Second Aparesh Bhattacharya Memorial Lecture, 1997
Global Climatic Change and Its Impact on Biodiversity

Professor J.J. Ghosh
Department of Biochemistry, Calcutta University

The global climatic changes, due to unrestricted human activities have become a major scientific and political issue in the last few decades of the 20th century. The two major sources of the climatic changes are (a) the increase of green-house gases such as carbon dioxide, methane, nitrous oxide, chlorofluorocarbons etc. in the troposphere leading to global warming and (b) depletion of stratospheric ozone layer associated with increased exposure of UV-B radiations to the earth’s biosphere.

Global Warming and the Rising Atmospheric Carbon dioxide Level :

It has now been realized that green house gases namely carbon dioxide, methane, nitrous oxide etc. although occurring in ppm or ppb ranges, play a crucial role in maintaining the atmospheric temperature by regulating the escape of thermal radiations from the earth’s surface layers. In the absence of theses gases, earth’s temperature will go down to -18° C to form a frozen planet, whereas if the level of these gases be on the higher side, the ambient temperature of the planet will be too high to permit the existence of any life. As a matter of fact, climatologists are greatly concerned about the future of the planet earth because of the rising trend of atmospheric CO2 level from 280 ppm in 1750’s to 310 ppm in 1980 and 353 ppm in 1990. If the present trend continues, by the end of the 21st century, the atmospheric CO2 level on earth is expected to be double (above 600 ppm) and earth’s temperature is predicted to rise by 2 to 5° C, as revealed by the climate modeling data of the Goddard Institute of Space Studies (GISS) at New York and the Climate Research Unit (CRU) of the National Center for Atmospheric Research of Colorade, USA.

Uncertainty Factors in the Doubling CO2 Budgeting of the Earth :

Although the increasing human activities, associated with the burning of fossil fuels for energy purposes, deforestation and land use for agriculture and human settlements, are no doubt responsible for the mounting increase in CO2 level and warming up of the atmosphere, there are many uncertainty factors underlying the doubling CO2 effect. Doubling the CO2 level is going to affect not only the global temperature but also other physical phenomena, including rainfall, or precipitation, wind velocity, ocean currents, storm pathways, cloud cover, ice cap melting, sea level rise etc. Hence what remains controversial about the green house effect in the century is about.

(i) the rate of global warming
(ii) its regional distribution
(iii) what to do about the problem

Regarding regulation, policy matter decisions etc. unless the fuel spectrum of the earth’s CO2 budgeting is done with special reference to the role of air-land-ocean as the sources and sinks of CO2 , it is difficult to make any precise conclusion about the extent of global warming in the next century.

Earth’s Carbon Dioxide Budget During the Industrial Era :

A major emphasis has been placed on stock-taking the earth’s CO2 budget, in view of the international urgency for restricting the green house gas emission in future. Atmospheric CO2 budgeting not only helps to keep a track on the source-versus-sink interaction of the released CO2 but also to construct models for forecasting the future trends. Table 1 shows a typical picture of the annual CO2 budget for 1980-1989, as summarized by the Intergovernmental Panel of Climate Change (IPCC), an UN Organization, containing representatives from about the 100 nations in the world.

Table 1 : IPCC CO2 Budget of the Earth (1980-1989)
(Taken From Climate Change The IPCC Scientific Assessment, Cambridge University Press, 1990)

Reservoir of CO2 Average Flux Gigaton (GT) of C/year1GT=10gm
SourcesFossil FuelsDeforestation & Land Use

5.4 ± 0.5
1.6 ± 1.0
7.0 ± 1.2
SinksAtmosphereOceans (Steady State Models) 3.2 ± 0.1
2.0 ± 0.8
5.2 ± 0.8
Imbalance (Sources-Sinks) 1.8 ± 1.4

