Have you ever
heard of GLOBAL DIMMING?
Not global
warming.
Global dimming.
Come along to
find out what it is.
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 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 in 1991, a very slight reversal in the
overall trend has been observed.[1]
Global dimming
is thought to have been caused by an increase in particulates 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 effect that may have partially
counteracted the effect of greenhouse gases on global warming.
In basic terms,
less sunlight is reaching the Earth because of visible air pollution, which is
reflecting the light back into space.
CAUSES AND EFFECTS
It is thought
that global dimming is probably due to the increased presence of aerosol particles in the atmosphere caused by human action.[2]
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.[3] 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.
This same
effect also reflects radiation from below, trapping it in the lower atmosphere.
In models, these smaller droplets also decrease rainfall.[4]
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.
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.[27]
BACKDROP
In the
late-1960s, Mikhail
Ivanovich Budyko worked with
simple two-dimensional energy-balance climate models to investigate the
reflectivity of ice.[5] 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.[6][7]
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 appeared to
contradict global
warming—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.[8] This was soon followed by others:
Viivi Russak in 1990 "Trends of solar radiation, cloudiness and
atmospheric transparency during recent decades in Estonia",[9] and Beate Liepert in 1994 "Solar
radiation in Germany — Observed trends and an assessment of their causes".[10] Dimming has also been observed in
sites all over the former Soviet Union.[11] Gerry Stanhill who studied these
declines worldwide in many papers coined the term "global dimming".[12]
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.
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,[13][32][33] and Pinker et al.[34] found that slight dimming continued
over land while brightening occurred over the ocean.[35] 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, 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.[31][36]
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 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 six principal air pollutants dropped by 53% in
the US. In 1975, the masked effects of trapped greenhouse gases finally started
to emerge and have dominated ever since.[37]
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.
GLOBAL DIMMING
AND 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.[43] 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."[44] 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.[43]
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.[45] 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.
Source:
http://www.en.wikipedia.org/wiki/Global_dimming
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