Global dimming
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224591280
2008-07-09T15:40:56Z
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{{Pollution}}
'''Global dimming''' is the gradual reduction in the amount of global direct [[irradiance]] at the [[Earth]]'s surface that was observed for several decades after the start of systematic measurements in 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 1960–1990. However, since 1990, the trend has reversed.<ref name=AR4_9_2_2_2/>
It is thought to have been caused by an increase in [[particulates]] such as sulphate aerosols in the atmosphere due to human action. The switch from a "global dimming" trend to a "brightening" trend in 1990 happened just as global aerosol levels started to decline.
Global dimming has interfered with the [[hydrological cycle]] by reducing evaporation and may have reduced rainfall in some areas. Global dimming also creates a [[global cooling|cooling]] effect that may have partially masked the effect of [[greenhouse gas]]es on [[global warming]].
==Causes and effects==
{{See|Albedo|irradiance|insolation}}
It is thought that global dimming was probably due to the increased presence of [[Particulate|aerosol]] particles in the [[Earth's atmosphere|atmosphere]] caused by human action.<ref>{{cite web
|url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter7.pdf
|title=Couplings between changes in Climate System and the Biogeochemistry, 7.5.3
|author=Keneth L. Denman and Guy Brasseur, et al.
|publisher=IPCC
|date=2007
|language=English
|accessdate=2008-04-09
}}</ref> Aerosols and other particulates absorb solar energy and reflect sunlight back into space. The pollutants can also become [[Cloud condensation nuclei|nuclei]] for cloud droplets. Water droplets in [[cloud]]s [[Coalescence|coalesce]] around the particles.<ref>{{cite web
|url=http://www.atmos-inc.com/weamod.html
|title=The Physical Basis for Seeding Clouds
|author=
|publisher=Atmospherics Inc.
|date=1996
|language=English
|accessdate=2008-04-03
}}</ref> Increased pollution causes more particulates and thereby creates clouds consisting of a greater number of smaller droplets (that is, the same amount of water is spread over more droplets). The smaller droplets make clouds more [[albedo|reflective]], so that more incoming sunlight is reflected back into space and less reaches the earth's surface.
Clouds intercept both heat from the sun and heat radiated from the Earth. Their effects are complex and vary in time, location, and altitude. Usually during the daytime the interception of sunlight predominates, giving a cooling effect; however, at night the re-radiation of heat to the Earth slows the Earth's heat loss.
{{Clear}}<!--- do not put anything past here. It will mess up the formatting -->
==Research==
{{See|Climate model|pyranometer}}
[[Image:Pollution over east China.jpg|thumb|left|270px|Eastern [[China]]. Dozens of fires burning on the surface (red dots) and a thick pall of smoke and haze (greyish pixels) filling the skies overhead. Photo taken by MODIS aboard [[NASA]]'s Aqua satellite]]
In the late-1960s, [[Mikhail Ivanovich Budyko]] worked with simple two-dimensional energy-balance climate models to investigate the reflectivity of ice.<ref name="Budyko 1969">
{{cite journal
|author=Budyko, M.I.
|title=The effect of solar radiation variations on the climate of the Earth
|journal=[[Tellus]]
|volume = 21
|issue=
|year=1969
|pages=pp. 611–619
| issn =
| doi =
| id =
| url = http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A7021919AH&q=&uid=790417110&setcookie=yes
| language = English
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}}</ref> He found that the ice-albedo feedback created a positive feedback loop in the Earth's climate system. The more snow and ice, the more solar radiation is reflected back into space and hence the colder Earth grows and the more it snows. Other studies found that pollution or a volcano eruption could snap Earth into an ice age.<ref>
{{cite journal
| quotes =
| author = Rasool, Ichtiaque, S. and Schneider, Stephen H.
| date =
| year = 1971
| month = July
| title = Atmospheric Carbon Dioxide and Aerosols: Effects of Large Increases on Global Climate
| journal = [[Science]]
| volume = 173
| issue = 3992
| pages = 138–141
| issn =
| pmid = 17739641
| doi = 10.1126/science.173.3992.138
| id =
| url = http://www.sciencemag.org/cgi/content/abstract/173/3992/138
| language =
| format =
| accessdate =
| laysummary =
| laysource =
| laydate =}}</ref><ref>
{{cite book
| last = Lockwood
| first = John G.
| title = Causes of Climate
| publisher = New York: [[John Wiley & Sons]]
| series = Lecture notes in mathematics 1358
| year = 1979
| pages = p. 162}}</ref>
In the mid-1980s, [[Atsumu Ohmura]], a geography researcher at the [[Swiss Federal Institute of Technology]], found that solar radiation striking the Earth's surface had declined by more than 10% over the three previous decades. His findings are in apparent contradiction to [[global warming]]—the global temperature has steadily been going up. Less light reaching the earth would mean that it would have to cool. Ohmura published his findings "Secular variation of global radiation in Europe" in 1989.<ref name="Ohmura and Lang 1989">
{{cite book
|author=Ohmura, A. and Lang, H.
|editor=Lenoble, J. and Geleyn, J.-F. (Eds)
|title=Secular variation of global radiation in Europe. In IRS '88: Current Problems in Atmospheric Radiation, A. Deepak Publ., Hampton, VA
|year=1989
|month=June
|publisher=Deepak Publ.
|location=, Hampton, VA
|language=English
|isbn=ISBN 978-0937194164
|doi=
|url=
|pages=(635) pp. 298-301}}</ref> This was soon followed by others. Viivi Russak in 1990 "Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia",<ref name="Russak 1990">
{{cite journal
| quotes =
| author = Russak, V.
| date = 1990
| year =
| month =
| title = Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia
| journal =[[Tellus]] B
| volume =42
| issue = 2
| pages = p.206
| issn =
| doi = 10.1034/j.1600-0889.1990.t01-1-00006.x
| id = 1990TellB..42..206R
| url = http://adsabs.harvard.edu/abs/1990TellB..42..206R
| language =
| format =
| accessdate =
| laysummary =
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| laydate =
| quote =
}}</ref> and Beate Liepert in 1994 "Solar radiation in Germany — Observed trends and an assessment of their causes".<ref name="Liepert et al 2004">
{{cite journal
|author=Liepert, B. G., P. Fabian, et al.
