If Singapore wants to prevent a decline in its citizen population from 2025, it will need 20,000 to 25,000 new citizens each year.
That is assuming no big increase in the number of Singaporean babies born here.
The Total Fertility Rate (TFR) is now 1.2, one of the lowest in the world. If it stays as it is and Singapore shuts out new migrants from this year, the citizen population will start shrinking in 13 years' time.
This means the pool of working age citizens will also drop steadily from today's 2.1 million to about 1.5 million in 2060.
These are some of the scenarios highlighted in a paper that the National Population and Talent Division (NPTD) released on April 24.
The five scenarios drawn up by NPTD, using data from the Department of Statistics, are based on the following assumptions: TFR is raised to 2.1 with immediate effect and no immigration.
TFR stays at 1.2 and the number of new citizens each year is zero.
TFR remains at 1.2 and there are 15,000 new citizens per year.
TFR remains at 1.2 with 20,000 new citizens per year.
TFR remains at 1.2 with 25,000 new citizens per year.
A comparison shows that only with an annual injection of 20,000 to 25,000 new citizens a year can the citizen population size be kept at a constant level of four million. In all other scenarios, the total number of citizens will dwindle.
From 2007 to 2010, Singapore's intake of new citizens ranged from over 17,334 to 20,513. Last year saw 15,777 additions to the citizen population.
The citizen population does not include permanent residents or foreigners.
The paper shows that the entry of new citizens into Singapore will supplement the shortfall in births and mitigates the decline in population, while also slowing down the decline in the pool of working-age citizens as well as the rate of ageing.
A key point from the paper is that, regardless of which scenario comes to pass, Singapore's citizen population will continue to age.
In the best-case scenario, median age rises from 39 in 2011 to 42 in 2060. In the worst case, it jumps to as high as 55.
From now to 2030, Singapore will also see "an unprecedented age shift, as over 900,000 baby boomers will retire from the workforce and enter their silver years", NPTD said in the paper.
The NPTD is releasing information to spur study and discussion in the lead-up to a White Paper on population matters, due by the end of the year.
It has launched focus group discussions with different segments of the community. From the middle of the year, it will engage the public in a variety of avenues, including dialogues and online channels, to discuss population issues holistically.
"Through this process, we hope to bring to light issues that are important to Singapore and Singaporeans, and develop a shared understanding of our strategies to build a sustainable population that secures Singapore's future," the NPTD told The Straits Times.
Members of the public may access the paper and give their comments at www.nptd.gov.sg