CLIMATE CHANGE
Topic:
There is a firm belief that climate has changed due
to human activity. Discuss this hypothesis in the
Zimbabwe context. Use droughts in Zimbabwe as
illustrations.
INTRODUCTION
The subject of climate change is one in which there is no agreement on whether climate is
changing or not. The same applies for the causes of climate change. A lot of evidence from
the study of past climates, like pollen in deep ice caves, geological features, documents on
floods, yields and drought, carbon dating, oxygen isotope analysis, dune formation,
glaciological evidence, dendrochronology and geomorphological observations show that
climate is changing. On the other hand, the school of thought that believes climate is not
changing considers the current changes as small cycles in large cycles which smooth
themselves out over a long period to effect no change in climate.
The view that climate is changing will be supported in this document, taking into account
that climate is average weather conditions for a period of at least 30 years. Causes of
climate change can be divided into two parts namely changes due to natural variability and
changes due to human (anthropogenic) activities.
Climate change due to natural variability
The most popular theories of changes due to natural variability include the following:
(a)
Variations in solar output.
(b)
Milankovitch Theory (variations in earth's orbit) and
(c)
Volcanic eruptions.
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Variations in the solar output of 0.1% can occur, this was noticed in 1980 to 1986. The real
causes of this variation is not known. One famous theory to explain this variation is the
sunspot activities. Studies have shown that sunspot maxima occur at 11 year cycles and the
1980-86 decrease in solar output is linked to a sunspot maxima. This decrease in solar
radiation can cause climate change.
Variations in the earth's orbit were studied by Milankovitch in 1930. The changes are due
to:(i)
change in the shape of the earth's orbit (eccentricity) which has a periodicity of
100 000 years.
(ii)
wobbling of the earth on its axis of rotation periodicity 22 000 years
(iii)
changes in the tilt (obliquity) of the earth's axis which is presently at
23periodicity 41 000 years.
The mentioned orbital changes affect the
distribution of the amount of solar energy on the earth-atmosphere systems and
hence cause climate change.
Volcanic Eruptions:
Violent volcanoes inject large quantities of dust and ash in the
atmosphere and these alter the earth's radiation balance, for example, Mount Pinetubo in
1992, Krakatoa (1883) and Katumani (1912). These were followed by cooler weather for 23 years.
Climate change due to Human Activities
The theories for climate change due to anthropogenic activities include:(i)
greenhouse effect (increasing levels of CO2 and other greenhouse gases).
(ii)
surface modification that is deforestation, farming practices and environmental
management, and
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(iii)
changes in the distribution and concentration of dust or solid particles in the lower
atmosphere for example, nuclear war products. Soot from war products absorb solar
radiation and this would cool the earth's atmosphere.
Prior to the Industrial Revolution, the concentration of greenhouse gases such as CO2,
Methane CH4 and nitrous oxide was fairly constant and maintained the earth at a stable
temperature. However after the revolution, industrialisation and increased population have
markedly increased these gases. In 1987 CO2 concentration was 345 ppm and by 1990 it
had gone up to 351 ppm and it is projected to be at 560 ppm by 2030 (Miller, 1996).
Models predict that the levels of CO2 concentration will double by the next century and this
will cause an increase in the global mean temperature of 3C to 4C. This will have
disastrous effects like:
(i)
increased droughts and floods
(ii)
changes in sea-levels from melting of ice caps.
(iii)
shift of climatic zone and increasing health problems just to mention a few.
WMO in 1988 established IPCC to assess present scientific knowledge, socio-economic
impacts and find options for policy responses. Surface modifications include deforestation,
farming practices and environmental management practices. These change the albedo of the
earth and cause radiation imbalance that will cause climate change.
The school of thought which does not believe that climate is changing dismiss the current
global warming as part of a natural alternate warm and cold episodes. Climate has been
warm and cold in the past they say. For example between 700 and 5000 years ago, the
climate was 2-3C warmer than today. This period is referred to as the climate optimum
whose warm conditions allowed life on earth to develop. The continuous increase of CO2 in
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the atmosphere is dismissed on the idea that alternate high and low levels of CO2 resulted in
periods between the ice ages and periods without ice ages. So this is argued to be a natural
cycle dismissing the effect of human induced carbon dioxide as insignificant.
Drought in Zimbabwe will be used in this essay to illustrate that climate is changing. There
are many definitions of drought non of which is universal. However, three main definitions
of drought have been used namely Meteorological drought, Hydrological drought and
Agricultural drought. Meteorological drought is when there is well below evaporation in a
large area for an extended period. Hydrological drought occurs when there is sustained
deficits in surface run-off below normal conditions or when there is depletion of ground
water level. Agricultural drought occurs when rainfall amounts and distribution, soil water
reserves and evaporation losses combine to cause crop or livestock yields to diminish
markedly.
Drought can be defined in terms of a number of meteorological and
agrometeorological factors, but rainfall had been found to be the most important single
factor.
Before a study of drought in Zimbabwe, it is appropriate to list down the synoptic features
which generate the weather over Zimbabwe. The main features are:(a)
ITCZ ( Inter-tropical Convergence Zone).
(b)
Tropical Cyclones.
(c)
Westerly and easterly waves.
(d)
Infusion of middle level air into the north west of Zimbabwe.
(e)
Indirect effects of cold frontal systems.
(f)
Botswana Upper High (BUH) and development of upper and surface vortices.
The failure of each of the factors to develop fully will lead to prolonged dry weather or wet
weather depending on whether the factor is for active weather generation or dissipation. For
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example prolonged and intense BUH give a drought, and prolonged intense ITCZ give very
wet conditions.