From the table, it will be evident that the amount of CO2 produced by human activities (combustion of fossil fuels and destruction of forests) significantly exceeds the amount of CO2 absorbed by the ocean and atmosphere. To account for the budgetary imbalance it is presumed that there is a so-called “missing sink” of CO2 (to the extent of 20-25%) existing somewhere in the earth’s terrestrial forest zones and soils which still needs identification. Another important point emerging from the budgetary data is that the industrial activities connected with energy consumption, are the major contributor as the sources of CO2. Thus international attention has been focused on minimizing or limiting this particular source of anthropogenic CO2 emission for stabilizing the CO2 budget of the planet earth. Although the budgetary data give an approximate picture of the source vs sink interaction of atmospheric CO2, there are several points of uncertainty – particularly regarding the ability of ocean to absorb the anthropogenic CO2 released in the northern and southern hemispheres and under varying conditions of oceanic drift, currents and temperature prevailing different regions, which are difficult to stimulate in the conventional steady state ocean model study – for which carbon cycling in the water has to be studied. Another area of uncertainty is the amount of CO2 released by changes in land use and deforestation – the magnitude of which is subject to variations from one region to the other on earth. The “Unaccounted CO2 sink” is also another important uncertainties in the CO2 budget. Indications have been made to numerous hypothetical regions of the earth. Very recently the Terrestrial Initiative in Global Environment Research (TIGER), an U.K. based environmental organization has tracked down the certain pockets in the Amazonian rain forest areas in Brazil which may serve as the exclusive CO2 sink (Nature 383, Octo 31, 1996p. 747). To make a realistic assessment of the Earth’s CO2 budget problem, one has to take into account the tremendous diversity in processes and feed backs that control and respond to atmospheric CO2 – it seems that both land and ocean have the intrinsic ability of responding dynamically, not passively, to man-made increase in atmo-increase in atmospheric CO2 levels.

Future Impact of Enriched Carbon Dioxide Atmosphere on the Agricultural and Plant


World

Though the future impact of green house warming due to the doubling of CO2 (2XCO2 ) level (expected towards the end of the next century) can not yet be modeled dependably, it may reasonably be assumed that the climatic change due to green house warming will be gradual and not abrupt and the impacts will more markedly be felt in the temperature zones and in the higher altitude regions of the earth than in the tropical regions. Using 2XCO2 climate simulation model developed by the Goddard Institute for Space Studies (GISS), New York, Several investigators have made many interesting findings about the future trends. Parry and Carton (1988) found that in the middle and high latitude regions where available water is not a major constraint, for example in Northern Europe and North Japan, Cereal Crop yields could increase, in North Japan, the yield of rice varieties would probably increase by 4% with the 35% increase in cropping seasonal days, whereas adverse yield effects of Canadian wheat production in the Saskatchewan region would be expected because of increased evapotranspiration loss in wheat plants due to warming conditions. Possible effects of climate change in doubled CO2 atmosphere will not only be felt on the crop yield pattern in different regions of the world but also on the geographical distribution of crops partly determined by the crop physiological responses and partly by the farm management strategies in future.
According to the plant physiologists and ecologists, the rising CO2 level in the atmosphere will have starting effect on the plant world. CO2 being the primary raw material used by plants in manufacturing carbohydrates and other constituents is expected to give a boost in plant growth at higher atmospheric concentration, although not all plants will respond alike. Photosynthesis efficiency in C3 plants (like rice, wheat etc.) will be markedly increased than the C4 plants (maize, sugar cane etc.). Plant growth in general will be stimulated by 30-34%. Scientists have found that plants growing amid higher CO2 levels are hardier in some respects – they use water more efficiently with minimum evaporation loss – because of the fewer stomatal openings in higher level of CO2 . Because of the spreading of the warming zones, shifts in the distribution of beach, brich, pines to further northward is expected to take place to remain in a suitable climate and habitat. Increased CO2 may alter the timing of pollination and flowering. Many of the endemic medicinal plant species from the Himalayan region may disappear. Along with the changes in the growth of insects and plant parasites which will be coming in larger number and varieties. Public health Scientists are also apprehending the greater emergence of vector borne diseases like malaria, encephalitis etc.

References

1. Jones P.D. and TML Wigley (1990) Global Warming Trends, Scientific American 263 (2) 84
2. Peters R L and D. Darling (1885) The Green house Effect and Nature Reserves Bioscicence 35(11) 707.
3. Post W.M. et al (1990) The Global Carbon Cycle, American Scientist 78, 310.
4. P.J. Michaels et al (1996) Human Effect on Global Climate, Nature 384, Dec. 1996p. 522.
5. Global Warming and Biological Diversity-ed. by R.L. Peters & T.E. Lovejoy (1992) Yale Univ. Press, New Haven + London.