Year	Kg per Square Metre
1980	20.95
1981	20.94
1982	20.94
1983	20.93
1984	20.94
1985	20.95
1986	20.96
1987	20.98
1988	21.01
1989	21.05
1990	21.09
1991	21.12
1992	21.15
1993	21.14
1994	21.14
1995	21.16
1996	21.19
1997	21.23
1998	21.27
1999	21.32
2000	21.37
2001	21.44
2002	21.52
2003	21.6
2004	21.69
2005	21.81
2006	21.94
2007	22.09
2008	22.25