Global dimming

Global dimming

Overview
Global dimming is the gradual reduction in the amount of global direct irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...

 at the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's surface that was observed for several decades after the start of systematic measurements in the 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, after discounting an anomaly caused by the eruption of Mount Pinatubo
Mount Pinatubo
Mount Pinatubo is an active stratovolcano located on the island of Luzon, near the tripoint of the Philippine provinces of Zambales, Tarlac, and Pampanga. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is west of the dormant and...

 in 1991, a very slight reversal in the overall trend has been observed.

Global dimming is thought to have been caused by an increase in particulates
Particulates
Particulates – also known as particulate matter , suspended particulate matter , fine particles, and soot – are tiny subdivisions of solid matter suspended in a gas or liquid. In contrast, aerosol refers to particles and/or liquid droplets and the gas together. Sources of particulate matter can be...

 such as sulfate aerosols in the atmosphere due to human action.

It has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas.
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Encyclopedia
Global dimming is the gradual reduction in the amount of global direct irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...

 at the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's surface that was observed for several decades after the start of systematic measurements in the 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, after discounting an anomaly caused by the eruption of Mount Pinatubo
Mount Pinatubo
Mount Pinatubo is an active stratovolcano located on the island of Luzon, near the tripoint of the Philippine provinces of Zambales, Tarlac, and Pampanga. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is west of the dormant and...

 in 1991, a very slight reversal in the overall trend has been observed.

Global dimming is thought to have been caused by an increase in particulates
Particulates
Particulates – also known as particulate matter , suspended particulate matter , fine particles, and soot – are tiny subdivisions of solid matter suspended in a gas or liquid. In contrast, aerosol refers to particles and/or liquid droplets and the gas together. Sources of particulate matter can be...

 such as sulfate aerosols in the atmosphere due to human action.

It has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. Global dimming also creates a cooling
Global cooling
Global cooling was a conjecture during the 1970s of imminent cooling of the Earth's surface and atmosphere along with a posited commencement of glaciation...

 effect that may have partially masked the effect of greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

es on global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

.

Deliberate manipulation of this dimming effect is now being considered as a geoengineering
Geoengineering
The concept of Geoengineering refers to the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry The Intergovernmental Panel on Climate Change concluded in 2007 that geoengineering options, such...

 technique to reduce the impact of global warming.

Causes and effects


It is thought that global dimming is probably due to the increased presence of aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

 particles
Particulates
Particulates – also known as particulate matter , suspended particulate matter , fine particles, and soot – are tiny subdivisions of solid matter suspended in a gas or liquid. In contrast, aerosol refers to particles and/or liquid droplets and the gas together. Sources of particulate matter can be...

 in the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 caused by human action. Aerosols and other particulates absorb solar energy and reflect sunlight back into space. The pollutants can also become nuclei
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...

 for cloud droplets. Water droplets in cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

s coalesce around the particles. 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
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

, so that more incoming sunlight is reflected back into space and less reaches the Earth's surface. In models, these smaller droplets also decrease rainfall.

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. 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 provoke the onset of an ice age.

In the mid-1980s, Atsumu Ohmura
Atsumu Ohmura
is a Japanese climatologist, known for his contributions to the theory of global dimming.Ohmura was born in the Bunkyō ward of Tokyo in 1942. In 1965 he graduated with a B.Sc from the University of Tokyo and in 1969 received an M.Sc from McGill University...

, a geography researcher at the Swiss Federal Institute of Technology
ETH Zurich
The Swiss Federal Institute of Technology Zurich or ETH Zürich is an engineering, science, technology, mathematics and management university in the City of Zurich, Switzerland....

, found that solar radiation striking the Earth's surface had declined by more than 10% over the three previous decades. His findings appeared to contradict global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

—the global temperature had been generally rising since the 70s. Less light reaching the earth seemed to mean that it should cool. Ohmura published his findings "Secular variation of global radiation in Europe" in 1989. This was soon followed by others: Viivi Russak in 1990 "Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia", and Beate Liepert in 1994 "Solar radiation in Germany — Observed trends and an assessment of their causes". Dimming has also been observed in sites all over the former Soviet Union. Gerry Stanhill who studied these declines worldwide in many papers (see references) coined the term "global dimming".

Independent research in Israel
Israel
The State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...

 and the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

 in the late 1980s showed an apparent reduction in the amount of sunlight, 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.
It is difficult to make a precise measurement, due to the difficulty in accurately calibrating
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....

 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.

