What exactly is global dimming?
Global dimming is a recently discovered phenomenon that reduces the amount of solar radiation reaching the surface of our planet. Until recently, scientists had dismissed the published evidence, since it directly opposed the evidence and observations for global warming.
What is the cause of global dimming?
There is evidence that increased numbers of particulates in the air are responsible for global dimming. These pollutants include sulfur dioxide, ash and soot. These pollutants have increased as the rate of burning of fossil fuels has increased. These aerosol particles in the atmosphere have been associated with global dimming. The pollutants form nuclei for cloud droplets. It is thought that the water droplets in clouds coalesce around the particles. As air pollution increases more particulates are available for the formation of droplets. This makes them more reflective, resulting in less solar radiation (sunlight) reaching Earth's surface.
So what is the process?
Water droplets in clouds are formed by water vapour condensing on particulates. The increased number of particulates in the air increases the number of water droplets in clouds. As a result, there is an increased amount of reflection of solar radiation back into space. This reduction of heat reaching the earth is known as global dimming.
What other contributions are there to global dimming?
Air transport is also contributing to global dimming. The vapour from aircraft, known as contrails, also form water droplets, which increase the amount of solar radiation reflected back into space.
A NASA photograph of aircraft contrails, by NASA scientist Louis Nguyen.
Evidence for this was observed by scientists during the three days after the 9/11 attacks on the World Trade Centre. During that time all commercial flights were grounded, and gave scientists the opportunity to study the effects on temperature without contrails. The effect was that the range in temperature increased by 1 degree centigrade. This was considered to be a significant and substantial effect for such a short time.
What evidence is there for Global Dimming?
Less energy from the Sun should make the Earth become cooler. However, the measurements reveal that the average temperature or Earth is rising. This has been the result of many years of research into global warming, and is one of the main reasons why so many scientists found it difficult to accept the phenomenon of global dimming.
(a) Dr.Gerry Stanhill, a meteorologist, made some startling observations when he repeated sunlight experiments first made in the 1950's . He discovered that there was a 22% reduction in sunlight between the 1950's and 1980's . His published results made little impact on the scientific community. Many scientists dismissed the 'evidence' since it conflicted with the effect of global warming. They were skeptical about his measurements.
(b) Long-term measurements of Pan Evaporation Rates* indicate that evaporation rates have fallen. Dr Farquhar ( Australia) and Dr. Roderick provide evidence for this phenomenon. This is and apparent paradox : as the average global temperature increases the pan evaporation rate actually decreases. Yet, it was found that the rise of temperature is not the main factor in the evaporation rate. The amount of sunlight reaching the surface of our planet was found to be the main contributory factor for the evaporation process.
(*Pan Evaporation Rate is measured by topping up a 'pan' with water, to determine the amount of evaporation which has occured in the previous 24 hours. )
(c) Further evidence was provided in a Nature article, entitled "Evaporation losing its strength" , which revealed that Russia, United States and Eastern Europe all exhibited reduced evaporation rates. This amounted to 100 mm of water in the last 30 years. When Dr Farquhar ( Australia) and Dr. Roderick analysed this data they arrived at the conclusion that the drop in evaporation rate was consistent with the reduction in the solar radiation reaching the surface of Earth.
(d) Atmospheric experiments carried out in the Maldives in the 1990s revealed that the effect of particulates in the atmosphere from India resulted in a 10% reduction in sunlight reaching the surface beneath the pollution cloud.
(d) Evidence for Global Dimming was found immediately after the attacks on September 11, 2001 . All commercial flights were grounded for three days. Without the contrails from aircraft, the scientists found that the average temperature rose by approximately 1 degree centigrade. This is considered to be a staggering change in such a short period of time.
Other reports of evidence of global dimming have been published, such as Atsumo Ohmura "Secular variation of global radiation in Europe" (1989), Vivii Russak "Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia" (1990), and Beate Liepert "Solar radiation in Germany - Observed trends and an assessment of their causes" (1994).
What about Global Warming?
Until quite recently the main concern with scientists and governments has been on the Greenhouse Gases which have been emitted as a result of burning fossil fuels. These reduce the amount of radiated heat from the Earth from leaving the atmosphere. However, the discovery of global dimming provides evidence that the models for global warming have seriously underestimated the power of global warming. In effect, the phenomenon of global dimming has masked the overall effect of global warming.
Are these two effects related?
The effect of global dimming have led to fears that global dimming has been masking the true potential of global warming. In effect the observed increases in temperature have occured as less radiation has been received on Earth. At the moment a number of predictions provide a prediction of 5 degrees increase in temperature for the next hundred years. Taking into account of global dimming would suggest that the true power of global earning is much higher than these estimates.
Global Warming : The level of agreement between a climate model driven by five factors and the historical temperature record. The negative component identified as "sulfate" is associated with the aerosol emissions blamed for global dimming. (Image: Globalwarmingart.com )
Can Global Dimming be reduced?
Global dimming can be reduced by cleaning up the emissions from burning fossil fuels and the emissions from vehicles. However, a focus on particulates only would result in an increased effect of global warming. In essence, the sources of both greenhouse gases and air particulates must be addressed. It has been speculated that the heat waves in Europe in 2003, resulting in thousands of deaths, could have been the result of the action of the European Community to reduce particulate emissions, without considering a reduction in greenhouse gases.
Some other scenarios have been presented. One predication is that irreversible damage would be only be about 30 years away. The effects on a global scale have also been predicted, if global dimming is reduced without reducing the greenhouse gases. including the melting of ice in Greenland, and the drying of tropical rain forests. This, in turn will reduce the natural carbon dioxide sinks available, thereby increasing carbon dioxide levels even further. 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.
Why not use an increased number of particulates to reduce global warming?
This is a short-sighted solution and it would increase the effects on human health by an increased amount of soot and smog. Other effects include environmental effects such as acid rain, and a change in rainfall patters around the world. This could have the effect of killing millions of people.
How should we approach a solution to global dimming and global warming?
Climatologists emphasise that the source of global warming and global dimming must be tackled together. Drastic reductions of the burning of fossil fuels is required to reduce the effects of greenhouse gases and the particulates which are giving rise to global dimming.
Need to find out more about Global Dimming?
There is a superb presentation available online, by Beite Liepert. Nicely constucted, with animations, it requires Macromedia Flash Player to be installed.
The presentation can be found at:
Other articles avaialbe, include:
Dimming the Sun (NOVA)
Global Dimming and Climate Models
Why the Sun seems to be dimming
Global Dimming (Wikipedia)