Rajbir Saha
(rajbirsaha1995@gmail.com)
The IPCC Fourth Assessment Report 'Climate Change 2007', based on the accumulated knowledge of scientific evidence about climate change, confirmed the GHGs induced climate change. In general, climate change would lead to a rise in air and ocean water temperature, melting of ice and snow, more extreme events (storms, cyclones, floods and droughts) and a rise in average sea level. About climate change and water resources, the report has underscored several important aspects.
It is common knowledge that scientists across the world are working on renewable sources of energy, particularly in the context of climate change. Equally important, or perhaps more important, is the search for technological breakthroughs for restoring renewable components of freshwater because water is essential for human civilization. A brief explanation of the hydrological cycle is important for understanding the supply and demand for freshwater. Nearly seventy per cent of Earth's surface is the sea. The solar heat energy reaches the Earth's surface and gets converted into latent heat/water vapours from the seawater, wet soil, rivers and lakes, through evaporation and from plant leaves by transpiration. Evaporation from the sea leaves behind salt. The water vapours reach the upper atmosphere where they get condensed and return to the Earth as precipitation. There is more evaporation than precipitation over the sea and the reverse is true over the land as precipitation is more than evaporation. The difference between precipitation and evaporation on land is the additional water available to feed the lakes, streams, springs and groundwater. The balance is the run-off back to the sea. The water that falls into oceans does not constitute a water resource.
According to the UNESCO Water Report, the land surface of the Earth receives annually on an average 42,700 cu km3 of water (precipitation – evaporation). This is the amount of water renewed annually and constitutes the basic source of freshwater. In the long run, the stock of water available in surface reservoirs and groundwater should be treated as reserve water. The main function of the reserve water is to stabilize the water supply due to the variability of run-off water over time. The global water demand is around 3750 km3 /year and constitutes only 8.4% of the annual freshwater resources. This is suggestive of the availability of sufficient global freshwater supplies. However, the devil lies in the details. The interplay of excessive mal-distribution of world run-off water sources and the anthropogenic causes, mainly climate change, have intensified water scarcity. The global stock of water on the Earth's surface is a very large volume of water: 1.386 million cubic km. However, 97.5 % of this stock of water is saline water in the seas, and only 2.5 % is freshwater. Of this stock of freshwater, 68.7 % is in the form of ice and snow cover in the Antarctic, Arctic, and mountainous regions. Further, 29.6% exists as fresh groundwater, and only 0.3 % of the available freshwater is in lakes, reservoirs, and rivers and is accessible for human requirements. The distribution of global freshwater across continents is highly skewed. Asia and South America receive 31.6 % and 28.1 5% of freshwater annually. Europe and Australia with Oceania receive the smallest volume of annual freshwater at 6.8 % and 5.6 %. North America and Africa receive 18.4 % and 9.5 % of the annual volume of freshwater water. The percentage of the world population that these six continents inhabit is as follows: Asia – 59.76 %; South America 5.53; Africa – 16.76 %; North America – 4.75; Europe – 10.7%; and Australia and Oceania – 0.54%. The distribution of freshwater is skewed not only about the continents but also between the Northern hemisphere (developed countries) and the Southern hemisphere (developing countries) divide. The South has about 82% of the world population and approximately only 68% of the world freshwater supply annually. The North has about 18% of the world population and 32% of the world freshwater supply annually. There are three important reasons for the increase in the consumption of fresh water in the future, particularly in the South. First, the population of the world is increasing and is likely to increase by 2 million by 2050. In other words, the current population is 7.7 billion that would increase to 9.7 billion by 2050. And it would further increase to 11 billion by 2100. The increasing population is leading to extra consumption of water. In addition, the per capita consumption of water is increasing. Second, the increase in population would require increased agricultural production of food, and this would require additional consumption of water in the agricultural sector. Third, an increase in population would also lead to an increase in energy consumption and production in the industrial sector and that would also require extra consumption of water. It was estimated that the total global water withdrawal in 1995 was about 3750 cu km3 /year.
The actual consumption was about 2,270 cu km3 /year, and this annual consumption amounts to 61% of the annual water withdrawal. The scientific assessment projected that the water withdrawal would grow by 10-12% every ten years, and would reach approximately 5,100 cu km3 /year by 2025, representing a 1.38-fold increase in water withdrawal. In addition, water consumption would also increase by 1.26 times due to an increase in per capita consumption, world population, agricultural production (mainly food), and industrial production (mainly energy). Another scientific report of 2000 extrapolates that the global water requirements would grow from the current 4,500 cu km3 in 2000 to 6,900 cu km3 per year by 2030. In 2000, the global water availability was only 4,100 cu km3, and the consumption was 4,500 cu km3. The shortfall was made up perhaps by overdrawing groundwater. As a result, the groundwater level is falling across the continents. In sum, the global water supply would decline with global warming and the demand would go up due to an increase in population, increase in per capita consumption of water, increase in food production, and increased water requirement in the industrial sector for increased production of energy. Climate change impinges upon water security and poses a serious threat to humanity.