Year	Kg per Square Metre
1980	19.7
1981	19.71
1982	19.72
1983	19.73
1984	19.75
1985	19.77
1986	19.79
1987	19.81
1988	19.84
1989	19.86
1990	19.88
1991	19.91
1992	19.94
1993	19.97
1994	20
1995	20.04
1996	20.08
1997	20.13
1998	20.16
1999	20.21
2000	20.26
2001	20.31
2002	20.36
2003	20.42
2004	20.48
2005	20.56
2006	20.64
2007	20.72
2008	20.8