| Year | million km² | Trend |
|---|---|---|
| 1979 | 1.5 | 4.8178 |
| 1980 | 5.0 | 6.5483 |
| 1981 | 3.9 | 8.205 |
| 1982 | 10.9 | 9.7879 |
| 1983 | 12.7 | 11.297 |
| 1984 | 14.2 | 12.7323 |
| 1985 | 17.5 | 14.0938 |
| 1986 | 14.9 | 15.3815 |
| 1987 | 22.4 | 16.5954 |
| 1988 | 14.0 | 17.7355 |
| 1989 | 21.8 | 18.8018 |
| 1990 | 20.8 | 19.7943 |
| 1991 | 21.8 | 20.713 |
| 1992 | 24.0 | 21.5579 |
| 1993 | 22.2 | 22.329 |
| 1994 | 22.6 | 23.0263 |
| 1995 | 22.9 | 23.6498 |
| 1996 | 24.5 | 24.1995 |
| 1997 | 22.2 | 24.6754 |
| 1998 | 27.0 | 25.0775 |
| 1999 | 25.3 | 25.4058 |
| 2000 | 28.4 | 25.6603 |
| 2001 | 26.1 | 25.841 |
| 2002 | 18.1 | 25.9479 |
| 2003 | 26.9 | 25.981 |
| 2004 | 21.5 | 25.9403 |
| 2005 | 25.7 | 25.8258 |
| 2006 | 26.2 | 25.6375 |
| 2007 | 24.0 | 25.3754 |
| 2008 | 26.0 | 25.0395 |
| 2009 | 24.2 | 24.6298 |
| 2010 | 21.8 | 24.1463 |
| 2011 | 24.5 | 23.589 |
| 2012 | 19.3 | 22.9579 |
| 2013 | 21.9 | 22.253 |
| 2014 | 22.6 | 21.4743 |
| 2015 | 27.9 | 20.6218 |
| 2016 | 21.8 | 19.6955 |
| 2017 | 17.5 | 18.6954 |
| 2018 | 24.1 | 17.6215 |
| 2019 | 9.3 | 16.4738 |