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)
  • 2%/decade over 1964–93 (Gilgen et al., 1998)
  • 2.7%/decade (total 20 W/m²); up to 2000 (Stanhill and Cohen, 2001)
  • 4% over 1961–90 (Liepert 2002)


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. A 2009 review by Wild et al. found that widespread variation in regional and time effects. There was solar brightening beyond 2000 at numerous stations in Europe, the United States, and Korea. The brightening seen at sites in Antarctica during the 1990s, influenced by recovering from the Mount Pinatubo volcanic eruption in 1991, fades after 2000. The brightening tendency also seems to level off at sites in Japan. In China there is some indication for a renewed dimming, after the stabilization in the 1990s. A continuation of the long-lasting dimming is also noted at the sites in India. Overall, the available data suggest continuation of the brightening beyond the year 2000 at numerous locations, yet less pronounced and coherent than during the 1990s, with more regions with no clear changes or declines. Therefore, globally, greenhouse warming after 2000 may be less modulated by surface solar variation
Solar variation
Solar variation is the change in the amount of radiation emitted by the Sun and in its spectral distribution over years to millennia. These variations have periodic components, the main one being the approximately 11-year solar cycle . The changes also have aperiodic fluctuations...

s than in prior decades. The largest reductions are found in the northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

 mid-latitudes. Visible light and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 radiation seem to be most affected rather than the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 part of the spectrum.

Pan evaporation data



Over the last 50 or so years, pan evaporation
Pan evaporation
Pan evaporation is a measurement that combines or integrates the effects of several climate elements: temperature, humidity, rain fall, drought dispersion, solar radiation, and wind. Evaporation is greatest on hot, windy, dry days; and is greatly reduced when air is cool, calm, and humid...

 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
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

. 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.

BBC Horizon producer David Sington
David Sington
David Sington is a British-born producer, screenwriter, director, author and journalist. He studied natural science at Trinity College, Cambridge, graduating in 1981. In 1983, had the opportunity to go to film school but instead, he started his career at the BBC World Service as a journalist. In...

 believes that many climate scientists regard the pan-evaporation data as the most convincing evidence of solar dimming. 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. The ambient temperature turns out to be a negligible factor. The pan evaporation data corroborates the data gathered by radiometer
Radiometer
A radiometer is a device for measuring the radiant flux of electromagnetic radiation. Generally, the term radiometer denotes an infrared radiation detector, yet it also includes detectors operating on any electromagnetic wavelength....

 and fills in the gaps in the data obtained using pyranometer
Pyranometer
A pyranometer is a type of actinometer used to measure broadband solar irradiance on a planar surface and is a sensor that is designed to measure the solar radiation flux density from a field of view of 180 degrees...

s. With adjustments to these factors, pan evaporation data has been compared to results of climate simulations.

Probable causes


The incomplete combustion of fossil fuels (such as diesel) and wood releases black carbon
Black carbon
In Climatology, black carbon or BC is a climate forcing agent formed through the incomplete combustion of fossil fuels, biofuel, and biomass, and is emitted in both anthropogenic and naturally occurring soot. It consists of pure carbon in several linked forms...

 into the air. Though black carbon, most of which is soot
Soot
Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...

, 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.

Experiments in the Maldives
Maldives
The Maldives , , officially Republic of Maldives , also referred to as the Maldive Islands, is an island nation in the Indian Ocean formed by a double chain of twenty-six atolls oriented north-south off India's Lakshadweep islands, between Minicoy Island and...

 (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
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

) caused about a 10% reduction in sunlight reaching the surface in the area under the pollution
Pollution
Pollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...

 cloud — a much greater reduction than expected from the presence of the particles themselves.
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
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

 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.

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 melting occurs. It can also darken Arctic ice reducing reflectivity and increasing absorption of solar radiation.

Some climate scientists have theorized that aircraft contrail
Contrail
Contrails or vapour trails are artificial clouds that are the visible trails of condensed water vapour made by the exhaust of aircraft engines...

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 air traffic
Civil aviation
Civil aviation is one of two major categories of flying, representing all non-military aviation, both private and commercial. Most of the countries in the world are members of the International Civil Aviation Organization and work together to establish common standards and recommended practices...

 during the three days following the September 11, 2001 attacks
September 11, 2001 attacks
The September 11 attacks The September 11 attacks The September 11 attacks (also referred to as September 11, September 11th or 9/119/11 is pronounced "nine eleven". The slash is not part of the pronunciation...

 afforded a unique opportunity in which to observe the climate of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 absent from the effect of contrails. During this period, an increase in diurnal
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...

