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IS INCREASING CARBON DIOXIDE COOLING OUR EARTH?
By
Dr. Nitish Priyadarshi
Carbon Dioxide
Carbon dioxide is probably the most important of the greenhouse gases as it accounts for the largest proportion of the 'trace gases' and is currently responsible for 60% of the 'enhanced greenhouse effect'. It is thought that it's been in the atmosphere for over 4 billion of the Earth's 4.6 billion year geological history and in much larger proportions (up to 80%) than today.
Most of the carbon dioxide was removed from the atmosphere as early organisms evolved photosynthesis. This locked away carbon dioxide as carbonate minerals, oil shale and coal, and petroleum in the Earth's crust when the organisms died. This left 0.03% in the atmosphere today.
The natural carbon dioxide cycle
Atmospheric carbon dioxide comes from a number of natural sources, mainly the decay of plants, volcanic eruptions and as a waste product of animal respiration.
It is removed from the atmosphere by photosynthesis in plants and by dissolving in water, especially on the surface of oceans.
Carbon dioxide stays in the atmosphere for approximately 100 years.
The amount of carbon dioxide taken out of the atmosphere by plants is almost perfectly balanced with the amount put back into the atmosphere by respiration and decay. Small changes as a result of human activities can have a large impact on this delicate balance.
The impact of human activities
Over the past three centuries, concentration of carbon dioxide is reported to be increasing in the Earth’s atmosphere, due to human activities like the burning of fossil fuels and deforestation that cause the release of carbon dioxide into the atmosphere.
Burning fossil fuels releases the carbon dioxide stored millions of years ago. We use fossil fuels to run vehicles (petrol, diesel and kerosene), heat homes, businesses and power factories. Deforestation releases the carbon stored in trees and also results in less carbon dioxide being removed from the atmosphere.
From the early 1700s, carbon dioxide has increased from 280 ppm to 360 ppm in 1990. many studies suggest that higher concentration of carbon dioxide in the atmosphere will enhance the green house effect making the earth warmer. It is likely that the thinning of ice masses, and retreat of glaciers of Himalaya and trans-Himalayan region during the last 150 years or so is an affect of such warming.
Most computer models suggest that the globe will warm up 1.5 degree centigrade to 4.5 degree centigrade if carbon dioxide reaches the predicted level of 600 ppm by the 2050. although this may be largely true, there are other possible mechanisms that could act in the opposite direction. For instance the sulphate aerosols-the tiny bit of dust that are also added to the atmosphere when fossil fuels are burned by human activity, may cool the climate. The aerosols reflect away the sun’s radiation. Thus, they partially counter the warming that may be caused by the greenhouse gases. However, the degree to which these emissions might reduce the impact of greenhouse gases is not yet fully understood.
Studies suggest that an increase in ocean temperatures associated with global warming could lead to increase in emission of dimethyl sulfide by photosynthesis. This would in turn increase the concentration of sulphate aerosols in the marine boundary layer causing an increase in the number of cloud drops. The net result of the cloud will be to cool the Earth to offset the warming by the greenhouse effect.
Sulphate aerosol is a type of solid compound commonly found in the atmosphere. Sulphate particles play an important role in reflecting, absorbing, and scattering incoming solar energy. The source of these compounds is both natural and man-made. Most of the man-made particles come from combustion of fossil fuels.
It is therefore possible that increased levels of trace gases in the atmosphere would lead to cooling of the Earth rather than to a net heating.

Reference:
• http://www.bbc.co.uk/climate/evidence/carbon_dioxide.shtml
• Mathur, U.B. 2005. Quaternary Geology Indian Perspective. Geological Society of India, Bangalore.