When ozone's hit...

warming The ozone layer is a protective shield that filters out ultraviolet radiation. What happens when it is depleted? Getty Images

Global warming, the heavy penalty the earth may have to pay for filling the atmosphere with greenhouse gases, is feared to cause melting of Arctic ice, raise ocean levels and alter global climate. This would cause not only economic and demographic upheavals, but also affect biodiversity and life forms in the sea. 

According to a paper by Michaela Hegglin and Theodore Shepherd, of Toronto, Canada, in Nature Geoscience, the levels of ozone in the stratosphere and the intensity of ultraviolet light striking the earth would also change.

The ozone layer is known to shield the earth from high energy radiation. Had high levels of UV light been coming in, the survival of most living things and the beginning of life itself on earth would not have been possible. When it was found that this layer was getting depleted by the effect of chemicals called chloroflourocarbons (CFCs), the world got together (Montreal Protocol, 1989) to limit emissions.
 
Ozone and its uses
Ozone is a form of oxygen. Oxygen gas consists of two atoms of oxygen paired to form a molecule. The oxygen atom consists of a nucleus and 16 electrons in orbit around it. The electrons are distributed as 2,8 and 6, which is to say that there are 6 electrons in the outer orbit. Now, atoms are most stable when they have 8 electrons in the outer orbit and they tend to combine with other atoms so that they can, together, get as close to this ‘8 electron state’ as possible. Two oxygen atoms get together and ‘share’ 2 of their outer electrons, so that each of them can create an impression of 8.

Another form that oxygen can take is that 3 atoms get together – with one atom sharing 2 electrons with another, but the second also passing on one electron to a third. The last two atoms thus only approximate to 7 electrons, but this combination is a little stable too, and does get created when the conditions are right.

The useful feature of ozone is that it can absorb ultraviolet light at the frequency that is the most harmful. Ozone acts as a filter and is formed in good quantity by the action of UV light on oxygen. But ozone also combines with free oxygen atoms to form ordinary oxygen. This breakdown of ozone is slow in the stratosphere, the part of the atmosphere after the first 16 km and till 50 km from the earth. It is in this region that ozone accumulates. The trouble is that even in the stratosphere, the breakdown of ozone hastens when some chemical groups are present, particularly of CFCs present in aerosol sprays and refrigerants.  

Atmospheric currents
There are currents in the larger atmosphere, apart from winds and monsoons and cyclones.  The atmosphere has a peculiar structure; it grows cooler as one goes higher, for the first 16 km or so. This region is called the troposphere. The main process is that with the falling pressure as one goes higher, air expands and cools. But when one ventures higher, the effect of expansive cooling reduces and the warming due to the sun increases as also absorption of UV, and temperature rise takes over. The atmosphere is thus ‘stratified’ and this region is called the stratosphere. These two regions of the atmosphere are separated by a layer called the tropopause. One model of flow across the boundary, which also explains the accumulation of ozone in the stratosphere is the Brewer-Dobson circulation, which relies on the earth’s radiation equilibrium and the behaviour of waves in the atmosphere when they pass over deformities on the surface. When these waves break, they release energy, which propels air masses upwards. This movement is separate for each hemisphere and is more marked in the northern hemisphere, perhaps because there are larger land mass promontories in the north. It is this circulation that carries the ozone up to the stratosphere and then away to higher latitudes. The result is the distribution of ozone within the stratosphere and the concentration at higher latitudes, although the production is at lower latitudes.

Global warming  
Global temperature rise results in warming of the troposphere and cooling of the stratosphere. The boundary is 16 km high in the tropics but is only about 10 km high at the mid latitudes. The warming of the troposphere and the cooling of the stratosphere then create gradients across latitudes, which give rise to winds, breaking of waves and more Brewer-Dobson circulation.  
Hegglin and Shepherd, at Toronto, developed a model of climate and stratospheric chemistry to simulate the effect of climate change on ozone distribution. The significant finding is that there would be greater transfer of ozone from the stratosphere to the troposphere as the globe warms. Down in the troposphere, ozone becomes effective as a greenhouse gas. With its larger molecular structure than oxygen, ozone, like CO2 or methane, can store energy and adding this ozone to the troposphere would spur the pace of global warming.

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