Year	Kg per Square Metre
1980	21.49
1981	21.47
1982	21.45
1983	21.44
1984	21.43
1985	21.41
1986	21.4
1987	21.38
1988	21.34
1989	21.3
1990	21.25
1991	21.2
1992	21.14
1993	21.06
1994	20.98
1995	20.91
1996	20.85
1997	20.81
1998	20.79
1999	20.75
2000	20.7
2001	20.63
2002	20.6
2003	20.59
2004	20.58
2005	20.58
2006	20.59
2007	20.6
2008	20.62