|year= 1994
|title= Solar radiation in Germany - Observed trends and an assessment of their causes. Part 1. Regional approach
|journal=[[Contr. Atm. Physics]]
|volume=67
|pages=15–29
|doi=
|url=}}</ref> Dimming has also been observed in sites all over the Former Soviet Union.<ref name="Abakumova 1996">
{{cite journal
| author = Abakumova, G.M. et al.
| date =
| year = 1996
| month =
| title = Evaluation of long-term changes in radiation, cloudiness and surface temperature on the territory of the former Soviet Union
| journal = [[Journal of Climate]]
| volume = 9
| issue = 6
| pages = pp. 1319–1327
| issn =
| doi =
| id =
| url = http://ams.allenpress.com/archive/1520-0442/9/6/pdf/i1520-0442-9-6-1319.pdf
| language =
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}}</ref> Gerry Stanhill who studied these declines worldwide in many papers (see references) coined the term "Global dimming".<ref name="Stanhill and Moreshet 1992">
{{cite journal
| quotes =
| author = Stanhill, G. and Moreshet, S.
| date = 2004-11-06
| year =
| month =
| title = Global radiation climate changes in Israel
| journal = [[Climatic Change]]
| volume = 22
| issue =
| pages = pp. 121–138
| issn =
| doi = 10.1007/BF00142962
| id =
| url = http://www.springerlink.com/content/ut2520p2471wk486/
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}}
</ref>
{{Clear}}
[[Image:Lightmatter Golden gate bridge.jpg|thumb|right|300px|[[Golden Gate Bridge]] with [[California]]'s characteristic brown cloud in the background — a likely contributor to global dimming.]]
Independent research in [[Israel]] and the [[Netherlands]] in the late 1980s showed an apparent reduction in the amount of sunlight,<ref name=LightensUp>
{{cite web
|title=Earth lightens up
|work=Pacific Northwest National Laboratory
|url=http://www.pnl.gov/news/release.asp?id=20
|accessmonthday=May 8
|accessyear=2005}}
</ref> despite widespread evidence that the climate was actually becoming hotter. The rate of dimming varies around the world but is on average estimated at around 2–3% per decade. The trend reversed in the early 1990s. <!-- excessive citation - i know but its as close to the recommended citation template in the report, as we can come
*** This citation is used elsewhere in the article - so if editing please ensure that the citation still works *** --><ref name=AR4_9_2_2_2>{{citation
| chapter-url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter9.pdf
| first1=G. C. | last1=Hegerl
| first2=F. W. | last2=Zwiers
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| first5=Y. | last5=Luo
| first6=J.A. | last6=Marengo Orsini
| first7=N. | last7=Nicholls
| first8=J.E. | last8=Penner
| first9=P.A. | last9=Stott
| chapter=Chapter 9, Understanding and Attributing Climate Change – Section 9.2.2 Spatial and Temporal Patterns of the Response to Different Forcings and their Uncertainties
| editor1-last=Solomon
| editor2-first=D. | editor2-last=Qin
| editor3-first=M. | editor3-last=Manning
| editor4-first=Z. | editor4-last=Chen
| editor5-first=M. | editor5-last=Marquis
| editor6-first=K.B. | editor6-last=Averyt
| editor7-first=M. | editor7-last=Tignor
| editor8-first=H.L. | editor8-last=Miller
| publisher=[[Cambridge University Press]]
| place=[[Cambridge, United Kingdom]] and [[New York City|New York, NY, USA]].
| title=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
| url=http://www.ipcc.ch/ipccreports/ar4-wg1.htm
| accessdate=2008-04-13
| year=2007
| series=[[Intergovernmental Panel on Climate Change]]
| quote=See 9.2.2.2
}}</ref>
It is difficult to make a precise measurement, due to the difficulty in accurately [[Calibration|calibrating]] the instruments used, and the problem of spatial coverage. Nonetheless, the effect is almost certainly present.
The effect (2–3%, as above) is due to changes within the Earth's atmosphere; the value of the solar radiation at the top of the atmosphere has not changed by more than a fraction of this amount.<ref name="Eddy et al 1982">
{{cite journal
| quotes = Spacecraft measurements have established that the total radiative output of the Sun varies at the 0.1−0.3% level
| author = Eddy, John A. Gilliland, Ronald L. & Hoyt, Douglas V.
| date = 1982-12-23
| year = 1982
| month = December
| title = Changes in the solar constant and climatic effects
| journal = [[Nature]]
| volume = 300
| issue =
| pages = pp. 689–693
| issn =
| doi = 10.1038/300689a0
| id =
| url = http://www.nature.com/nature/journal/v300/n5894/abs/300689a0.html
| language =
| format =
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}}</ref>
The effect varies greatly over the planet, but estimates of the terrestrial surface average value are:
* 5.3% (9 W/m²); over 1958–85 (Stanhill and Moreshet, 1992)<ref name="Stanhill and Moreshet 1992">
{{cite journal
|author=Stanhill, G. and Moreshet, s.
