Environmental Engineering Reference
In-Depth Information
sessment, Elizabeth Weatherhead described Arctic ozone depletion as resembling the type
of Swiss cheese that is riddled with bubbles. Overall, average ozone levels in the Arctic
spring remain comparable to 1990 levels. We will return to the nature of Arctic depletion
events later in the present chapter and again when we look at climate change in the Arctic.
If the Montreal Protocol has arrested erosion of the ozone layer, when can we expect a
full recovery to pre-1970 values? It will not be soon, but the short answer is that by the end
of the present century, the layer should have recovered. This is assuming that the protocol
continues to be implemented and is allowed to respond positively to any further scientific
findings on the behaviour of ozone-depleting substances.
There are several reasons that make a general statement such as this very difficult.
The first is rather obvious: It depends on latitude. The second is that ozone-depleting sub-
stances are greenhouse gases. In addition, a healthy ozone layer absorbs ultraviolet radi-
ation. Our depleted ozone layer is allowing more of this highly energetic radiation to reach
Earth's surface and to warm the troposphere. In other words, ozone-depleting substances
are directly and indirectly contributing to global warming. It is ironic that by taking action
on ozone-depleting substances, the Montreal Protocol has probably achieved much more
concrete action to slow global warming than has the Framework Convention on Climate
Change and its accompanying Kyoto Protocol!
Global warming is relevant to projecting timetables for ozone recovery because it im-
pacts the rate of this recovery. This is where things become rather complicated. As the tro-
posphere warms, the stratosphere cools. The WMO has reported that between 1980 and
1995, the global mean lower stratosphere cooled by 1-2°C and the upper stratosphere by
4-6°C. At the same time, modelling studies and new observations suggest that the strato-
spheric circulation (known as the
Brewer-Dobson circulation
) is increasing. It is believed
that such acceleration will increase the poleward transport of ozone away from the tropics
(where most ozone is produced), resulting in decreased column ozone levels in the tropics
and increased levels elsewhere.
Global warming can also lead to tropospheric changes that are expected to enhance
stratospheric ozone levels. For example, troposphere warming accelerates the processes of
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