| date | Cumulative area (square km) | Cumulative number of sites |
|---|---|---|
| 1838 | 1.11 | 2 |
| 1858 | 7.97 | 3 |
| 1895 | 8.21 | 5 |
| 1899 | 8.58 | 6 |
| 1900 | 74.34 | 10 |
| 1903 | 92.03 | 11 |
| 1905 | 94.86 | 13 |
| 1908 | 95.07 | 14 |
| 1909 | 3757.23 | 24 |
| 1910 | 3757.24 | 25 |
| 1911 | 3757.24 | 28 |
| 1912 | 3765.48 | 30 |
| 1913 | 3771.34 | 39 |
| 1914 | 3941.67 | 46 |
| 1915 | 3941.67 | 51 |
| 1916 | 3941.73 | 56 |
| 1917 | 4108.13 | 79 |
| 1918 | 4109.07 | 96 |
| 1919 | 4109.82 | 116 |
| 1920 | 4163.55 | 148 |
| 1921 | 4332.17 | 214 |
| 1922 | 5090.89 | 239 |
| 1923 | 5614.77 | 281 |
| 1924 | 6456.09 | 311 |
| 1925 | 6466.79 | 342 |
| 1926 | 6509.5 | 374 |
| 1927 | 6515.92 | 391 |
| 1928 | 6765 | 428 |
| 1929 | 6784.79 | 462 |
| 1930 | 6942.55 | 517 |
| 1931 | 6972.38 | 555 |
| 1932 | 7297.46 | 615 |
| 1933 | 7397.02 | 747 |
| 1934 | 7766.05 | 798 |
| 1935 | 9548.07 | 869 |
| 1936 | 9625.46 | 941 |
| 1937 | 9909.19 | 1111 |
| 1938 | 10324.12 | 1335 |
| 1939 | 10612.32 | 1552 |
| 1940 | 10808.1 | 1686 |
| 1941 | 11008.67 | 1795 |
| 1942 | 11680.94 | 1872 |
| 1943 | 11819.37 | 1918 |
| 1944 | 11834 | 1968 |
| 1945 | 11842.27 | 2058 |
| 1946 | 11873.81 | 2185 |
| 1947 | 11976.83 | 2319 |
| 1948 | 13286.84 | 2503 |
| 1949 | 14586.87 | 2648 |
| 1950 | 14814.61 | 2808 |
| 1951 | 22012.6 | 2997 |
| 1952 | 25013.66 | 3214 |
| 1953 | 25530.95 | 3465 |
| 1954 | 30324.27 | 3767 |
| 1955 | 32408.58 | 4115 |
| 1956 | 39283.79 | 4407 |
| 1957 | 41954.69 | 4716 |
| 1958 | 43843.52 | 5044 |
| 1959 | 47733.15 | 5339 |
| 1960 | 49488.99 | 5643 |
| 1961 | 51407.7 | 6214 |
| 1962 | 55981.89 | 6581 |
| 1963 | 63853.8 | 6980 |
| 1964 | 66372.71 | 7353 |
| 1965 | 69442 | 7821 |
| 1966 | 73072.6 | 8153 |
| 1967 | 78847.02 | 8601 |
| 1968 | 84182.93 | 9192 |
| 1969 | 89523.74 | 9689 |
| 1970 | 111141.36 | 10164 |
| 1971 | 119215.5 | 10739 |
| 1972 | 128859.69 | 11337 |
| 1973 | 141010.04 | 11926 |
| 1974 | 159166.31 | 12664 |
| 1975 | 170367.3 | 13092 |
| 1976 | 179781.41 | 13767 |
| 1977 | 193576.48 | 14568 |
| 1978 | 199712.3 | 15204 |
| 1979 | 216420.34 | 15888 |
| 1980 | 242373.94 | 16728 |
| 1981 | 264202.38 | 17701 |
| 1982 | 275364.29 | 18760 |
| 1983 | 290334.65 | 19994 |
| 1984 | 297701.15 | 21790 |
| 1985 | 312197.55 | 23352 |
| 1986 | 342118.68 | 25101 |
| 1987 | 356479.51 | 26671 |
| 1988 | 388022.66 | 28395 |
| 1989 | 425935.65 | 29926 |
| 1990 | 460251.29 | 32028 |
| 1991 | 499179.74 | 33990 |
| 1992 | 551893.89 | 36723 |
| 1993 | 567003.85 | 38351 |
| 1994 | 584243.89 | 40870 |
| 1995 | 616540.41 | 43025 |
| 1996 | 633978.13 | 45382 |
| 1997 | 664147.15 | 47066 |
| 1998 | 700853.25 | 49029 |
| 1999 | 714844.09 | 51241 |
| 2000 | 742473.76 | 55123 |
| 2001 | 789162.45 | 61048 |
| 2002 | 817603.39 | 64685 |
| 2003 | 837972.68 | 67740 |
| 2004 | 892237.22 | 71469 |
| 2005 | 924292.56 | 75161 |
| 2006 | 950004.04 | 78347 |
| 2007 | 979754.14 | 81678 |
| 2008 | 1010168.67 | 84459 |
| 2009 | 1036835.06 | 86767 |
| 2010 | 1058132.09 | 91751 |
| 2011 | 1193639.56 | 94236 |
| 2012 | 1216155.33 | 95876 |
| 2013 | 1232846.54 | 98812 |
| 2014 | 1305572.73 | 101001 |
| 2015 | 1335565.86 | 102485 |
| 2016 | 1506690.58 | 103663 |
| 2017 | 1506714.05 | 103670 |