|title=Global radiation climate change in Israel
|journal=[[Climate Change]]
|volume = 22
|issue=
|year=1992
|pages=pp. 121–138
|doi=10.1007/BF00142962}}</ref>
* 2%/decade over 1964–93 (Gilgen ''et al'', 1998)<ref name="Gilgen et al 1998">
{{cite journal
|author=H. Gilgen, M. Wild, and A. Ohmura
|title=Means and trends of shortwave irradiance at the surface estimated from global energy balance archive data
|journal=[[Journal of Climate]]
|volume = 11
|issue= 8
|year=1998
|pages=pp. 2042–2061
|doi=10.1175/1520-0442(1998)011<2042:MATOSI>2.0.CO;2
|url=http://ams.allenpress.com/archive/1520-0442/11/8/pdf/i1520-0442-11-8-2042.pdf
|language = English
|format =
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|laysummary =
|laysource =
|laydate =
|doi_brokendate=2008-06-20 }}</ref>
* 2.7%/decade (total 20 W/m²); up to 2000 (Stanhill and Cohen, 2001)<ref name="Stanhill and Cohen 2001">
{{cite journal
|author=Stanhill, G. and S. Cohen
|title=Global dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences | journal=[[Agricultural and Forest Meteorology]]
|volume = 107
|issue=
|year=2001
|url=http://vortex.suiri.tsukuba.ac.jp/~asanuma/.papers/PanEvaporation/Stanhill_2001_AFM.pdf
|pages=pp. 255–278
|doi=10.1016/S0168-1923(00)00241-0
|format={{dead link|date=June 2008}} – <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3A+intitle%3AGlobal+dimming%3A+a+review+of+the+evidence+for+a+widespread+and+significant+reduction+in+global+radiation+with+discussion+of+its+probable+causes+and+possible+agricultural+consequences&as_publication=%5B%5BAgricultural+and+Forest+Meteorology%5D%5D&as_ylo=2001&as_yhi=2001&btnG=Search Scholar search]</sup>}}</ref>
* 4% over 1961–90 (Liepert 2002)<ref name="Liepert 2002">
{{cite journal
|author=Liepert, B. G.
|title=Observed Reductions in Surface Solar Radiation in the United States and Worldwide from 1961 to 1990
|journal=[[Geophysical Research Letters]]
|volume = 29/12
|issue=2002-05-02
|year=2002
|url=http://www.ldeo.columbia.edu/~liepert/pdf/2003GL019060.pdf
|pages=pp. 1421
|doi=10.1029/2002GL014910}}</ref><ref name=gbd2>
{{cite web
|url=http://www.realclimate.org/index.php?p=110
|title=RealClimate: Global dimming II
|accessdate=2006-06-12}}</ref>
Note that these numbers are for the terrestrial surface and not really a global average. Whether dimming (or brightening) occurred over the ocean has been a bit of an unknown though a specific measurement (see below, Causes) measured effects some 400 miles (643.7 km) from India over the Indian Ocean towards the Maldives Islands. Regional effects probably dominate but are not strictly confined to the land area, and the effects will be driven by regional air circulation.
The largest reductions are found in the [[Northern Hemisphere]] mid-latitudes.<ref>
{{cite journal
| quotes =
| author = R. E. Carnell, C. A. Senior
| date = 1998-04
| year = 1998
| month = April
| title = Changes in mid-latitude variability due to increasing greenhouse gases and sulphate aerosols
| journal = [[Climate Dynamics]] Springer Berlin / Heidelberg
| volume = 14
| issue = 5
| pages = pp. 369–383
| issn = (Print) 1432-0894 (Online)
| doi = 10.1007/s003820050229
| id =
| url = http://www.springerlink.com/content/rreh3q3kkf79f65x/
| language =
| format =
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}}
</ref> The region of the [[spectrum]] of light radiation most affected seems to be the [[visible]] and [[infrared]] rather than the [[ultraviolet]] part of the spectrum.<ref>[http://www.guardian.co.uk/life/feature/story/0,13026,1108853,00.html Guardian Unlimited - Science - Goodbye sunshine, Thursday December 18, 2003]</ref>
==Pan evaporation data==
{{See|Pan evaporation}}
Over the last 50 or so years, [[pan evaporation]] has been carefully monitored. For decades, nobody took much notice of the pan evaporation measurements. But in the 1990s in Europe, Israel, and North America, scientists spotted something that at the time was considered very strange, the rate of evaporation was falling although they had expected it to increase due to [[global warming]].<ref name="Roderick and Farquhar 2002">
{{cite journal
| quotes =
| author = Roderick, Michael L. and Farquhar, Graham D.
| date=
| year = 2002
| month =
| title = The Cause of Decreased Pan Evaporation over the Past 50 Years
| journal = [[Science]]
| volume = 298
| issue = 5597
| pages = pp. 1410–1411
| issn =
| pmid = 12434057
| doi = 10.1126/science.1075390
| id =
| url = http://www.sciencemag.org/cgi/content/abstract/298/5597/1410
| language = English
| format =
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|quote= }}</ref> The same trend has been observed in China over a similar period. A decrease in solar irradiance is cited to be the driving force. However, unlike in other areas of the world, in China the decrease in solar irradiance was not always accompanied by an increase in cloud cover and precipitation. It is believed that [[Particulate|aerosols]] may play a critical role in the decrease of solar irradiance in China.<ref name ="Liu et al 2004">
{{cite journal
|author=Liu B., Xu M., Henderson M. & Gong W.
|year = 2004
|title = A spatial analysis of pan evaporation trends in China, 1955-2000
|journal = [[Journal of Geophysical Research]]
|volume = 109
|issue = D15102
|doi = 15110.11029/12004JD004511
|url = http://vortex.suiri.tsukuba.ac.jp/~asanuma/.papers/PanEvaporation/Liu_2004_JGR.pdf
|language = English
|format = {{dead link|date=June 2008}} – <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3A+intitle%3AA+spatial+analysis+of+pan+evaporation+trends+in+China%2C+1955-2000&as_publication=%5B%5BJournal+of+Geophysical+Research%5D%5D&as_ylo=2004&as_yhi=2004&btnG=Search Scholar search]</sup>
|doi_brokendate = 2008-06-20}}</ref>
[[BBC Horizon]] producer [[David Sington]] believes that many climate scientists regard the pan-evaporation data as the most convincing evidence of solar dimming.<ref name="Sington 2005">
{{cite news
|last=Sington
|first=David
|url=http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_qa.shtml
|title=TV&Radio follow-up
|publisher=[[BBC]] - Science & Nature - Horizon
|date=[[January 15]], [[2005]]
|accessdate=}}</ref> Pan evaporation experiments are easy to reproduce with low-cost equipment, there are many pans used for agriculture all over the world and in many instances, and the data has been collected for nearly a half century. However, pan evaporation depends on some additional factors besides net radiation from the sun. The other two major factors are vapor pressure deficit and wind speed.<ref name="pe attribution">{{cite journal
| last =Roderick
| first =Michael L.
