Global dimming

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.

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 cooling effect that may have partially masked the effect of greenhouse gases on global warming.

Causes and effects

It is thought that global dimming was probably due to the increased presence of aerosol particles in the atmosphere caused by human action. [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
year=2007
accessdate=2008-04-09
format=PDF] Aerosols and other particulates absorb solar energy and reflect sunlight back into space. The pollutants can also become nuclei for cloud droplets. Water droplets in clouds coalesce around the particles. [cite web
url=http://www.atmos-inc.com/weamod.html
title=The Physical Basis for Seeding Clouds
author=
publisher=Atmospherics Inc.
year=1996
accessdate=2008-04-03] 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 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.

Research

In the late-1960s, Mikhail Ivanovich Budyko worked with simple two-dimensional energy-balance climate models to investigate the reflectivity of ice.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
format =
accessdate =
laysummary =
laysource =
laydate = ] 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. [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 =] [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]

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.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
isbn=ISBN 978-0937194164
doi=
url=
pages=(635) pp. 298-301] This was soon followed by others: Viivi Russak in 1990 "Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia",cite journal
quotes =
author = Russak, V.
year = 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 =
laysource =
laydate =
quote = ] and Beate Liepert in 1994 "Solar radiation in Germany — Observed trends and an assessment of their causes".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=] Dimming has also been observed in sites all over the former Soviet Union.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 =
format =
accessdate =
laysummary =
laysource =
laydate = ] Gerry Stanhill who studied these declines worldwide in many papers (see references) coined the term "global dimming".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/
language =
format =
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laysummary =
laysource =
laydate =
quote = ] Independent research in Israel and the Netherlands in the late 1980s showed an apparent reduction in the amount of sunlight,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] 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. citation
chapter-url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter9.pdf
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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, 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] It is difficult to make a precise measurement, due to the difficulty in accurately 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.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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laysummary =
laysource =
laydate = ]

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)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]
* 2%/decade over 1964–93 (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
format =
accessdate =
laysummary =
laysource =
laydate =
doi_brokendate=2008-06-20 ]
* 2.7%/decade (total 20 W/m²); up to 2000 (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 &ndash; ^{[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]} ]
* 4% over 1961–90 (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|format=PDF] cite web
url=http://www.realclimate.org/index.php?p=110
title=RealClimate: Global dimming II
accessdate=2006-06-12]

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. [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 =
accessdate =
laysummary =
laysource =
laydate = ] 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. [ [http://www.guardian.co.uk/life/feature/story/0,13026,1108853,00.html Guardian Unlimited - Science - Goodbye sunshine, Thursday December 18, 2003] ]

Pan evaporation data

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.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
format =
accessdate =
laysummary =
laysource =
laydate =
quote= ] The same trend has been observed in China over a similar period. A decrease in solar irradiance is cited as 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 aerosols may play a critical role in the decrease of solar irradiance in China.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
format = dead link|date=June 2008 &ndash; ^{[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]}
doi_brokendate = 2008-06-20]

BBC Horizon producer David Sington believes that many climate scientists regard the pan-evaporation data as the most convincing evidence of solar dimming.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=] 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 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.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 ] The ambient temperature turns out to be a negligible factor. The pan evaporation data corroborates the data gathered by radiometer and fills in the gaps in the data obtained using pyranometers. With adjustments to these factors, pan evaporation data has been compared to results of climate simulations.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|format=PDF]

Probable causes

of the effect of thin ice clouds at the Earth's surface.cite journal
title=Contrails reduce daily temperature range
first=David J.
last=Travis
author=Carleton, Andrew M. & Lauritsen, Ryan G
journal=Nature
pages=601
volume=418
year=2002
url=http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf
doi=10.1038/418601a|format=PDF] ] 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. [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=]

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 &mdash; a much greater reduction than expected from the presence of the particles themselves. [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 = ] 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 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. [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]

However some research shows that black carbon will actually increase global warming being second only to CO₂. 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. [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]

Some climate scientists have theorized that aircraft contrails (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 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 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.

