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Tuesday, April 6, 2010

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Could It Be This Easy?

Instead of talking about global warming, let’s take simple and practical steps to counter it

Arun K Attri

Notwithstanding the IPCC’s role in creating cynicism about the global warming debate, the issue is real and alive. Simply, the laws of physics can be used to assess the partial trapping of infrared radiations emitted by the Earth’s surface from constituents present in atmosphere. The main concern is increasing atmospheric concentration of carbon dioxide, an end-product of energy use in any form. Wishing a cap on emissions by countries is, at best, a fantasy immersed in a black hole of convoluted discussions to no end. One casualty of never-ending parleys – within countries, between countries, between groups of countries and diverse interest groups – has been the search for alternate solutions to cool the planet’s temperature.
Almost all incoming solar radiation, known as short-wave radiation, passes through the atmosphere and reaches the Earth’s surface. The bulk atmospheric constituents (99 per cent) – nitrogen and oxygen – are transparent to short-wave solar radiation. On reaching surfaces, short-wave radiations undergo reflection and absorption. ‘Fractionation’ between these two is associated with the nature of the surface. That is, the surface composition’s texture and colour determine what fraction of incident radiation gets reflected and goes back.
The characteristics of shortwave solar radiation do not
change when reflected, and essentially the atmosphere remains transparent on its return journey too. Albedo, or surface reflectivity, determines the fraction of radiation absorbed by the surface, and thus its temperature. Low albedo implies a larger fraction will be retained. In short, tinkering with the albedo of surfaces can help lower surface temperature by reflecting a larger fraction of incident solar short-wave radiation back.
Take the example of the surfaces amenable to albedo tinkering in a large city like Delhi. About 64 sq km (12.5 per cent of the city’s total area) is occupied by roads made of asphalt. When freshly laid, the colour is black

and the albedo 0.05. With time, the colour changes to grayish black and the albedo to 0.15. In the first case, the black surface will only reflect 5 per cent of the incident solar radiation and absorb 95 per cent. Later, the grey reflects 15 per cent and absorbs 85 per cent.
Now, if instead of asphalt the road surface is made of cement, the albedo jumps to between 0.3 and 0.4, implying that additional
short-wave solar radiation can be sent back (30 to 40 per cent). In the middle of June in Delhi, mid-day asphalt road surface temperature can be about 65 degrees centigrade, much higher than the Met department’s daily record, where the maximum tempera-ture is never more than 45 degrees as it is measured in the shade at a height of about four feet.
Short-wave radiation absorbed during the day transforms into heat, and this heat is sent back into the atmosphere as infra-red radiation partially trapped by green house gases, the essence of the global warming problem. This area is large enough to alter the albedo substantially by bouncing a large chunk of short-wave solar radiation back. By the simple expedient of raising the albedo from
0.15 to 0.4, an additional 25 per cent of the short-wave radiation can be reflected away from the Earth’s surface, lowering the city’s overall average temperature by about 2 degree Celsius. This decrease would send less infra-red radiation back to be trapped by green house gases.
Almost 90 sq km area of Delhi is covered by high and low density dwelling units. If even 50 per cent of the roof area of these buildings is appropriated for change in albedo by painting roof surfaces white (an allweather exterior coat), an additional 40 per cent of short-wave solar radiation will be reflected back. The results can be so substantial as to be startling. A roof that is cooler prevents conduction of heat to rooms on the floors below, ensuring a decrease in power requirements for cooling.
This measure alone would lower the entire city’s temperature by 2.5 degrees Celsius.
Planting trees along roads would also help lower temperature by reflecting a major fraction of short-wave solar radiation spectrum back. Plants are green because they reflect this range of radiation. Almost 12 per cent of incoming solar shortwave radiation energy comprises this fraction. The efficacy of this step will depend on the number of trees planted and their canopy size but, in any case, the cooling effect is assured.
If these steps are taken, rough calculations indicate that the average reduction in city temperature can be between 3 to 5 degrees Celsius. Given the direct correlation between power consumption and higher daytime temperatures during summer, summertime energy consumption will be considerably lowered. Lower power consumption implies less emission of dreaded carbon dioxide, or a saving of precious carbon credits.
This seems a simple, logical way to counter global warming: introducing measures based on established scientific facts, thus transforming large cities into the first frontier of the battle against global warming, without imposing carbon dioxide emission caps on nations. Or we could continue to think nations will somehow agree to decrease emissions of global warming gases, a dream indeed.
The writer is professor, atmospheric chemistry, School of Environmental Sciences, JNU.


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