| Year | gm∙cm−2 |
| 1996 | 1.96 |
| 1997 | 2.01 |
| 1998 | 2.06 |
| 1999 | 2.01 |
| 2000 | 2.00 |
| 2001 | 2.03 |
| 2002 | 2.05 |
| 2003 | 2.05 |
| 2004 | 2.04 |
| 2005 | 2.07 |
| 2006 | 2.05 |
| 2007 | 2.06 |