Year	Kg per Square Metre
1980	20.52
1981	20.56
1982	20.61
1983	20.64
1984	20.65
1985	20.65
1986	20.66
1987	20.66
1988	20.66
1989	20.67
1990	20.67
1991	20.68
1992	20.67
1993	20.67
1994	20.68
1995	20.69
1996	20.7
1997	20.72
1998	20.76
1999	20.8
2000	20.83
2001	20.86
2002	20.9
2003	20.94
2004	20.97
2005	21.03
2006	21.08
2007	21.15
2008	21.22