: A Review of the Evidence, G Stanhill and S Cohen
Agricultural and Forest Meteorology Volume 107 (2001), pages 255-278
The Cause of Decreased Pan Evaporation Over the Past 50 Years, M Roderick
and G Farquhar Science Volume 298 (2002), pages 1410-1411
Observed Reductions of Surface Solar Radiation at Sites in the US and
Worldwide, B Liepert Geophysical Research Letters Volume 29 (2002), pages
1421-1433 |
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