Total darkening
The total darkening or total obscuration (English: Global Dimming ) is a gradual reduction, since the beginning of the Fifties, the Luminous intensity of the diurnal light which reaches terrestrial surface.
This effect was highlighted, inter alia by Gerry Stanhill, an English researcher installed in Israel, which compared the intensity of the solar radiations since the years 1950 until the years 1980. The total darkening creates a cooling effect which perhaps led the scientists to underestimate the Greenhouse effect on the Climate warming.
By 1950 to 1985, the solar radiation on the surface of the ground decreased by 8% to 30%. These reductions are subjected to regional and seasonal fluctuations. Thus, the greatest darkenings of 30% were measured in Russia. On the continents African and American, reductions in the sunning of 15% were measured. The weakest darkenings were measured in Europe of north and Australia.
Causes and effects
The cause would be the increase in the average rate of Aérosol S in the atmosphere following the emissions of various dependant particles:- with forest fires
- with motorized transport: Gasoline and Gas oil in particular, emissions against which filters particles were installed on the recent vehicles diesel
- with the heating and more generally with the combustion of fossile fuels: Wood, coal, Oil and in a less way Gas.
In the atmosphere, the microparticles or aerosols are used as cores of nucleation with the steam which is transformed then into droplets. Usually, they are the natural aerosols, pollens and sea salts in particular, which play this part; however by mulitipliant their rate by 10 following the emissions resulting from the human activity (soots, ashes and sulfur dioxide), the droplets are smaller and more numerous, making the clouds more reflective. Moreover, some of these particles (soots, ashes, microparticles of ground resulting from wind erosion) are opaque or of dark color. The clouds are formed then in places and with abnormal heights, including with very high-altitude starting from the trailed condensation of plane, which have been in very strong increase for 30 years. The more and more opaque clouds, reflect a greater proportion of light towards space. The Smog S which decreased in the big cities of the rich countries, became almost permanent with the top of certain towns of countries known as emergent .
The solar radiation on the ground is decreased by it, involving a cooling of the low layers of the atmosphere which could mask or delay the impact of gases with greenhouse effect (warming). Locally, a reduction in condensation on ground (less dew) and in the low layers (less rains) could result from it. The total darkening could be one of the causes of the Sécheresse S in Africa at the time of the years 1980 (with the famine of 1984 in Ethiopia for example, that causes hunger 50 million people and causing 1 million dead).
Thus, the total warming (+ 0,6°C) noted at the 20th century would have perhaps been quite higher if the cooling effects of the total darkening had not masked the Climate warming. The increase in the warming due to the Gaz with greenhouse effect seems nevertheless undeniable, and it could paradoxically be accelerated by a generalization of the depollution of the human rejections, which proves to be necessary in particular for health reasons: the aerosols contribute in particular to cancers and many lung diseases.
The models usually used to simulate the weather evolution and of the climate are based on an assumption of reduction of radiation of about 1%, whereas taken measurements show a weakening of the radiation to be multiplied by 10 (about 10%). This reduction of the radiation (more important than envisaged, and which created an important cooling) thus masked the increase in the greenhouse effect in an equivalent proportion.
The forecasts of warming will have most probably to thus be re-examined with the rise (according to Peter Cox + 8 or +10°C for 2100, instead of +4 with 6°C according to the GIEC) if the pollution which creates the darkening is fought effectively in the world, as it started to be it since the years 1980 in Europe. The reduction in the darkening recorded in Europe could already contribute to the warming noted these last years in Western Europe.
The effects vary according to the areas of the world, but of the evaluations of the median value of the reduction in the received radiation were calculated in a rather convergent way:
- 5,3% (9 W/m ²) of 1958 to 1985 (Stanhill and Moreshet, 1992)
- 2% per decade of 1964 to 1993 (Gilgen and others, 1998)
- 2,7% per decade (on the whole 20 W/m ²) up to 2000 (Stanhill and Cohen, 2001)
- 4% of 1961 to 1990 (Liepert, 2002).
Work of the famous climatologist V. Ramanathan (oceanographical Institute Scripps de San Diego) had made it possible to establish as of the middle of the years 1990 a reduction in the solar radiation on vast zones. An international experiment campaign (Indian Experiment Ocean, Indoex) was led to the Maldives between January and March 1999 to compare the radiation on the islands of the north and the south of the archipelago, by a team of 150 scientists of various nationalities led by the Nobel Prize of P.Crutzen Chemistry and V.Ramanathan.
This study, which cost 25 million dollars, showed that the cloud of macroscopic pollutants pulled by the winds blowing of the South of India because a reduction from approximately 10% of the intensity of solar light in the islands of north, compared to the intensity measured in the islands of the south, which profit from the fresh air of the Pacific Ocean.
De more this reduction proves much more important than that awaited on the basis of presence simple of the particles.
The question of the climate change appears more complex in any event still than envisaged. Very recently, another research was carried out by the team of professor V. Ramanathan. Results, published in " Nature" , show how the combination of gases with greenhouse effect and the brown clouds (soot compounds, of particles of metal and residues caused by the activities urban, industrial and agricultural), would have been at the origin of the retreat of the glaciers of the Himalayas during the 50 last years. Because the effect of the polluted air is double, different on all the height from the layers of air:
- it contributes to the warming of the atmosphere, because the particles absorb the sunlight,
- it cools on the other hand the surface of the ground because the particles reduce the quantity of light which reaches the ground.