| coauthors =Leon D. Rotstayn, Graham D. Farquhar, Michael T. Hobbins
| title =On the attribution of changing pan evaporation
| journal =[[Geophysical Research Letters]]
| volume =34
| issue =L17403
| date =[[2007-09-13]]
| url =http://www.agu.org/pubs/crossref/2007/2007GL031166.shtml
| doi =10.1029/2007GL031166
| pages =L17403 }}</ref> The ambient temperature turns out to be a negligible factor. The pan evaporation data corroborates the data gathered by [[radiometer]]<ref name="Stanhill and Cohen 2001"/><ref name="Roderick and Farquhar 2002"/> and fills in the gaps in the data obtained using [[pyranometer]]s. With adjustments to these factors, pan evaporation data has been compared to results of climate simulations.<ref name="Rotstayn et al 2006">
{{cite journal
|author=Rotstayn L.D., Roderick M.L. & Farquhar G.D.
|title=A simple pan-evaporation model for analysis of climate simulations: Evaluation over Australia
|journal=[[Geophysical Research Letters]]
|volume=33
|issue=L17715
|year=2006
|url=http://www.rsbs.anu.edu.au/Profiles/Graham_Farquhar/documents/235doiRotstaynpanGRL2006.pdf
|doi=10.1029/2006GL027114
|pages=L17403}}</ref>
==Probable causes==
{{see|Particulate|Black carbon|Contrail|Volcanic ash}}
[[Image:Sfc.contrail.1.26.01.JPG|thumb|300px|right|[[NASA]] photograph showing aircraft [[contrails]] and natural clouds. The temporary disappearance of contrails over North America due to plane groundings after the [[September 11, 2001 attacks]], and the resulting increase in [[Day|diurnal]] [[temperature range]] gave [[empirical evidence]] of the effect of thin ice clouds at the Earth's surface.<ref name=contrails>
{{cite journal
| title=Contrails reduce daily temperature range
| first=David J.
| last=Travis
| author=Carleton, Andrew M. & Lauritsen, Ryan G
| journal=[[Nature (journal)|Nature]]
| pages=601
| volume=418
| year=2002
| url=http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf
| doi=10.1038/418601a}}</ref>]]
The incomplete combustion of fossil fuels (such as [[diesel]]) and wood releases [[black carbon]] into the air. Though black carbon, most of which is [[soot]], is an extremely small component of air pollution at land surface levels, the phenomenon has a significant heating effect on the atmosphere at altitudes above two kilometers (6,562 ft). Also, it dims the surface of the ocean by absorbing solar radiation.<ref>{{cite news
|last=
|first=
|url=http://www.sciencedaily.com/releases/2007/03/070314134655.htm
|title=Transported Black Carbon A Significant Player In Pacific Ocean Climate
|publisher=[[Science Daily]]
|date=2007-03-15
|access=}}</ref>
Experiments in the [[Maldives]] (comparing the atmosphere over the northern and southern islands) in the 1990s showed that the effect of macroscopic pollutants in the atmosphere at that time (blown south from [[India]]) caused about a 10% reduction in sunlight reaching the surface in the area under the [[pollution]] cloud — a much greater reduction than expected from the presence of the particles themselves.<ref>
{{cite journal
| quotes =
| author = J. Srinivasan et al.
| date =
| year = 2002
| month =
| title = Asian Brown Cloud – fact and fantasy
| journal = [[Current Science]]
| volume = 83
| issue = 5
| pages = pp. 586–592
| issn =
| doi =
| id =
| url = http://www.ias.ac.in/currsci/sep102002/586.pdf
| language =
| format =
| accessdate =
| laysummary =
| laysource =
| laydate =
}}</ref>
Prior to the research being undertaken, predictions were of a 0.5–1% effect from [[particulate]] matter; the variation from prediction may be explained by cloud formation with the particles acting as the focus for droplet creation.
Clouds are very effective at [[Reflection (physics)|reflecting]] light back out into space.
The phenomenon underlying global dimming may also have regional effects. While most of the earth has warmed, the regions that are downwind from major sources of air pollution (specifically sulfur dioxide emissions) have generally cooled. This may explain the cooling of the eastern United States relative to the warming western part.<ref>
{{cite web
| url=http://www.ucsusa.org/global_warming/science/crichton-thriller-state-of-fear.html
| title=Crichton's Thriller State of Fear: Separating Fact from Fiction
| accessdate=2006-06-12
}}</ref>
However some research shows that black carbon will actually increase global warming being second only to CO<sub>2</sub>. They believe that soot will absorb solar energy and transport it to other areas such as the Himalayas where glacial melt occurs. It can also darken Arctic ice reducing reflectivity and increasing absorption of solar radiation.<ref>
{{cite web
| url=http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo156.html
| title=Nature Geoscience: Global and regional climate changes due to black carbon
| accessdate=2008-03-26
}}</ref>
Some climate scientists have theorized that aircraft [[contrail]]s (also called ''vapor trails'') are implicated in global dimming, but the constant flow of air traffic previously meant that this could not be tested. The near-total shutdown of [[Civil aviation|civil air traffic]] during the three days following the [[September 11, 2001 attacks]] afforded a rare opportunity in which to observe the climate of the [[United States]] absent from the effect of contrails. During this period, an increase in [[Day|diurnal]] temperature variation of over 1 [[°C]] (1.8 [[°F]]) was observed in some parts of the U.S., i.e. aircraft contrails may have been raising nighttime temperatures and/or lowering daytime temperatures by much more than previously thought.<ref name=contrails />
Airborne [[volcanic ash]] can reflect the Sun's rays back out into space and cool the planet. Dips in earth temperatures have been observed from large volcano eruptions such as [[Mount Agung]] in Bali that erupted in 1963, El Chichon (Mexico) 1983, [[Nevado del Ruiz|Ruiz]] (Colombia) 1985, and [[Mount Pinatubo|Pinatubo]] (Philippines) 1991. But even for major eruptions, the ash clouds remain only for relatively short periods.<ref name=NASA>
{{cite news
|last=
|first=
|url=http://www.nasa.gov/centers/goddard/news/topstory/2007/aerosol_dimming.html
|title=Global 'Sunscreen' Has Likely Thinned, Report NASA Scientists
|publisher=[[NASA]]
|date=2007-03-15
|access=}}</ref>
==Recent reversal of the trend==
{{see|Clean Air Act}}
[[Image:Aerosol dimming.jpg|thumb|500px|right|Sun-blocking aerosols around the world steadily declined (red line) since the 1991 eruption of Mount Pinatubo, according to satellite estimates. Credit: Michael Mishchenko, NASA]]
Wild ''et al'', using measurements over land, report brightening since 1990,<ref name=LightensUp /><ref name="Wild et al 2005">
{{cite journal
| author=Wild, M et al.