The causes of drought in Zimbabwe can be given as persistence BUH, below normal
incursions of a well developed ITCZ, too many tropical cyclones, causing diffluent flow
over Zimbabwe, below normal incursions of easterly and westerly cloud bands, abnormally
northwards displacement of the subtropical high pressure belt in summer and the El Nino
Beside the El Nino and the factors mentioned above, drought can be caused by
teleconnection events like anomalously
warm sea surface temperatures(SST)over the
Equatorial Pacific, Easterly phase Quasi-Bienal Oscillation(QBO),very low Southern
Oscillation Index(SO) and unfavourable wind characteristics in winter just to mention a
few.
Anthropogenic activities like deforestation, farming, land use and environmental
policy practices also contribute to drought indirectly. These activities make the effects of
drought more severe and can even delay its cessation making droughts more frequent. For
example take bad land use practices like cultivation on river banks, source and cutting down
of trees. This will lead to soil erosion, river and dam siltation, e.g. Save area is getting drier
and drier. This cuts down the amount of water available for evaporation and hence less
cloud formation which can cause or modify drought. Deforestation, increased use of fossil
fuel can intensify the severity of droughts by altering the energy budget resulting in less
convection and CO2 from fossil fuel can give global warming with the possible effects
discussed earlier. Forests and oceans are a sink to carbon dioxide so deforestation will
encourage which in turn cause droughts.
A brief review of the relationship between El Nino and drought in Zimbabwe will be made
and this will be related to climate change. In studies of drought it should be borne in mind
that drought is a natural phenomenon and therefore part of the climate. A study by Dr.
Makarau in his PH.D thesis for the period 1901 to 1992 revealed that drought years were
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1912, 1914, 1916, 1922, 1924, 1947, 1960, 1964, 1968, 1973, 1982, 1983, 1987 and 1992.
A total of 14 cases. 1991/92 was the worst drought with annual average rainfall of 335 mm
followed by 1946/47 with 365 mm. In terms of severity, 1982/83 was the worst, this was
because two successive droughts had occurred. In this study there were 45 El Nino
Southern Oscillation events both low and high. The correlation between high El Nino
events and drought was very high, about 53%. It is clear that Zimbabwe's droughts are
more associated with high ENSO events than low ENSO events.
A time series analysis of rainfall, (see Fig 1 ) from 1901 up to 1992 which show a general
decreasing trend in rainfall which is given by;
R=698 - 0.8T where R = rainfall, T is number of years after 1901.
From this about 100 mm reduction in precipitation have occurred (Makarau, 1992 Ph.D
Thesis). See Fig 2 for rainfall time series and El Nino ENSO events. From this study it is
clear that climate is changing from the decrease in rainfall. At the moment it is difficult to
say whether the change is from natural variability or human induced. See Fig 3 for the
global distribution of adverse effects during the El Nino episodes. Figures 1 and and
figure 2 enclosed indicate Zimbabwe's El Nino events with the Southern Oscillation Index
(SOI) for the periods 1951 to 1996 and 1901 to 1996 respectively.
A time series analysis of maximum, mean and minimum temperatures (see figures 9, 10,
11) reveal that from 1897 to 1988, the three temperature variables show an increasing trend
for Bulawayo, Harare and Chipinge.
The warming trend is more pronounced over
Bulawayo and over drier areas of Zimbabwe as compared to Harare and Chipinge. The
higher warming trend over the dry areas might be due to deforestation and the
encroachment of Kalahari desert. Desertification rate is dependent on land use practices so
this increased rate of warming is a good example of human induced climate change. A study
by Mr. Chipindu in a paper entittled “Green House effects Over Southern Africa” revealed
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that mean temperatures have been rising from 1897 to 1988 with rates varying from 0.3 to
0.5degrees Celcius and increased to 0.7degrees from 1977. In the same paper abnormally
warm years were found to be related to drought. Rainfall also was found to decrease
generally from 1897 up to 1988. More research is needed to conclude that this warming
and decrease in rainfall is due to human or natural causes.
It is clear that the climate is changing but whether the changes are temporary or most will be
known as time goes on. The causes of climate change comprise of natural variability and
anthropogenic activities. As for drought it is agreed that human activities do not cause
drought but make it more severe and more frequent. If drought occurs without any adequate
mitigation measures in place, the negative impacts of this drought will be worsened.
REFERENCES:
1. P.D Tyson (1987); Climate change and variability in Southern Africa;Oxford Press
Cape Town.
2. G.Miller (1996); Living in the environment. principles, connections and solutions
9th Edition.Nadsworth Publishing Company,Belmont Calfornia
USA.
3. B.Bolin,R.Doos,J,Jager and A.Warrick (1989);The Green house Effect ,Climate
Change and Ecosystems; John Wiley and Sons ;Toronto
Canada.
4. IPCC (1995) ; Climate Change ;WMO.
5. IPCC (1990); Climate Change; WMO.
6. Makarau and Marume ;Drought management in Zimbabwe;The 1981|82
Experience.
7. B.Chipindu (1994);Green House Effect Over Southern Africa,Nairobi Kenya.
8. C.Matarira (1989);Drought over Zimbabwe in a Regional and Global Context
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9. A. Makarau and M. Jury (1997); Seasonal cycles of convective spells over southern
Africa during Austral Summer.
9. Impact No 5 June 1992;Newsletter of climate Netwrk; Climate Change by Impact
Team.
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