Sunday, January 25, 2009

Can global warming be tackled with white paint?

Can white paint reduce global warming?
by
Dr. Nitish Priyadarshi


(Above pictures shows small houses made with the help of soil as cementing material. These houses are cool even during peak summer season in Jharkhand State of India.)
In a new study, a scientist has come up with the suggestion that global warming may be tackled with white paint.
According to a report in The Guardian, the scientist in question is Hashem Akbari from the Lawrence Berkeley National Laboratory in California, US, who suggests painting roads and the rooftops of buildings with the color white to fight global warming.
Theory is based on the concept that white colour reflects enough of sunlight and heat back to the atmosphere which will help to delay global warming.
He wants dozens of the world's largest cities to unite in an effort to replace the dark-coloured materials used to cover roads and roofs with something a little more reflective.

Study has shown that buildings with white roofs stay cooler during the summer. The change reduces the way heat accumulates in built-up areas (known as the urban heat island effect) and allows people who live and work inside to switch off power-hungry air conditioning units.
Dark roofs reflect about 10-20 percent of sunlight, while white surfaces tend to send back at least half.

We all know that Greenhouse gases, which include water vapor, carbon dioxide and methane, warm the atmosphere by efficiently absorbing thermal infrared radiation emitted by the Earth’s surface by the atmosphere itself, and by clouds. As a result of its warmth, the atmosphere also radiates thermal infrared in all directions, including downward to the Earth’s surface. Thus, greenhouse gases trap heat within the surface-troposphere system.
The Earth receives energy from the Sun mostly in the form of visible light. The bulk of this energy is not absorbed by the atmosphere since the atmosphere is transparent to visible light. 50% of the sun's energy reaches the Earth which is absorbed by the surface as heat. Because of its temperature, the Earth's surface radiates energy in infrared range. The Greenhouse gases are not transparent to infrared radiation so they absorb infrared radiation. Infrared radiation is absorbed from all directions and is passed as heat to all gases in the atmosphere. The atmosphere also radiates in the infrared range (because of its temperature, in the same way the Earth's surface does) and does so in all directions. The surface and lower atmosphere are warmed because of the greenhouse gases and makes our life on earth possible.
Suggesting painting roads and the rooftops of the building white looks impressive and true at first glance. But the side effects may be different. According to the theory white colours reflects half of the heat from its surface. If we colour our buildings with white, chances of reflecting more heat from the white surface will be high which will ultimately increase the temperature of the surrounding atmosphere and space many folds thrusting people to leave inside the house during day time. In other words we will dump more heat into the atmosphere.

It is true that people leaving inside the house painted with white have comfortable summer. But what about the surrounding atmosphere where the temperature may rise due to reflection of the sunlight from the white paint. It will not only affect the humans but ecology too will be affected.
I have seen small hut like houses (see the above pictures) build by soils as cementing material by the tribal of the Jharkhand state of India are very comfortable from inside even during the peak summer where the temperature reaches up to 44 degree centigrade. It may be due to the roughness of the soil surface. Theoretical considerations indicate that increasing surface roughness results in greater transport of energy from the soil surface to the atmosphere.
So before implementing this research it is essential to carry more research on this topic.
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