Climate
Change
Ozone and Solar UV-B over India
Global climate change is not
uncommon in the long history of our planet. What will be uncommon is the
extremely fast rate of change over the next few decades, as predicted by
computer modelling studies.
The Chemical composition of
the atmosphere is rapidly changing on a global scale, due to the cumulative
impact of human activities. Concentrations of trace gases, including carbon
dioxide (CO2) methane (CH4), nitrous oxide (N2O)
and CFCs, are rising. These trace gases are all greenhouse gases and
contribute to global warming. They also deplete the ozone layer, most
dramatically to date over the Antarctic.
The bulk of the trace gas emissions (75 percent) come from the developed
countries, yet owing to their long atmospheric life, their effects will be
global, irrespective of their geographical origins. The developing countries,
because of resource constrains, will be most severely affected by global
climate change.
The sun emits radiation over a broad range of wavelengths, some of which the
human eye perceives. The solar ultraviolet wavelengths that cannot be seen
with the naked eye are classed as UV-A, UV-B and UV-C, in decreasing order of
wavelengths. The atmosphere absorbs virtually all UV-C, and will continue to
do so under all foreseeable circumstances, even if the ozone in the atmosphere
decreases by 50 percent. UV-B radiation is partially absorbed by ozone, but
UV-A is not absorbed at all.
If ozone concentration in the stratosphere (the middle atmosphere which, over
the tropics, exists at 18-50 km above the earth’s surface) decreases by even
as little as one percent, UV-B radiation can be expected to increase by two
percent. This would result in an increase in the incidence of skin cancer,
cataracts and other eye diseases. Scientific evidence suggests that UV-B
radiation will also have an effect on the human immune system. Leading
scientists believe that the effects on plants and micro-organisms may be
potentially more serious than the direct effects on human health. Many crop
varieties, for instance, are very sensitive to ultraviolet radiation.
Ultraviolet-induced reduction in plant yields, of both crops and wild species,
may have profound consequences for the global food supply.
|
Air
quality would be further degraded, too, since the reduced
photosynthetic activity of plants would push up carbon dioxide levels
in the atmosphere |
In India, not
much work has been done in understanding the scientific causes, impacts and
response strategies of these global changes. Development Alternatives has
therefore established a Global Environment Group to undertake this type of
study. The first task has been to understand and assess the magnitude of the
problem in the Indian context.
An initial assessment of atmospheric ozone and solar UV-B over India has been
taken up with ozone data between latitude 100N and 340N,
spanning a 25-year period between 1964-1988. Data of UV-B measurements over
New Delhi during 1985-1987 have also been utilised. A scientific tool has been
developed to derive UV-B from ozone data over any station. The development of
such a tool was necessary to fill the UV-B data gaps and to utilise India’s
long ozone data series covering the entire country.
The focus on ozone data over the period 1964-1988 was for two reasons. A long
series of well-calibrated ozone data was required. Moreover, the series
contained two complete solar cycles, thereby minimising the effects of natural
variability, including 11-year solar cycles. The salient findings of this
study are summarised in the following paragraphs.
Ozone Over India
In India, the ozone
layer is thinnest over the southern part of the country during December,
January and February, resulting in higher UV-B radiation reaching the earth
surface in this region during winter. The data also shows that the ozone layer
is thinner over south India than over north India. During the post-monsoon
(autumn) season levels, or no change, over south India. Ozone levels tend to
decrease over the northern part of the country in winter.
UV-B Over India
UV-B radiation in
general is highest over the entire country (south or north) during the month
of June. But the shorter wavelengths of UV-B, the most harmful to life on
Earth, are highest during the month of April. This may mean that India is most
vulnerable to UV-B radiation during April and June. UV-B levels over the
entire country are highest during the monsoon season and lowest during winter.
One of the reasons may be that the presence of pollution, aerosols and
turbidity, which tend to reduce incoming UV-B, is comparatively low following
rain. Total ozone is typically less over hill stations than over the plains at
the same latitude.
In contrast, UV-B over New Delhi and adjoining areas is likely to be reduced
by the heavy pollution, aerosols, turbidity, vehicular pollution, and dust
from Rajasthan. These features are reflected in the annual trends of UV-B over
these stations.
Development Alternatives has established a Global Environment Group to
understand the scientific causes, impacts and response strategies in the
Indian context.
Other important findings :
Relationship between
ozone and UV-B
An inverse relationship
exists between ozone and UV-B, particularly so at its longer wavelengths ,
during most months over the entire country. A decrease in ozone will certainly
result in an increase in solar UV-B reaching the earth’s surface, more so if
the sky is free from turbidity and suspended particles.
Environmental
implications of ozone and UV-B over India
Considerable
variability of ozone layer thickness is possible during certain weather
conditions, such as the Western disturbances, which pass over the western
Himalayan region during winter. On some occasions, ozone over Srinagar was
depleted by over 14 percent with the passage of the passage of the Western
disturbances, resulting in higher levels of UV-B reaching the earth’s
surface.
In winter, frequent weather disturbances over the western Himalayan region
could have serious environmental implications. The hill stations, with less
ozone cover, are especially prone to increased UV-B penetration and the
adverse impacts on human and plant life, and air quality.
Impact on agriculture
The impact on
agriculture may be severe during the monsoon months when the sky is
predominantly overcast. The reasons are not fully understood, but it may be
analogous to the workings of the human eye. Experiments indicate that, under
low levels of visible light, UV-B sensitivity increases. At low visible light
levels the eye makes maximum use of available light, allowing maximum
radiation to enter the eye. This subjects the eye to damage by the
unexpectedly high levels of UV-B.
Similarly, a plant may also be damaged in low light situations, being
"programmed" through evolution to expect low levels of ultraviolet
radiation. The plant has not developed mechanisms against UV-B and so is
adversely affected. This phenomenon could have widespread consequences for
crops, particularly during the monsoon months when it is overcast.
These are some of the preliminary findings of Development Alternatives’
study. Detailed analysis is still continuing. Surveys are planned in
localities which have high ambient rates of UV-B, to attempt to establish a
link between UV-B and its effects on human health and food security. Other
future work by the Global Environmental Issues Group will focus on global
warming, tropospheric ozone, and the socio-economic impacts of sea-level rise
in coastal regions of the country.
By K. Chatterjee