Global dimming is the ironic counterpoint to global warming, both caused from burning fossil fuels, and both a serious potential threat to our future climate.
The sun shines down on the Earth, providing two of the essential elements of life: light and heat. But we live in a world of “quanta”; of measurable matter and energy. Thus, the sunlight radiating upon the earth is a matter of specific measurement.
These specific measurements of the quantity of sunlight reaching the Earth’s surface did not begin in any scientifically viable way until the 1950’s. Since then, the ongoing measurement shows a steady decline of solar irradiance. A "Global Dimming" of the sun of an average of 4% between 1960 and 1990.
Global dimming was first discovered by English Scientist Gerry Stanhill. Working in Israel, Stanhill compared records from the 1950’s with recent measurements and found, to his astonishment, a 22% decrease in sunlight . This led to further investigations and just about everywhere he looked he found the same phenomenon: a 10% decrease over the United States, nearly 30% over the former Soviet Union, 16% over portions of the British Isles.
Despite the wide regional variation of decreased sunlight, the decline has been averaged to 1-2% per decade from the 1950’s to the 1990’s.
Causes of Global Dimming
The cause of the dimming is believed to be from the increased presence in the atmosphere of particulate matter and aerosols from burning fossil fuels and other human activity. This particulate matter absorbs and reflects sunlight.
The effect of solar “dimming” by virtue of dirty air is somewhat expected. More unexpected and surprising to scientists is how these particles alter the reflective properties of clouds. The reflectivity of a cloud depends on the surface area of the droplets within it. The aerosols and other particles in polluted clouds create smaller water droplets than in unpolluted clouds; up to six times as many droplets. The actual amount of water vapor does not change, but the increased number of droplets makes the clouds more reflective, thus radiating more sunlight back into space before reaching the Earth’s surface.
Pan Evaporation Data
For the past fifty years, a very simple measurement known as the “pan evaporation rate” has been extensively monitored throughout the world. Used to schedule irrigation and budget water use, this data was otherwise largely ignored.
However, in the 1990’s scientists began to note something strange in the data; the rate of evaporation was decreasing in spite of an expected increase due to global warming. There are three elements that effect evaporation: humidity, wind, and sunlight. Given relatively consistent measures of humidity and wind, the indication was clear that a decrease in sunlight was the principal indicator of this reduced evaporation. More evidence confirming global dimming.
Jet Contrails and Global Dimming
Jet contrails, the water vapor streaming from the engines of high-flying jet aircraft, was another suspected cause of global dimming, but there was no practical way to test the theory – until September 11th, 2001.
For three days virtually all civil air traffic ceased over most of North America providing an opportunity for scientists to finally test this theory. To their surprise, in those short three days there was a marked increase in diurnal temperature variation; in other words an increase in the range of temperature between day and night averaging 1 degree Celsius (warmer during the day, cooler at night), providing some evidence that these contrails contribute to Global Dimming.
The Consequences of Global Dimming
Drought, acid rain, and lung disease are all potential risks from the airborne particulates thought to cause global dimming. Perhaps the most insidious consequence, however, is the masking effect it has had on global warming.
In the tug of war between cooling and warming, a warming climate is the net winner. The irony is that as regional air pollution is reduced through concerted efforts to clean particulate matter generated from human processes out of the atmosphere, we may be “letting go of the rope”, thus allowing global warming to lurch ahead to levels not currently expected.
Is this certain to happen? Nobody knows for sure. A reason itself to consider the implications of the dangerous game we play when altering our global atmosphere.
