The population of India is expected to reach about 160 crore by 2051 before the 'steady-state' level is reached.
The total population, which stood at 102.7 crore in 2001, "is likely to be in the vicinity of 140 crore by 2026 and could well approach 160 crore by 2051," Prof Tim Dyson of the London School of Economics and Political Sciences says in his study on India's population growth in the 21st century.
Around half of the growth in total population during the period, Dyson says, is likely to occur in the northern states of Bihar, Madhya Pradesh, Rajasthan and Uttar Pradesh.
"The populations of these states will increase by 45-55 per cent during 2001-26. Uttar Pradesh and Uttaranchal will have about 27 crore people by 2026.
"The future fertility declines in these four states will be particularly crucial in determining by how much India's population will grow in the decades beyond 2026," he notes.
Dyson, a professor of population and development in the Development Studies Institute of LSE, came up with these figures at a seminar on 'India in the 21st century', organised by the Indian Institute of Management (Calcutta) and LSE.
The projections are based on official data and census reports. They "employ what some would regard as fairly optimistic assumptions about the future demographic progress - namely the total fertility rate will fall to around 2.1 per cent births per woman by 2016-21."
Noting that alternative projections suggest that the population could reach 170 crore by 2051 and still be growing, Dyson says, "The only way a population of 150 crore will not be reached is through an event such as an unexpectedly severe HIV/AIDS epidemic or perhaps, a full-scale nuclear war."
The overall decline in fertility rate over the last few decades, however, suggests that the population growth after 2026 would be modest and could reach a steady-state level around 2051.
Dyson cautions that the population growth would put unprecedented burden on 70 major cities, worsen the employment scenario and the environment.
The good news is that the productivity of the agriculture sector would match the demand of the growing population through an increase in gross harvested area and diversification in food consumption patterns.
Another positive factor is the 'demographic bonus' that results when the labour force grows faster than the dependent population of younger people, permitting increased savings and investment, he says.
He predicts life expectancy to become 67 years for males and 70 years for females two decades from now.
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