Global Dimming

Two independent recent studies in Science magazine suggest that the amount of sunlight reaching the Earth’s surface is increasing. Using different methods, scientists find that solar radiation at the surface has risen for at least the last decade. This theory clearly negates the concept of global dimming as wee know it. Scientists across the world have started studying global dimming with a new objective hence this column.

What is global dimming? As per Wikipedia, “Global dimming is a term describing the gradual reduction in the amount of sunlight observed reaching the Earth’s surface since the 1950s. The effect varies by location but globally is of the order of 5 percent reduction over three decades.

Here is the explanation of global dimming. Clouds change as we emit more particles into the atmosphere. Clouds are made of droplets, which form by latching onto tiny particles called condensation nuclei. These occur naturally in the atmosphere, but by emitting more particulate pollution into the atmosphere, human activities help make even more condensation nuclei. The result: Instead of fewer, larger water droplets forming, many, smaller water droplets form. The reason for the loss of sunlight is that particulate pollution such as soot plugs up clouds, so that when it is cloudy, it is darker than before. In effect, this is like the difference between two sieves, one coarse and the other fine. Like a coarse sieve, the cloud with fewer, larger particles lets more solar radiation through to the ground, whereas like the fine sieve, the cloud with lots of very small particles lets less sunlight pass through.

Clouds intercept both heat from the sun and heat radiated from the Earth. Their effects are complex and vary in time and location and height. Usually, during the day 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. As with many other issues relevant to environment science, the answers researchers seek are not easily obtained, because previous generations did not build the instruments and set up the experiments that present-day investigators now suddenly need. As per records since early 1950s (when the measurement first started), the average amount of sunlight reaching the ground has gone down by almost 3 percent a decade in past 50 years. It is too small an effect to see with the naked eye, but it has implications for everything from climate change to solar power and even the future sustainability of plant may be seismic. Global dimming may cause large scale changes in weather patterns, the failure of the monsoon, famines, unpredicted impact on temperatures and sea levels; all speculation so far.

Global dimming has been hard to study definitively. There are still so many unanswered questions. Scientists are working to see the extant global dimming, its impact on our plant and if it is really going reverse as suggested in recent studies. With the growing realization that climate change may be a major hazard for the planet and for human society, the new research is likely to bridge the gaps, which are left in this area.

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