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Du er her: Skole > Global environment: Ozone hole

Global environment: Ozone hole

Ozonhullets utvikling, hva kan man gjøre for å forhindre dette?

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Engelsk
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26.09.2006


The influence of the human race on climate is still a matter for study and speculation, but the ability to perturb the ozone layer is an established fact.

 

“For nearly a billion years, ozone molecules in the atmosphere have protected life on Earth from the effects of ultraviolet rays. In the past 60 years or so human activity has contributed to the deterioration of the ozone layer.” (direct from the papers)

 

Chemical reactions on the surfaces of ice crystals in the clouds release active forms of CFCs. Ozone depletion begins, and the ozone hole appears. The “hole” is not technically a “hole” where no ozone is present, but a region with thinner ozone in the stratosphere.

 

Over the course of two to three months, approximately 50% of the total column amount of ozone in the atmosphere disappears. At some levels, the losses approach 90%. This has come to be called the Antarctic ozone hole.

 

However, less-well-known is that ozone depletion has been measured everywhere outside the tropics, and that it is, in fact, getting worse. in the middle latitudes (most of the populated world), ozone levels have fallen about 10% during the winter and 5% in the summer. Since 1979, they have fallen about 5% per decade when averaged over the entire year. Depletion is generally worse at higher latitudes, i.e. further from the Equator.


 

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Background

The ozone hole over Antarctic were discovered by British scientist in 1985. It appears every year in September and October and it manifests itself by a strong thinning of the ozone amount between 14 and 20km. Through November and December the hole breaks up, and one returns to a normal state. The hole is caused by manmade emissions wich decay the ozone. The atmospheric concentrations of these compounds have increased during the last several decades as a consequence of human activity.

 

Model calculations that take into consideration the coupling between ozone and climate show that the situation will deteriorate the next 10-20 years before one can expect any improvement.

 

In 2002 the Antarctic vortex split in two parts and the degree of ozone loss was more modest than for many years. This was caused by dynamics rather than chemistry.

 

The 2003 ozone hole size and persistence developed similarly to the year 2000, with an early rapid growth observed during August, a record size observed in September and finally its disappearance in mid November.

 

The 2004 ozone hole continues to be much smaller than the average size over the past decade, and for late October, nearly as small as it was in 2002 after the ozone hole had split into two parts in late September. This year, the temperatures sufficiently low for the formation of PSCs ended about 2 to 3 weeks earlier than in most recent years.

 

The 2005 south polar vortex was close to the average of the last decade in terms of vortex area and temperature conditions. However, minimum temperatures inside the vortex were in early September near the coldest recorded since 1979. The area where total ozone was less than 220 DU (also called "ozone hole area") was larger than ever before, at that time of the year, during the first two weeks of August. During the last two weeks of August and the first three weeks of September the ozone hole area continued to increase at a pace close to the average of the last 10 years.

 

Why does the ozone hole form over Antarctica?

The answer is essentially 'because of the weather in the ozone layer'. In order for rapid ozone destruction to happen, clouds (known as PSCs, Stratospheric Clouds Mother of Pearl or Nacreous Clouds) have to form in the ozone layer. In these clouds surface chemistry takes place. This converts chlorine or bromine (from CFCs and other ozone depleting chemicals) into an active form, so that when there is sunlight, ozone is rapidly destroyed. Without the clouds, there is little or no ozone destruction. Only during the Antarctic winter does the atmosphere get cold enough for these clouds to form widely through the centre of the ozone layer. Elsewhere the atmosphere is just too warm and no clouds form. The northern and southern hemispheres have different 'weather' in the ozone layer, and the net result is that the temperature of the Arctic ozone layer during winter is normally some ten degrees warmer than that of the Antarctic. This means that such clouds are rare, but sometimes the 'weather' is colder than normal and they do form. Under these circumstances significant ozone depletion can take place over the Arctic, but it is usually for a much shorter period of time and covers a smaller area than in the Antarctic.

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