(a) Each annuitant, who retired at age 55 or over and after the completion of at least 15 years of creditable service, whose status as an employee terminated before January 1, 1970, is entitled to a monthly supplemental annuity effective January 1, 1970, or on January 1 nearest the annuitant’s 65th birthday, whichever is later. Such supplemental annuity shall be 1-1/2% of the monthly retirement annuity, multiplied by the number of full years which elapsed from the date of the member’s latest retirement to the effective date of the supplemental annuity. This monthly supplemental annuity shall be increased on each January 1 thereafter during the lifetime of the annuitant by 1-1/2% of the monthly retirement annuity disregarding any supplemental annuity previously granted. Beginning January 1, 1972, the rate of increase in the supplemental annuity shall be 2%. Beginning January 1, 1979, the rate of increase in the supplemental annuity shall be 3%.
     The supplemental annuity under this subsection is payable only if the annuitant pays to the System, in a single sum, an amount equal to 1% of his monthly final average compensation multiplied by the number of full years of creditable service.
     (b) Any member who retired with less than 15 years of creditable service whose status as an employee terminated before January 1, 1970, shall be entitled to an increase of 3% of the original monthly retirement allowance, effective January 1, 1982, or on January 1 nearest the annuitant’s 65th birthday, whichever is later. On each January 1 thereafter during the lifetime of the member, he shall be entitled to an additional increase of 3% of the original monthly retirement allowance. No qualifying contribution is required for the supplemental annuity under this subsection.
     (c) Beginning January 1, 1990, all automatic annual increases payable under this Section shall be calculated as a percentage of the total monthly amount of annuity payable at the time of the increase, including any supplemental annuity or other increase previously granted under this Article.
     (d) Notwithstanding any other provision of this Section, all increases payable under this Section on or after the effective date of this amendatory Act of the 98th General Assembly shall be calculated as 3% of the lesser of (1) the total annuity payable at the time of the increase, including previous increases granted, or (2) $800 ($1,000 for portions of the annuity based on service as a noncovered employee) multiplied by the number of years of creditable service upon which the annuity is based.
     Beginning January 1, 2016, the $800 ($1,000 for portions of the annuity based on service as a noncovered employee) referred in item (2) of this subsection (d) shall be increased on each January 1 by the annual unadjusted percentage increase (but not less than zero) in the consumer price index-u for the 12 months ending with the preceding September; these adjustments shall be cumulative and compounded. For the purposes of this subsection (d), “consumer price index-u” means the index published by the Bureau of Labor Statistics of the United States Department of Labor that measures the average change in prices of goods and services purchased by all urban consumers, United States city average, all items, 1982-84 = 100. The new dollar amount resulting from each annual adjustment shall be determined by the Public Pension Division of the Department of Insurance and made available to the System by November 1 of each year.
     This subsection (d) is applicable without regard to whether the person is in service on or after the effective date of this amendatory Act of the 98th General Assembly.