Global Climate Change
An important descriptor of climate is temperature.
Sunlight heats up the sea and land. The warmed
surface of the earth then radiates heat back towards
space. On its way out, some of this heat (infrared
radiation) is absorbed by trace gases in the
atmosphere, notably CO2 and water vapour, and thereby
keeps the earth's temperature suitable for life.
Without this natural greenhouse effect of CO2 and
water vapour, the temperature at the earth's surface
would be some 33 Celsius cooler than it is today, i.e.
below the freezing point. The natural concentration
of CO2 in the atmosphere is controlled by the
interactions of the at mosphere, the oceans and the
biosphere in what is known as the geochemical carbon
cycle. Human activities can disturb this cycle by
injecting carbon dioxide into the atmosphere. This
leads to a net increase in carbon dioxide
concentration in the atmosphere, which enhances the
natural greenhouse effect.
It had been thought that CO2 was the only
greenhouse gas. However, research over the last two
decades has identified other gases such as nitrous
oxide, methane, chlorofluorocarbons and tropospheric
ozone as potential greenhouse gases.
The atmospheric CO2 concentration is now 353 parts
per million by volume (ppmv), 25 per cent greater
than the pre-industrial (1750-1800) value of about
280 ppmv, and it is currently rising at about 0.5 per
cent per year owing to anthropogenic emissions. The
latter are estimated to amount to about 5,700 million
tonnes of carbon per year due to fossil fuel burning,
plus 600-2,500 million tonnes of carbon per year due
to deforestation. Between 40 and 60 per cent of the
CO2 emitted into the atmosphere remains there, at
least for the short term; the rest is taken up by
natural sinks, particularly the oceans but also
forests. Future atmospheric CO2 concentrations depend
on the amounts of CO2 released from fossil fuel
burning, which will be determined by the amount and
type of energy sources to be used; the CO2 released
from biotic sources, which is determined by the rate
of future deforestation and changes of other
vegetative cover; and the uptake of CO2 by various
natural sinks. The Intergovernmental Panel on Climate
Change (IPCC) has estimated that if anthropogenic
emissions of CO2 could be kept at present-day rates,
atmospheric CO2 would increase to 460-560 ppmv by the
year 2100 because of the long residence time of CO2
in the atmosphere and the long lead-time for its
removal by natural sinks.
Over the past 100 years, the atmospheric CO2
concentration increased by about 25 per cent. A range
of model calculations suggests that the corresponding
equilibrium temperature rise should be 0.5-1.0
Celsius. If this is corrected for the effects of the
thermal inertia of the oceans, which slows down
climate change for a period of 10-20 years, the
changing composition of the atmosphere should have
produced a warming of 0.35-0.7 Celsius superimposed
on the natural fluctuations of the atmosphere.
Detailed analysis of temperature records of the
past 100 years indicates that the global mean
temperature has risen by 0.3-0.6 Celsius. Much of the
warming since 1900 has been concentrated in two
periods, the first between about 1910 and 1940 and
the other since 1975; the five warmest years on
record were all in the 1980s. The size of the warming
over the last century is broadly consistent with the
predictions of climate models, but is also of the
same magnitude as natural climate variability.
The main impacts of climate change are as
follows:
- Sufficient evidence is now available to
indicate that changes in climate would have
an important effect on agriculture and
livestock. Negative impacts could be felt at
the regional level as a result of changes in
weather (e.g. more frequent and more severe
storms) and the arrival of pests associated
with climate change, necessitating
innovations in technology and agricultural
management practices. There may be a severe
decline in production in some regions (e.g.
Brazil, the Sahel region of Africa, South
East Asia, the Asian region of the former
Soviet Union and China), but there may be an
increase in other regions because of a
prolonged growing season.
- Natural terrestrial ecosystems could face
significant consequences as a result of
climate changes. Their evolution would lag
ehind these climate shifts: they might
survive in their location but flora and fauna
could find themselves, in effect, in a
different climatic regime. These regimes may
be more or less h ospitable and could
increase the productivity of some species and
decrease that of others.
- Relatively small changes in climate can cause
large water resource problems in many areas,
especially in semi arid regions and in those
humid areas where demand or pollution has led
to water scarcity.
- Global warming will accelerate the rise in
sea level, modify ocean circulation and
change marine ecosystems, with considerable
socio economic consequences. The IPCC
predicted that under the business as usual
scenario, an average rise in the global mean
sea level of about 6 cm per decade could
occur over the next century. The predicted
rate would mean a 20 cm rise in global mean
sea level by 2030 and 65 cm by the end of the
century.
(Source: Unido and Industrial
Sustainable Studies)