| title=From Dimming to Brightening: Decadal Changes in Solar Radiation at Earth’s Surface
| journal=[[Science]]
| volume = 308
| issue=2005-05-06
| year=2005
| url=http://www.sciencemag.org/cgi/content/abstract/308/5723/847
| doi=10.1126/science.1103215
| pages=pp. 847–850
| pmid=15879214}}
</ref><ref name="Wild et al 2007">
{{cite journal
| author=Wild, M., A. Ohmura, and K. Makowski
| title=Impact of global dimming and brightening on global warming
| journal=[[Geophysical Research Letters]]
| volume=34
| issue=L04702
| year=2007
| url=http://www.agu.org/pubs/crossref/2007/2006GL028031.shtml
| doi=10.1029/2006GL028031
| pages=pp. }}
</ref> and Pinker ''et al''<ref name="Pinker et al 2005">
{{cite journal
| author=Pinker, et al.
| title=Do Satellites Detect Trends in Surface Solar Radiation?
| journal=[[Science]]
| volume = 308
| issue=6 May 2005
| year=2005
| url=http://www.sciencemag.org/cgi/content/abstract/308/5723/850
| doi=10.1126/science.1103159
| pages=pp. 850–854
| pmid=15879215}}
</ref> found that slight dimming continued over land while brightening occurred over the ocean.<ref>
{{cite web
| url=http://www.realclimate.org/index.php?p=154
| title=Global Dimming may have a brighter future
| accessdate=2006-06-12}}</ref> Hence, over the land surface, Wild ''et al'' and Pinker ''et al'' disagree. A 2007 [[NASA]] sponsored satellite-based study sheds light on the puzzling observations by other scientists that the amount of sunlight reaching Earth's surface had been steadily declining in recent decades, suddenly started to rebound around 1990. This switch from a "global dimming" trend to a "brightening" trend happened just as global aerosol levels started to decline.<ref name=NASA/><ref name="Kerr 2007">{{cite journal
|url=http://www.sciencemag.org/cgi/content/summary/315/5818/1480
|title=CLIMATE CHANGE: Is a Thinning Haze Unveiling the Real Global Warming?
|author=Richard A. Kerr
|pub=[[Science]]
|date=2007-03-16
|pmid=17363636
|doi=10.1126/science.315.5818.1480
|journal=Science
|volume=315
|pages=1480}}</ref>
It is likely that at least some of this change, particularly over Europe, is due to decreases in pollution. Most governments of [[developed nation]]s have done more to reduce aerosols released into the atmosphere, which helps reduce global dimming, than to reduce CO<sub>2</sub> emissions.
Sulfate aerosols have declined significantly since 1970 with the [[Clean Air Act]] in the United States and similar policies in Europe. The Clean Air Act was strengthened in 1977 and 1990. According to the EPA, from 1970 to 2005, total emissions of the six principal air pollutants, including PM’s, dropped by 53% in the US. In 1975, the masked effects of trapped greenhouse gases finally started to emerge and have dominated ever since.<ref>
{{cite web
| title = Air Emissions Trends - Continued Progress Through 2005
| url = http://www.epa.gov/airtrends/econ-emissions.html
| language = English
| format = HTML
}}</ref>
The [[Baseline Surface Radiation Network]] (BSRN) has been collecting surface measurements. BSRN was started in the early 1990s and updated the archives in this time. Analysis of recent data reveals that the surface of the planet has brightened by about 4% in the past decade. The brightening trend is corroborated by other data, including satellite analyses.
==Relationship to hydrological cycle==
[[Image:Climate Change Attribution.png|thumb|right|230px|This figure shows the level of agreement between a [[climate model]] driven by five factors and the [[historical temperature record]]. The negative component identified as "sulfate" is associated with the aerosol emissions blamed for global dimming.]]
{{See|Hydrological cycle}}
Pollution produced by humans may be seriously weakening the Earth's [[water cycle]] — reducing rainfall and threatening fresh water supplies. A 2001 study by researchers at the Scripps Institution of Oceanography suggests that tiny particles of soot and other pollutants have a significant effect on the hydrological cycle. According to Professor V. Ramanathan: "The energy for the hydrological cycle comes from sunlight. As sunlight heats the ocean, water escapes into the atmosphere and falls out as rain. So as aerosols cut down sunlight by large amounts, they may be spinning down the hydrological cycle of the planet."<ref>
{{cite web
|url=http://ens-newswire.com/ens/dec2001/2001-12-07-07.asp
|title=Aerosol Pollution Could Drain Earth's Water Cycle
|author=Cat Lazaroff
|pub=[[Environment News Service]]
|date=2007-12-07}}</ref>
Large scale changes in weather patterns may also have been caused by global dimming. [[Climate model]]s speculatively suggest that this reduction in sunshine at the surface may have led to the failure of the [[monsoon]] in sub-[[Sahara]]n [[Africa]] during the 1970s and 1980s, together with the associated [[famine]]s such as the [[Sahel drought]], caused by Northern hemisphere pollution cooling the [[Atlantic]].<ref name="Rotstayn and Lohmann 2002">
{{cite journal
| quotes =
| author = Rotstayn and Lohmann
| date =
| year = 2002
| month =
| title = Tropical Rainfall Trends and the Indirect Aerosol Effect
| journal = [[Journal of Climate]]
| volume =
| issue =
| pages = pp. 2103–2116
| issn =
| doi =
| id =
| url = http://ams.allenpress.com/perlserv/?request=get-abstract&issn=1520-0442&volume=015&issue=15&page=2103
| language =
| format =
| accessdate =
| laysummary =
| laysource =
| laydate =
}}</ref>
Because of this, the [[Tropical rain belt]] may not have risen to its northern latitudes, thus causing an absence of seasonal rains. This claim is not universally accepted and is very difficult to test.