 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
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...

 can reflect the Sun's rays back into space and thereby contribute to cooling the planet. Dips in earth temperatures have been observed after large volcano eruptions such as Mount Agung
Mount Agung
Mount Agung or Gunung Agung is a mountain in Bali, Indonesia. This stratovolcano is the highest point on the island. It dominates the surrounding area influencing the climate...

 in Bali that erupted in 1963, El Chichon (Mexico) 1983, Ruiz
Nevado del Ruiz
The Nevado del Ruiz, also known as La Mesa de Herveo or Kumanday in the language of the local pre-Columbian indigenous people, is a volcano located on the border of the departments of Caldas and Tolima in Colombia, about west of the capital city Bogotá. It is a stratovolcano, composed of many...

 (Colombia) 1985, and Pinatubo
Mount Pinatubo
Mount Pinatubo is an active stratovolcano located on the island of Luzon, near the tripoint of the Philippine provinces of Zambales, Tarlac, and Pampanga. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is west of the dormant and...

 (Philippines) 1991. But even for major eruptions, the ash clouds remain only for relatively short periods.

Recent reversal of the trend


Wild et al., using measurements over land, report brightening since 1990, and Pinker et al. found that slight dimming continued over land while brightening occurred over the ocean. Hence, over the land surface, Wild et al. and Pinker et al. disagree. A 2007 NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 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, began to reverse around 1990. This switch from a "global dimming" trend to a "brightening" trend happened just as global aerosol levels started to decline.

It is likely that at least some of this change, particularly over Europe, is due to decreases in airborne pollution. Most governments of developed nations have taken steps to reduce aerosols released into the atmosphere, which helps reduce global dimming.

Sulfate aerosols have declined significantly since 1970 with the Clean Air Act
Clean Air Act
A Clean Air Act is one of a number of pieces of legislation relating to the reduction of airborne contaminants, smog and air pollution in general. The use by governments to enforce clean air standards has contributed to an improvement in human health and longer life spans...

 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.

The Baseline Surface Radiation Network
Baseline Surface Radiation Network
Baseline Surface Radiation Network is a project of the World Climate Research Programme and the Global Energy and Water Cycle Experiment and as such is aimed detecting important changes in the Earth's radiation field at the Earth's surface which may be related to climate changes...

(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
Water cycle
The water cycle, also known as the hydrologic cycle or H2O cycle, describes the continuous movement of water on, above and below the surface of the Earth. Water can change states among liquid, vapor, and solid at various places in the 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 Veerabhadran Ramanathan
Veerabhadran Ramanathan
Veerabhadran Ramanathan is Victor Alderson Professor of Applied Ocean Sciences and director of the Center for Atmospheric Sciences at the Scripps Institution of Oceanography, University of California, San Diego. He has contributed to many areas of the atmospheric sciences including developments to...

, "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."

Large scale changes in weather patterns may also have been caused by global dimming. Climate modelers speculatively suggest that this reduction in solar radiation at the surface may have led to the failure of the monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...

 in sub-Sahara
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...

n Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

 during the 1970s and 1980s, together with the associated famine
Famine
A famine is a widespread scarcity of food, caused by several factors including crop failure, overpopulation, or government policies. This phenomenon is usually accompanied or followed by regional malnutrition, starvation, epidemic, and increased mortality. Every continent in the world has...

s such as the Sahel drought
Sahel drought
[[File:Greening Sahel 1982-1999.jpg|thumb|300px|Recent "Greening" of the Sahel: The results of trend analyses of time series over the Sahel region of seasonally integrated NDVI using NOAA AVHRR NDVI-data from 1982 to 1999...

, caused by Northern hemisphere pollution cooling the Atlantic.
Because of this, the Tropical rain belt
Tropical rain belt
The rainfall, and hence the tropical climate is dominated by the tropical rain belt, which oscillates from the northern to the southern tropics over the course of the year, roughly following the solar equator; it is largely a manifestation of the ITCZ...

 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. However a 2009 Chinese study of 50 years of continuous data found that though most parts of eastern China saw no significant change in the amount of water held by the atmosphere but light rains had decreased. In addition, where the atmosphere transported water vapor didn't coincide with light rain frequency. The researchers then modeled the effect of aerosols and also concluded the overall effect was that water drops in polluted cases are up to 50 percent smaller than in pristine skies. They concluded smaller size impedes the formation of rain clouds and the falling of light rain beneficial for agriculture; a different effect than reducing solar irradiance, but still a direct result from the presence of aerosols.

