"Global Dimming"

What is global dimming?
Two effects of fossil fuel productions are:
1>Greenhouse gases that cause global warming
2>By-products which are pollutants that cause global dimming

Fossil fuel use, as well as producing greenhouse gases, creates other by-products. These by-products are also pollutants, such as sulphur dioxide, soot, and ash. These pollutants however, also change the properties of clouds.

Clouds are formed when water droplets are seeded by air-borne particles, such as pollen. Polluted air results in clouds with larger number of droplets than unpolluted clouds. This then makes those clouds more reflective. More of the sun’s heat and energy is therefore reflected back into space.

This reduction of heat reaching the earth is known as Global Dimming.

In other words it is the gradual reduction in the amount of global direct irradiation 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.

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.

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Global Dimming...

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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. 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. 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.

Impacts of global dimming:
1>Health and environmental effects:::The pollutants that lead to global dimming also lead to various human and environmental problems, such as smog, respiratory problems, and acid rain.

2>Millions from Famines in the Sahel in the 70s and 80s::::Climatologists studying this phenomenon believe that the reflection of heat have made waters in the northern hemisphere cooler. As a result, less rain has formed in key areas and crucial rainfall has failed to arrive over the Sahel in Northern Africa.
In the 1970s and 1980s, massive famines were caused by failed rains which climatologists had never quite understood why they had failed.

3>Billions are likely to be affected in Asia from similar effects::::Scientists said that the impact of global dimming might not be in the millions, but billions. The Asian monsoons bring rainfall to half the world’s population. If this air pollution and global dimming has a detrimental impact on the Asian monsoons some 3 billion people could be affected.

As well as fossil fuel burning, contrails is another source
Contrails (the vapor from planes flying high in the sky) were seen as another significant cause of heat reflection.

During the aftermath of the September 11, 2001 terrorist attacks in the United States, all commercial flights were grounded for the next three days.

This allowed climate scientists to look at the effect on the climate when there were no contrails and no heat reflection.

What scientists found was that the temperature rose by some 1 degree centigrade in that period of 3 days.

Global Dimming is hiding the true power of Global Warming
The above impacts of global dimming have led to fears that global dimming has been hiding the true power of global warming.

Currently, most climate change models predict a 5 degrees increase in temperature over the next century, which is already considered extremely grave. However, global dimming has led to an underestimation of the power of global warming.
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Addressing global dimming only will lead to massive global warming
Global dimming can be dealt with by cleaning up emissions.

However, if global dimming problems are only addressed, then the effects of global warming will increase even more. This may be what happened to Europe in 2003.

In Europe, various measures have been taken in recent years to clean up the emissions to reduce pollutants that create smog and other problems, but without reducing the greenhouse gas emissions in parallel. This seems to have had a few effects:
This may have already lessened the severity of droughts and failed rains in the Sahel.
However, it seems that it may have caused, or contributed to, the European heat wave in 2003 that killed thousands in France, saw forest fires in Portugal, and caused many other problems throughout the continent.

The documentary noted that the impacts of addressing global dimming only would increase global warming more rapidly. Irreversible damage would be only about 30 years away. Global level impacts would include:
The melting of ice in Greenland, which would lead to more rising sea levels. This in turn would impact many of our major world cities
Drying tropical rain forests would increase the risk of burning. This would release even more carbon dioxide into the atmosphere, further increasing global warming effects. (Some countries have pushed for using “carbon sinks” to count as part of their emission targets. This has already been controversial because these store carbon dioxide that can be released into the atmosphere when burnt. Global dimming worries increase these concerns even more.)

These and other effects could combine to lead to an increase of 10 degrees centigrade in temperature over the next 100 years, not the standard 5 degrees which most models currently predict.

This would be a more rapid warming than any other time in history, the documentary noted. With such an increase,
Vegetation will die off even more quickly
Soil erosion will increase and food production will fail
A Sahara type of climate could be possible in places such as England, while other parts of the world would fare even worse.
Such an increase in temperature would also release one of the biggest stores of greenhouse gases on earth, methane hydrate, currently contained at the bottom of the earth’s oceans and known to destabilize with warming. This gas is eight times stronger than carbon dioxide in its greenhouse effect. As the documentary also added, due to the sheer amounts that would be released, by this time, whatever we would try to curb emissions, it would be too late.

“This is not a prediction,” the documentary said, “it is a warning of what will happen if we clean up the pollution while doing nothing about greenhouse gases.”

Root causes of global warming also must be addressed
If we were to use global dimming pollutants to stave off the effects of global warming, we would still face many problems, such as:
Human health problems from the soot/smog
Environmental problems such as acid rain
Ecological problems such as changes in rainfall patterns (as the Ethiopian famine example above reminds us) which can kill millions, if not billions.

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.

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