It is also concluded that the imbalance between global dimming and global warming at the surface leads to weaker turbulent heat fluxes to the atmosphere. This means globally reduced evaporation and hence precipitation occur in a dimmer and warmer world, which could ultimately lead to a more humid atmosphere in which it rains less.<ref>
{{cite news
| url=http://www.ldeo.columbia.edu/news/2006/04_14_06.htm
| title=Could Reducing Global Dimming Mean a Hotter, Dryer World?
| publisher=Lamont-Doherty Earth Observatory News
| author=Kostel, Ken and Oh, Clare
| date=2006-04-14
| year=2006
| accessdate=2006-06-12}}</ref>
A natural form of large scale environmental shading/dimming has been identified that affected the 2006 northern hemisphere [[hurricane season]]. The [[NASA]] study found that several major dust storms in June and July in the [[Sahara]] desert sent dust drifting over the [[Atlantic Ocean]] and through several effects caused cooling of the waters — and thus dampening the development of hurricanes.<ref>
{{cite news
|url=http://www.upi.com/NewsTrack/Science/2007/04/03/study_ties_hurricanes_to_sahara/
|title=Study ties hurricanes to Sahara
|publisher=[[United Press International]]
|date=2007-04-03
}}</ref><ref>
{{cite news
|url=http://www.nasa.gov/mission_pages/hurricanes/archives/2007/hurricane_dust.html
|title=Did Dust Bust the 2006 Hurricane Season Forecasts?
|publisher=NASA
|date=2007-03-28
}}</ref>
{{Clear}}
==Relationship to global warming==
{{See|Global warming}}
Some scientists now consider that the effects of global dimming have masked the effect of [[global warming]] to some extent and that resolving global dimming may therefore lead to increases in predictions of future temperature rise.<ref name=gbd2 /> According to Beate Liepert, "We lived in a global warming plus a global dimming world and now we are taking out global dimming. So we end up with the global warming world, which will be much worse than we thought it will be, much hotter."<ref>
{{Cite web
|url=http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_trans.shtml
|title=Global Dimming
}}</ref> The magnitude of this masking effect is one of the central problems in climate change with significant implications for future climate changes and policy responses to global warming.<ref name="Andreae et al 2005">
{{cite journal
| quotes =
| author = Andreae O. M., Jones C. D., Cox P. M.
| date = 2005-06-30
| year = 2005
| month = June
| title = Strong present-day aerosol cooling implies a hot future
| journal = Nature
| volume = 435
| issue =
| pages = pp. 1187–1190
| issn =
| doi =
| id =
| url = http://www.nature.com/nature/journal/v435/n7046/abs/nature03671.html
| language =
| format =
| accessdate =
| laysummary =
| laysource =
| laydate =
| quote =
}}</ref>
But it's much more complicated than an either warming or dimming issue. Global warming and global dimming are not mutually exclusive or contradictory. In a paper published on [[March 8]], [[2005]] in the American Geophysical Union's Geophysical Research Letters, a research team led by Anastasia Romanou of Columbia University's Department of Applied Physics and Mathematics, New York, also showed that the apparently opposing forces of global warming and global dimming can occur at the same time.<ref>
{{cite journal
| quotes =
| author = Alpert, P., P. Kishcha, Y. J. Kaufman, and R. Schwarzbard
| date =
| year = 2005
| month =
| title = Global dimming or local dimming?: Effect of urbanization on sunlight availability
| journal = [[Geophysical Research Letters|Geophys. Res. Lett.]]
| volume = 32
| issue = L17802
| pages = L17802
| issn =
| doi = 10.1029/2005GL023320
| id =
| url = http://www.agu.org/pubs/crossref/2005/2005GL023320.shtml
| language =
| format =
| accessdate =
| laysummary =
| laysource =
| laydate =
| quote =
}}</ref> Global dimming interacts with global warming by blocking sunlight that would otherwise cause evaporation and the particulates bind to water droplets. Water vapor is one of the greenhouse gases. On the other hand, global dimming is affected by evaporation and rain. Rain has the effect of clearing out polluted skies.
Brown clouds have been found to amplify global warming according to V. Ramanathan, an atmospheric chemist at the Scripps Institution of Oceanography in La Jolla, CA. "The conventional thinking is that brown clouds have masked as much as 50 percent of global warming by greenhouse gases through so-called global dimming," ... "While this is true globally, this study reveals that over southern and eastern Asia, the soot particles in the brown clouds are in fact amplifying the atmospheric warming trend caused by greenhouse gases by as much as 50 percent."<ref>{{cite web
|url=http://www.nsf.gov/news/news_summ.jsp?cntn_id=109712
|title=Press Release 07-087 "Brown Cloud" Particulate Pollution Amplifies Global Warming
|publisher=[[NSF]]
|date=2007-08-01
|language=English
|accessdate=2008-04-03
}}</ref>
==Possible use to mitigate global warming==
{{See|Mitigation of global warming|Albedo}}
Some scientists have suggested using aerosols to stave off the effects of global warming as an emergency measure. Russian expert [[Mikhail Budyko]] understood this relationship very early on. In 1974, he suggested that if global warming became a problem, the planet could be cooled by burning sulfur in the stratosphere, which would create a haze.<ref>
{{cite web
|url=http://www.aip.org/history/climate/aerosol.htm
|title=Aerosols: Effects of Haze and Cloud
|author=Spencer Weart
|date=July 2006}}</ref><ref>
{{cite journal
| quotes =
| author = Crutzen, P.