Effects on health and the environment
Where the rates of aerosols increased much in the low layers, the diseases associated with the air pollution increased, of which lung cancers and lung diseases. The acid rains can have increased, with impacts on the flora and the ground (mobilization and increased scrubbing of heavy metals).In these zones, the rate of ultra-violet (UV) can have strongly decreased, with loss of purifying capacity of solar light on water (UV kill out of many microbes épiphytes on the sheets, or on the surface of the ground and water). When the darkening is maximum, the productivity of terrestrial or watery photosynthesis could have decreased, but this topic seems to have been the subject of little research.
Conversely, where the efforts for the quality of the air were significant, in Europe in particular, or where the air remained clean because of a low level of industrialization and anthropisation (ex: South America), the probable increase in the holes in the layer of Ozone, to some extent related to the greenhouse effect (the cooling of the upper atmosphere induced by the trapping of the calories in the low layers supports the destruction of ozone there), the rate of UV significantly could increase, with the associated risks: cancers and premature ageing of the skin, earlier sunstrokes, etc These elements were in strong increase in Australia.
Seek
The first reports/ratios on the total darkening attracted only little interest, probably because the concept itself had not been still invented. It seems that the first reports/ratios come from Mr. Budyko: “The effect of the variations of the solar radiation on the climate of the ground” in 1969, published by Tellus. Starting from the end of the year 1980, the scientists started to work independently on the unit of the data concerning the solar radiation and discovered that it tended to be reduced in the whole world. It is Gerry Stanhill, which wrote many articles on this reduction in the whole world (see the references), which is the inventor of the term dimming (translated into French by obscuration or darkening ).The research led independently in Israel and to the the Netherlands towards end of the year 1980 showed an apparent reduction of the quantity of sunlight, in spite of the theory of climate warming. The rate of darkening varies according to the place, but it is estimated on average at approximately 2 to 3% per decade, the tendency having been able to be reversed with the beginning of the year 1990. Although it is difficult to carry out measurements with exactitude, in particular because of the very precise calibration necessary of the instruments used, the presence of this effect is practically certain.
The effect is due only to the changes of the composition of the terrestrial atmosphere, the value of the solar radiation to the top of the atmosphere not having changed significantly.
This effect was also confirmed by research undertaken independently on the rate of evaporation: the speed of evaporation, measured for a very long time in an identical way in all areas of the sphere, dropped in the years 1990, which seemed strange since one parallel to assisted a warming of planet. But the study of evaporation shows that the determining phenomena are, by decreasing order, the sunning, moisture and the wind, well before the temperature.
The two biologists (Michael L. Roderick and Graham D. Farquhar) calculated the fall of sunning corresponding and arrived at results similar so that found researchers Russian, who had established a fall of evaporation in Russia, Europe of the East and the United States over 30 years. Thus two completely independent studies (measurement of the sunning and the rate of evaporation) lead to identical results.
Local and recent inversions of the tendency
It is possible that the total darkening caused weather changes with large scales. The climatic models suggest that the reduction of intensity of the solar radiation could have been at the origin of the failure of monsoon in sub-Saharan Africa during the years 1970 and 1980, as well as associated famines, as that which followed the Sécheresse to the Sahel, caused by the cooling of the Atlantic Ocean related to the pollution of the northern hemisphere. That could be the reason for which the Ceinture of the rains of monsoon could not go up towards north, thus involving the absence of the seasonal rains. This assumption is not universally accepted and is very difficult to check.
Certain scientists specialists in the climate put forth the assumption that the trailed condensation of the planes (in English contrails for condensation trails ) would be implied in the total darkening. Although the continuity of the air traffic prohibits to check this assumption, the complete stop of the civil air traffic in the United States during the three days having followed the Attentats of September 11th, 2001 made it possible to the climatologist David Travis to observe the temperatures of 5000 weather stations distributed out of 48 states, in the absence of these trails of condensation. For this period, the measurement of the amplitude of temperature showed day at the following day an increase in 1°C, which was the greatest variation noted over 30 years. That showed that the trails of condensation could play an immediate part in the rise of the night temperatures and/or a fall of the diurnal temperatures, and that these variations could be much more important than it previously was thought.
Recent publications give a report on a return of sunning since 1985 in the areas where the particulate emission (with the source of the aerosols) was strongly reduced. This resumption of the radiation received by the ground can explain a share at least:
- of the fast increase in the temperatures noted recently. For example:
- heat wave in France in 2003
- forest fires in Portugal in 2005 and Greece in 2007.
- and more generally of the recent weather disordered states:
- particularly powerful cyclones and devastators in North America (Katrina, 2005) and in the Caribbean (Dean in 2007 p. e.g.)
- catastrophic floods in Europe: Germany (2002) and England (2007) p. e.g.; but also in sub-Saharan Africa in August 2007, precisely where the dryness had lasted of the decades, when the darkening was maximum in Europe
- cast iron of the ice-barrier in September 2007 much stronger than during the previous years, cast iron in 2007 qu ' none climatic models currently used had envisaged.
Relationship with climate warming
Certain scientists consider now that the effects of the total darkening masked, to a certain extent, the effect of climate warming, and that thus a possible reduction in the darkening could result in re-examining with the rise the forecasts of rise in the temperatures.
The increase in the atmospheric aerosols could also be seen like means of action against climate warming. However, the aerosols have negative effects (acid rains) which explain the efforts which were made to reduce them, and as they have moreover one very short life, about a few days or weeks (in opposition to several decades for CO2), their use appears not easily possible.
The total darkening can also have regional effects. Whereas the major part of the Earth was heated, the areas which receive great quantities of aerosols by atmospheric dynamics are generally cooled. This can explain for example the relative cooling of the areas of the east of the United States compared to those of the west.
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