(This image on the Eastern China was acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard of Aqua satellite of NASA, October 16, 2002. The scene shows dozens of fires burning on the surface (red dots) and a thick smoke and fog (gray pixels) filling the skies above. The smog filled valleys and streams around the contour land in areas of hills and mountains in China.)
The global dimming is a gradual reduction since the early 1950s of the light intensity of daytime light that reaches the Earth’s surface.
This effect was demonstrated, among others, by Gerry Stanhill, an English scientist installed in Israel, who compared the intensity of solar radiation since the 1950s to 1980. The global dimming creates a cooling effect which may conduct to underestimate the scientific greenhouse gases on global warming.
From 1950 to 1985, the solar radiation on the surface of the Earth has decreased by about 4%. These decreases are subject to availability and seasonal fluctuations. Thus, the greatest darkening of 30% were measured in Russia. On the African and American continents, sunshine decreases of 15% were measured. The lowest darkening were measured in northern Europe and Australia.
Causes and effects
The cause is the increase in the average rate of aerosols into the atmosphere due to the emissions of various particles:
- forest fire
- motorized transport: including petrol and diesel, emissions against which particulate filters were installed on the recent diesel vehicles
- heating and more generally to the combustion of fuel: wood, coal, oil and, lesser extent, gas.
In the atmosphere, the microparticles or aerosols serve as nucleation nuclei to water vapor which then turns into droplets. Usually these are natural aerosols, pollen and sea salts including that play this role. But by multiplying their rate by 10 following the emissions from human activity (soot, ash and sulfur dioxide), the droplets are smaller and more numerous, making them more reflective clouds. In addition, some of these particles (soot, ash, sol microparticles from wind erosion) are opaque or dark colored. Clouds then form in abnormal places and heights, including at high altitude from airplane condensation trails, which are very strong increased in 30 years. The clouds are more and more opaque, reflecting a greater proportion of light into space. The smog which decreased in major cities in rich countries, have become almost permanent over some cities in so-called emerging countries.
Solar radiation to the ground is reduced, resulting in a refresh of the lower layers of the atmosphere that could mask or delay the impact of greenhouse gas (global warming). Locally, reduced condensation on earth (less dew) and in the lower layers (less rain) could result. The global dimming may be one cause of droughts in Africa during the 1980s (with the 1984 famine in Ethiopia, for example, with 50 million people starving and causing 1 million deaths).
Thus, global warming (+ 0.6 °C) observed in the twentieth century might have been much higher if the cooling effects of global dimming hadn‘t masked global warming. The increased of warming due to greenhouse gases nevertheless seems indisputable, and could paradoxically be accelerated by a generalization of the clearance of human waste, which is necessary in particular for health reasons: aerosols contribute significantly to cancer and many lung diseases.
Models commonly used to simulate the weather and climate change are based on an assumption of radiation reduction of the order of 1%, while the measurements show an attenuation of the radiation to be multiplied by 10 (of the order 10%). The reduction of radiation (greater than expected, and that created an important cooling) has masked the increase in the greenhouse effect by an equivalent amount.
Warming forecasts therefore will most likely be revised upwards (by Peter Cox + 8 or 10 °C for 2100, instead of 4-6 °C according to the IPCC) if the pollution that creates the darkening is fought effectively in the world, as it began to be since the 1980s in Europe. The reduction in the darkening already registered in Europe could contribute to the observed warming in recent years in Western Europe.
The effects vary among regions, but assessments of the average value of the decrease of the radiation received were calculated in a fairly convergent manner:
- 5.3% (9 W/m²) for 1958-1985 (Stanhill and Moreshet, 1992)
- 2% per decade from 1964 to 1993 (Gilgen and others, 1998)
- 2.7% per decade (total 20 W/m²) to 2000 (Stanhill and Cohen, 2001)
- 4% from 1961 to 1990 (Liepert, 2002).
The largest decreases were in middle latitudes of the northern hemisphere, which correspond to regions with the highest emissions of particles (before the Asian countries, India and China in particular, will start to turn to pollute their atmosphere massively).
The work of renowned climatologist V. Ramanathan (Scripps Institution of Oceanography in San Diego) had helped to establish in the mid-1990s a decrease in solar radiation over large areas. An international campaign experience (Indian Ocean Experiment, INDOEX) was conducted in the Maldives between January and March 1999 to compare the radiation on the north and south islands of the archipelago, by a team of 150 scientists of different nationalities conduct by the Nobel Prize in Chemistry and V.Ramanathan P.Crutzen.
The study, which cost $25 million, showed that the cloud of macroscopic pollutants caused by winds blowing from the south of India caused a reduction of about 10% of the intensity of sunlight in the northern islands, compared to the measured intensity in the southern islands, enjoying the fresh air of the Indian Ocean.
Also this reduction is much larger than that expected based on the mere presence of the particles.
The issue of climate change appear anyway even more complex than expected. Very recently, another search was conducted by the team of Professor V. Ramanathan. The results, published in “Nature“, show how the combination of greenhouse gases and brown clouds (composed of soot, metal particles and caused by urban, industrial and agricultural residue), was to origin of glacial retreat in the Himalayas over the past 50 years. Since polluted air effect is double, different over the entire height of the air layers:
- it contributes to the global warming, because the particles absorb sunlight,
- however it cools the surface of the earth because the particles reduces the amount of light that reaches the ground.
Effects on health and the environment
Where aerosol rates have increased in the lower layers, the diseases associated with air pollution has increased, including lung cancer and lung disease. Acid rain may have increased, with impacts on flora and soil (mobilization and increased leaching of heavy metals).
In these areas, the rate of ultraviolet (UV) may have fallen sharply, with loss of purifying power of sunlight on the water (UV kill many microbes epiphytes on the leaves or on the surface of the soil and water). When darkening is maximal, the productivity of the land or water photosynthesis may have declined, but this theme seems to have been little research.
Conversely, where efforts to air quality were significant, particularly in Europe, or wherever the air is kept clean because of a low level of industrialization and human impacts (eg South America ), the likely increase in holes in the ozone layer, partly linked to the greenhouse effect (the upper atmosphere cooling induced trapping of calories in the lower layers it promotes ozone depletion), the UV rate could increase significantly, with the associated risks: cancers and premature aging of the skin, earlier sunburn, etc. These elements have been rising sharply in Australia.
Translated from Wikipedia