| date =
| year = 2006
| month = August
| title = Albedo enhancement by stratospheric sulfur injections: a contribution to resolve a policy dilemma?
| journal = [[Climatic Change]]
| volume = 77
| issue = 3-4
| pages = pp. 211–220
| issn =
| doi = 10.1007/s10584-006-9101-y
| id =
| url = http://www.heartland.org/pdf/19632.pdf
| language =
| format =
| accessdate =
| laysummary =
| laysource =
| laydate =
| quote =
}}</ref><ref>
{{cite journal
| quotes =
| author = Harshvardhan
| date = 06/1978
| year = 1978
| month = June
| title = Albedo enhancement and perturbation of radiation balance due to stratospheric aerosols
| journal =
| volume =
| issue =
| pages =
| issn =
| doi =
| id = 1978aepr.rept.....H
| url = http://adsabs.harvard.edu/abs/1978aepr.rept.....H
| language =
| format =
| accessdate =
| laysummary =
| laysource =
| laydate =
| quote =
}}</ref> According to Ramanathan (1988), an increase in planetary [[albedo]] of just 0.5 percent is sufficient to halve the effect of a CO<sub>2</sub> doubling.<ref>
{{cite journal
| quotes =
| author = Ramanathan, V.
| date = 1988-04-15
| year = 1988
| month = June
| title = The greenhouse theory of climate change: a test by an inadvertent global experiment
| journal = Science
| volume = 240
| issue = 4850
| pages = pp. 293–299
| issn =
| doi =
| id =
| url = http://adsabs.harvard.edu/abs/1988Sci...240..293R
| language = (English)
| format =
| accessdate =
| laysummary =
| laysource =
| laydate =
| quote =
}}</ref>
However, Earth would still face many problems, such as:
* Using sulfates causes environmental problems such as acid rain<ref name="Ramanathan V. 2006">
{{cite journal
| quotes =
| author = Ramanathan, V.
| date =
| year = 2006
| month =
| title = Atmospheric Brown Clouds: Health, Climate and Agriculture Impacts
| journal = Pontifical Academy of Sciences Scripta Varia (Pontifica Academia Scientiarvm)
| volume = 106
| issue = Interactions Between Global Change and Human Health
| pages = pp. 47–60
| issn =
| doi =
| id =
| url = http://www-ramanathan.ucsd.edu/PASScriptaVaria106.pdf
| language =
| format =
| accessdate =
| laysummary =
| laysource =
| laydate =
| quote =
}}</ref>
* Using carbon black causes human health problems<ref name="Ramanathan V. 2006"/>
* Dimming causes ecological problems such as changes in evaporation and rainfall patterns<ref name="Ramanathan V. 2006"/>
* Droughts and/or increased rainfall cause problems for agriculture<ref name="Ramanathan V. 2006"/>
* Aerosol has a relatively short lifetime
"Ideas that we should increase aerosol emissions to counteract global warming have been described as a '[[Deal with the Devil|Faustian bargain]]' because that would imply an ever increasing amount of emissions in order to match the accumulated greenhouse gas in the atmosphere, with ever increasing monetary and health costs."<ref name=gbd1>
{{cite web
|url=http://www.realclimate.org/index.php?p=105
|title=RealClimate: Global Dimming?
|date=2005-01-18
|accessdate=2007-04-05}}</ref> Climatologists are stressing that the roots of both global dimming-causing pollutants and global warming-causing greenhouse gases have to be dealt with together and soon.<ref>
{{cite web
|url=http://www.globalissues.org/EnvIssues/GlobalWarming/globaldimming.asp
|title=Global Dimming
|author=Anup Shah
|date=[[January 15]], [[2005]]
}}</ref>
==See also==
<!-- Please maintain ALPHABETIZATION of wikilinks -->
{{portal|energy}}
*[[Climate change]]
*[[Insolation]]
*[[Iris hypothesis]]
*[[Global cooling]]
*[[Snowball Earth]]
*[[Solar variation]]
*[[Sunshine recorders]]
*[[Ship tracks]]
==References==
{{reflist|2}}
==External links==
{{wikinews|PBS show asserts greenhouse gases, atmospheric pollutants dimming future}}
'''Bibliographies'''
<!-- ORDERED ALPHABETICALLY BY LAST NAME, FIRST --->
* {{cite web
|url=http://www.greenhouse.crc.org.au/research/c2_bibliog.cfm
|title=Global Dimming Bibliography
|last=Roderick
|first=Michael
|language=English}}
* {{cite web
|url=http://www.greenhouse.crc.org.au/crc/research/c2_bibliog.htm
|title=Global Dimming Bibliography
|last=Saunders
|first=Alison
|language=English}}
'''Notable web pages'''
<!-- Ordered Alphabetically by URL --->
* {{cite web
|url=http://www.globalissues.org/EnvIssues/GlobalWarming/globaldimming.asp
|title=Global Dimming summary
|accessdate=
|last=
|first=
|authorlink=
|coauthors=
|date=
|year=
|month=
|format=
|work=
|publisher= BBC Horizon
|pages=
|language=English
|archiveurl=
|archivedate=
|quote= }}
* {{cite web
|url=http://www.ldeo.columbia.edu/~liepert/media/flash/globalDimming.swf
|title=Global Dimming (requires flash)
|accessdate=
|last=Liepert
|first=Beate
|authorlink=
|coauthors=
|date=
|year=
|month=
|format=
|work=
|publisher=
|pages=
|language=English
|archiveurl=
|archivedate=
|quote= }}
*{{cite web
|url=http://www.realclimate.org/index.php?p=105
|title=Global Dimming - part 1
|publisher= Realclimate.org
|language=English
|quote= }}
*{{cite web
|url=http://www.realclimate.org/index.php?p=110
|title=Global Dimming - part 2
|publisher= Realclimate.org
|language=English
|quote= }}
*{{cite web
|url=http://www.realclimate.org/index.php?p=154
|title=Global Dimming may have a brighter future
|publisher= Realclimate.org
|language=English
|quote= }}
*[http://www.celsias.com/blog/2007/05/09/veiling-our-true-predicament-part-i-global-dimming/ Veiling Our True Predicament: Global Dimming] - Easy to Understand Primer on Global Dimming, also including BBC documentary on the subject.