The 2001 study by researchers at the Scripps Institution of Oceanography 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.

A natural form of large scale environmental shading/dimming has been identified that affected the 2006 northern hemisphere hurricane season. The NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 study found that several major dust storms in June and July in the Sahara
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...

 desert sent dust drifting over the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

 and through several effects caused cooling of the waters — and thus dampening the development of hurricanes.

Relationship to global warming


Some scientists now consider that the effects of global dimming have masked the effect of global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

 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." 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.

Interactions between the two theories for climate modification have also been studied, as global warming and global dimming are neither mutually exclusive nor 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. Global dimming interacts with global warming by blocking sunlight that would otherwise cause evaporation and the particulates bind to water droplets. Water vapor is the major greenhouse gas. 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 Veerabhadran 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."

Possible use to mitigate global warming


Some scientists have suggested using aerosols to stave off the effects of global warming as an emergency geoengineering measure. In 1974, Mikhail Budyko
Mikhail Budyko
Mikhail Ivanovich Budyko was a Russian climatologist and one of the founders of physical climatology. He pioneered studies on global climate and calculated temperature of Earth considering simple physical model of equilibrium in which the incoming solar radiation absorbed by the Earth's system is...

 suggested that if global warming became a problem, the planet could be cooled by burning sulfur in the stratosphere, which would create a haze. An increase in planetary albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 of just 0.5 percent is sufficient to halve the effect of a CO2 doubling.

The simplest solution would be to simply emit more sulfates, which would end up in troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 - the lowest part of the atmosphere. If this were done, Earth would still face many problems, such as:
  • Using sulfates causes environmental problems such as acid rain
    Acid rain
    Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...

  • Using carbon black
    Carbon black
    Carbon black is a material produced by the incomplete combustion of heavy petroleum products such as FCC tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of amorphous carbon that has a high surface-area-to-volume ratio, although its...

     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


The solution actually advocated is transporting sulfates into the next higher layer of the atmosphere - stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

. Aerosols in the stratosphere last years instead of weeks - so only a relatively smaller (though still large) amount of sulfate emissions would be necessary, and side effects would be less. This would require developing an efficient way to transport large amounts of gases into stratosphere, many of which have been proposed http://climate.envsci.rutgers.edu/pdf/GRLreview2.pdf though none are known to be effective or economically viable.

In a blog post, Gavin Schmidt
Gavin Schmidt
Gavin A. Schmidt is a climatologist and climate modeler at the NASA Goddard Institute for Space Studies in New York. He works on the variability of the ocean circulation and climate, using general circulation models . He has also worked on ways to reconcile paleo-data with models...

 stated that "Ideas that we should increase aerosol emissions to counteract global warming have been described as a 'Faustian bargain
Deal with the Devil
Deal With The Devil is the fifth studio album by the American heavy metal band Lizzy Borden released in 2000 .A return to form, featuring a cover by Todd McFarlane.2 covers were recorded...

' 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."

See also


  • Asian brown cloud
    Asian brown cloud
    The Asian brown cloud is a layer of air pollution that covers parts of South Asia, namely the northern Indian Ocean, India, and Pakistan. Viewed from satellite photos, the cloud appears as a giant brown stain hanging in the air over much of South Asia and the Indian Ocean every year between January...

  • Climate change
    Climate change
    Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

  • Global cooling
    Global cooling
    Global cooling was a conjecture during the 1970s of imminent cooling of the Earth's surface and atmosphere along with a posited commencement of glaciation...

  • Iris hypothesis
    Iris hypothesis
    The iris hypothesis is a hypothesis proposed by Professor Richard Lindzen in 2001 that suggested increased sea surface temperature in the tropics would result in reduced cirrus clouds and thus more infrared radiation leakage from Earth's atmosphere...

  • Ship tracks
    Ship tracks
    Ship tracks are clouds that form around the exhaust released by ships into the still ocean air. Water molecules collect around the tiny particles from exhaust to form a cloud seed. More and more water accumulates on the seed until a visible cloud is formed...

  • Snowball Earth
    Snowball Earth
    The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...

  • Sunshine recorders
    Sunshine recorders
    A sunshine recorder is a device that records the amount of sunshine at a given location. The results provide information about the weather and climate of a geographical area. This information is useful in meteorology, science, agriculture, tourism, and other fields.There are two basic types of...


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