Reflect more
............................................................................................................Pankaj shrivastava*
Summary
Global warming can be mitigated by increasing reflection of in coming solar radiation. It is possible to increase reflection by increasing surfaces of good reflectance.
Our present approach focused at decreasing proportion of green house gasses (GHG), is essential and must be followed. But it should be reinforced with the measures of increasing reflectivity.
Reducing GHG is a time taking, long-term measure. And there are chances of severe damage to occur by the time we succeed, in reduction of GHG to desired proportion.
Warming of earth to normal limits is necessary but when warming goes beyond the limits, it becomes damaging. Excessive warming is a function of increased absorption by GHG and decreased reflection by earth surface.
Decreasing reflection from earth surface has not been thoroughly worked out. reduction in, ice-covered surface, closed canopy (dense forest) surface, and increase in bare fallow degraded lands, increase in human dwellings, roads etc have all together contributed to decreased reflection of solar radiation.
Decreased reflection means increased retention of radiation by the system. And increased retention of radiation means increased level of warming. In - coming solar radiation (343 watt m2), goes out to the space by two ways.
(i) By reflection of short wave radiation : About 30 percent of in coming radiation (103 watt m2) is reflected back by the atmosphere and earth surface. This part of reflected radiation does not contribute to global warming process.
(ii) By radiation of long wave infrared radiation : About 49 percent of in coming radiation (168 watt m2), is absorbed by earth surface. It warms the surface and is released back as long wave (infra red) radiation. Most of thus emitted infrared goes out in the space, but some of the infrared radiation is absorbed and re-emitted by GHG; this part of radiation is held responsible for global warming.
Our present approach is focused on reduction of GHG, to reduce absorption of out going infrared. But we should also simultaneously and with equal importance focus on increasing surfaces of high reflectance.
Warming has increased not only because of increased GHG but also because of increase in surface area of poor-reflecting ( and high absorbing) surfaces (HAS). AS Increased quantum of GHG contributes to global warming, so the increase in area of high absorbing surface (HAS) is equally responsible for global warming.
(iii) Quantum of absorption of solar radiation has also increased. Absorption of radiation is proportional to the type of surface it falls on. A bright shining surface of high reflectance, like ice absorbs less and reflects more ( 90 % ), while dull rough surface like denuded dark earth, cement concrete, black top roads etc, absorbs more radiation and reflect less (10 %). We can call these surfaces as high absorbing surfaces (HAS).
Since last five, six decades area of earth surface under denuded earth, concrete roofs and roads etc (HAS), has increased many fold. The increase in HAS, is responsible for higher absorption and then higher emission of infrared. Higher emission of Infrared correspond to higher absorption by GHG, and consequently higher global warming.
(iv) there are surfaces that absorb moderate amount of radiation and use some part of absorbed radiation in photosynthesis. Vegetation come under moderately absorbing surfaces (MAS). Well stocked close canopy forests are good reflectors (25 %). New and shining leaves that sprout in spring season are very good reflectors, and they not only help the tree to stay cool in sun, but also contribute to regulate global warming by reflecting 25 % radiation and using some part of absorbed radiation in fixing carbon.
Warming of earth within normal limits is essential. But warming beyond the limit is damaging.
the earth temperature can be regulated either by reducing GHG or by increasing reflection or by both. Reducing GHG is essential and a long term solution. while increasing Reflection will relieve the system soon and will mitigate Greenhouse effects.
Albedo or the reflectivity of solar radiation by different surfaces has been shown in the table given below.
Table 1 : Reflectivity values of various surfaces
Surface /Average Reflectivity /Remarks
Soil Dark soils 5%
Light coloured / silt 40%
Sand White clean sand 45 %
Grass lands tall grasses 20% Some additional quantity of radiation used
in transpiration and photosynthesis
grass-land Short grasses 26 % - do -
Agricultural crops 25% - do -
Forests Deciduous 20% - do -
Forests Conifers 15% - do -
Forests Dense evergreen 28% - do -
Water bodies 30 % Depends on angle of in coming rays
Snow Clean fresh surface 95%
snow Old dusty 40%
Clouds Thick 90%
cloud Thin 50%
Increased reflection will lower the energy absorbed by the earth. And in turn emission of infrared from earth surface will come down. And low emission of infrared will cause lower absorption of infrared by GHG, and in turn normal warming of the biosphere.
we can increase reflection of radiation by adopting simple methods of increasing surfaces of high Albedo. Some of these methods are as follows.
(1)Roof Treatments: Roofs come in direct contact of radiation. It is important to increase reflection of HAS roofs.
Since last 3, 4 decades number of flat roofs of concrete has increased very fast, mostly in urban areas,. Flat concrete roofs with low albedo, absorb high quantity of solar radiation and in turn release more infrared. villages have roofs made of tiles and thatches the roofs are slant and absorbs comparatively less radiation than flat concrete roofs. like wise industries have roofs made of asbestos or iron corrugated sheets, painted with red oxide. These roofs have low albedo values and should be treated to reflect more.
Total roof area of the earth has roughly been estimated to be about 225000 square km. it is about 0.1516% of the total land area. Covering the roofs with materials of high reflectance will help increase reflection.
(a) White washing roof-top : White washing roof-top with high reflectance coats, is perhaps the easiest and cheapest method. We have to find out the best suited coating material.we can motivate people to either white-wash their roof top or adopt other means to reflect more from roof.
(i)Lime coating is the cheapest one. But it does not last longer.
(ii)Roof tops may be fixed with bright white tiles. although it is costlier, approximately @ Rs 50/= per sft. but a long lasting solution.
(iii)White cement may also be coated. it costs about Rs 10/= per sft. It is long lasting. White paint / distemper will also do.
(b)Roof top gardening : roof top gardening is a natural way of reflecting more, keeping room cool and trapping radiation and Co2 to wood. Grasses have 25 % albedo.
(c) Solar paneling : Solar panels of silica are suitable reflectors and uses solar radiation to generate electricity. Solar panels are very costly but are very useful and saves cost of electric bills.
(2) Field treatment: Denuded dark soil, crop less fields are very high radiation absorbing surface, following treatment should be ensured
(i) increase summer season cropping.
(ii) grow orchard.
(3) Black top roads : these road surfaces have very less reflectance. The roads must be covered with road side plantations.
(4) Vehicles: vehicles roof top should be coated with materials of high reflectance.
(5) various other possible solutions are there to increase reflectance, we have to find out most suitable of them.
* pankaj shrivastava. Email acfpankaj@gmail.com
Friday, August 21, 2009
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