'''Podcasts'''
* {{cite web
|url=http://www.ecoshock.org/downloads/ecoshock/Ecoshock_BrownCloud.mp3
|title=Brown Cloud
|accessdate=
|last=
|first=
|authorlink=
|coauthors=
|date=
|year=
|month=
|format=mp3
|work=
|publisher= Ecoshock
|pages=
|language=English
|archiveurl=
|archivedate=
|quote= }}
'''Q&A'''
* {{cite web
|url=http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_qa.shtml
|title=BBC Global Dimming Q&A
|accessdate=
|last=
|first=
|authorlink=
|coauthors=
|date=
|year=
|month=
|format=
|work=
|publisher=
|pages=
|language=English
|archiveurl=
|archivedate=
|quote= }}
'''News articles'''
<!-- ORDERED CHRONOLOGICALLY -->
* {{cite news
|last= Adam
|first= David
|url=http://www.guardian.co.uk/life/feature/story/0,13026,1108853,00.html
|title=Goodbye Sunshine
|publisher=[[The Guardian]]
|date=2003-12-18
|language=English
|access=}}
* {{cite news
|last=Chang
|first=Kenneth
|url=http://www.commondreams.org/headlines04/0513-01.htm
|title= Globe Grows Darker as Sunshine Diminishes 10% to 37%
|publisher=[[The New York Times]]
|date=2004-05-13
|language=English
|access=}}
* {{cite news
|last=Appell
|first=David
|url=http://www.sciam.com/article.cfm?articleID=000C3AAE-D82A-10F9-975883414B7F0000
|title= The Darkening Earth Less sun at the Earth's surface complicates climate models
|publisher=[[Scientific American]]
|date=2004-08-02
|language=English
|access=}}
* {{cite news
|last=Keen
|first=Kip
|url=http://www.grist.org/news/maindish/2004/09/22/keen-dimming
| title=Dim Sun Global dimming? Global warming? What's with the globe, anyway?
|publisher=[[Grist Magazine]]
|date=2004-09-22
|language=English
|access=}}
* {{cite news
|url=http://news.bbc.co.uk/1/hi/sci/tech/4171591.stm
|title=Why the Sun seems to be 'dimming'
|accessdate=
|last= Sington
|first= David
|authorlink=
|coauthors=
|date= 2005-01-13
|year= 2005
|month=
|format=
|work=
|publisher= [[BBC News]]
|pages=
|language=English
|archiveurl=
|archivedate=
|quote= }}
* {{cite news
|last=Onion
|first=Amanda
|url=http://abcnews.go.com/Technology/GlobalWarming/story?id=1566139
|title=Are Skies Dimming Over Earth? Data Suggest Human Pollution Can Lead to Darker Days
|publisher=[[ABC News]]
|date=2006-02-09
|language=English
|access=}}
* {{cite news
|last=
|first=
|url=http://www.sciencedaily.com/releases/2007/03/070314134655.htm
|title=Transported Black Carbon A Significant Player In Pacific Ocean Climate
|publisher=[[Science Daily]]
|date=2007-03-15
|language=English
|access=}}
* {{cite news
|last=
|first=
|url=http://www.nasa.gov/centers/goddard/news/topstory/2007/aerosol_dimming.html
|title=Global 'Sunscreen' Has Likely Thinned, Report NASA Scientists
|publisher=[[NASA]]
|date=2007-03-15
|language=English
|access=}}
* {{cite journal
| last =Seinfeld
| first =John
| title =Atmospheric science: Black carbon and brown clouds
| journal =[[Nature Geoscience]]
| volume =1
| issue =1
| pages =15–6
| publisher =
| date =January 2008
| url =http://www.nature.com/ngeo/journal/v1/n1/full/ngeo.2007.62.html
| doi =10.1038/ngeo.2007.62
| accessdate = }}
'''Slide decks'''
* {{cite web
|url=http://www.neespi.org/web-content/meetings/IGBPbrief/Briefing_Sokolik.pdf
|title=Atmospheric Aerosol and Air Pollution
|Author=Irina N. Sokolik
|Publisher=School of Earth and Atmospheric Sciences Georgia Institute of Technology Atlanta, GA, USA}}
'''Television programs'''
<!-- ORDERED CHRONOLOGICALLY -->
*{{cite web
|title=Report on another consequence of global warming: the dimming effect of clouds
|work=BBC2 TV [[Horizon (BBC TV series)|Horizon]]
|date=2005-01-15
|language=English
|url=http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_prog_summary.shtml}}
*{{cite web
|title=Dimming The Sun
|work=PBS WGBH Boston [[NOVA (TV series)|NOVA]]
|date=2006-04-18
|language=English
|url=http://www.pbs.org/wgbh/nova/sun/ }}
*{{cite web
|title=BBC Horizon - Global Dimming - Google Video
|work=BBC Horizon
|date=
|language=english
|url=http://video.google.com/videoplay?docid=39520879762623193 }}
<BR>
{{Global warming}}
[[Category:Climate change feedbacks and causes]]
[[ca:Enfosquiment global]]
[[cs:Globální stmívání]]
[[de:Globale Verdunkelung]]
[[es:Oscurecimiento global]]
[[fr:Assombrissement global]]
[[gl:Escurecemento global]]
[[it:Oscuramento globale]]
[[he:עמעום עולמי]]
[[hu:Globális homály]]
[[nl:Global dimming]]
[[ja:地球薄暮化]]
[[no:Global dimming]]
[[nn:Global dimming]]
[[pl:Globalne zaciemnienie]]
[[pt:Escurecimento global]]
[[ro:Întunecarea globală]]
[[ru:Глобальное затемнение]]
[[fi:Ilmakehän hämärtyminen]]
[[sv:Global fördunkling]]
[[zh:全球黯化]]