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, Ruiz (Colombia) 1985, and Pinatubo (Philippines) 1991. But even for major eruptions, the ash clouds remain only for relatively short periods.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=]

Recent reversal of the trend

Wild "et al", using measurements over land, report brightening since 1990, 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] 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. ] and Pinker "et al"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] found that slight dimming continued over land while brightening occurred over the ocean. [cite web
url=http://www.realclimate.org/index.php?p=154
title=Global Dimming may have a brighter future
accessdate=2006-06-12] 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.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]

It is likely that at least some of this change, particularly over Europe, is due to decreases in pollution. Most governments of developed nations have done more to reduce aerosols released into the atmosphere, which helps reduce global dimming, than to reduce CO₂ 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. [cite web
title = Air Emissions Trends - Continued Progress Through 2005
url = http://www.epa.gov/airtrends/econ-emissions.html
format = HTML]

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

Pollution produced by humans may be seriously weakening the Earth's water cycle &mdash; 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." [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]

Large scale changes in weather patterns may also have been caused by global dimming. Climate models speculatively suggest that this reduction in sunshine at the surface may have led to the failure of the monsoon in sub-Saharan Africa during the 1970s and 1980s, together with the associated famines such as the Sahel drought, caused by Northern hemisphere pollution cooling the Atlantic.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 = ] 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. [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]

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 &mdash; and thus dampening the development of hurricanes. [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] [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]

Relationship to 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. 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." [Cite web
url=http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_trans.shtml
title=Global Dimming] 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.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 =
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quote = ] 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. [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 = 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 =
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laysummary =
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quote =] 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." [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
accessdate=2008-04-03]

Possible use to mitigate global warming

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. [cite web
url=http://www.aip.org/history/climate/aerosol.htm
title=Aerosols: Effects of Haze and Cloud
author=Spencer Weart
month=July | year=2006] [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 = ] [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 =
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laydate =
quote = ] According to Ramanathan (1988), an increase in planetary albedo of just 0.5 percent is sufficient to halve the effect of a CO₂ doubling. [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 = ]

However, Earth would still face many problems, such as:
* Using sulfates causes environmental problems such as acid raincite 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 = ]
* Using carbon black causes human health problems
* Dimming causes ecological problems such as changes in evaporation and rainfall patterns
* Droughts and/or increased rainfall cause problems for agriculture
* Aerosol has a relatively short lifetime

"Ideas that we should increase aerosol emissions to counteract global warming have been described as a '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."cite web
url=http://www.realclimate.org/index.php?p=105
title=RealClimate: Global Dimming?
date=2005-01-18
accessdate=2007-04-05] 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. [cite web
url=http://www.globalissues.org/EnvIssues/GlobalWarming/globaldimming.asp
title=Global Dimming
author=Anup Shah
date=January 15, 2005]

ee also

*Climate change
*Global cooling
*Insolation
*Iris hypothesis
*Ship tracks
*Snowball Earth
*Solar variation
*Sunshine recorders

References

External links

Bibliographies
* cite web
url=http://www.greenhouse.crc.org.au/research/c2_bibliog.cfm
title=Global Dimming Bibliography
last=Roderick
first=Michael
* cite web
url=http://www.greenhouse.crc.org.au/crc/research/c2_bibliog.htm
title=Global Dimming Bibliography
last=Saunders
first=Alison

Notable web pages
* cite web
url=http://www.globalissues.org/EnvIssues/GlobalWarming/globaldimming.asp
title=Global Dimming summary
accessdate=
last=
first=
authorlink=
coauthors=
date=
year=
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publisher= BBC Horizon
pages=
archiveurl=
archivedate=
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* 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=
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*cite web
url=http://www.realclimate.org/index.php?p=105
title=Global Dimming - part 1
publisher= Realclimate.org
quote=
*cite web
url=http://www.realclimate.org/index.php?p=110
title=Global Dimming - part 2
publisher= Realclimate.org
quote=
*cite web
url=http://www.realclimate.org/index.php?p=154
title=Global Dimming may have a brighter future
publisher= Realclimate.org
quote=

Podcasts
* cite web
url=http://www.ecoshock.org/downloads/ecoshock/Ecoshock_BrownCloud.mp3
title=Brown Cloud
accessdate=
last=
first=
authorlink=
coauthors=
date=
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format=mp3
work=
publisher= Ecoshock
pages=
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archivedate=
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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=
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News articles
* 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
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
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
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
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=
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
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
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
access=
* cite news
last=Catherine
first=Brahic
url=http://environment.newscientist.com/channel/earth/dn12919-pollution-is-dimming-indias-sunshine.html
title=Pollution is dimming India's sunshine
publisher=New Scientist
date=2007-11-14
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 =
month =January | year =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|format=PDF

Television programs
*cite web
title=Report on another consequence of global warming: the dimming effect of clouds
work=BBC2 TV Horizon
date=2005-01-15
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
date=2006-04-18
url=http://www.pbs.org/wgbh/nova/sun/
*cite web
title=BBC Horizon - Global Dimming - Google Video
work=BBC Horizon
date=
url=http://video.google.com/videoplay?docid=39520879762623193