Pekka
Joining them by a relatively smooth curve as Girma has done gives a totally misleading impression of something that oscillates regularly.
I have not joined any thing!
They are all the 30-year trends for each successive years as below (for hadcrut3)
Period Trend (deg C/decde)
1850-1880 0.053
1851-1881 0.051
1852-1882 0.058
1853-1883 0.064
1854-1884 0.065
1855-1885 0.064
1856-1886 0.062
1857-1887 0.059
1858-1888 0.045
1859-1889 0.033
1860-1890 0.036
1861-1891 0.024
1862-1892 0.013
1863-1893 -0.012
1864-1894 -0.025
1865-1895 -0.044
1866-1896 -0.049
1867-1897 -0.042
1868-1898 -0.038
1869-1899 -0.040
1870-1900 -0.038
1871-1901 -0.031
1872-1902 -0.031
1873-1903 -0.035
1874-1904 -0.047
1875-1905 -0.063
1876-1906 -0.074
1877-1907 -0.077
1878-1908 -0.074
1879-1909 -0.064
1880-1910 -0.069
1881-1911 -0.070
1882-1912 -0.073
1883-1913 -0.069
1884-1914 -0.067
1885-1915 -0.059
1886-1916 -0.045
1887-1917 -0.039
1888-1918 -0.041
1889-1919 -0.034
1890-1920 -0.015
1891-1921 -0.009
1892-1922 0.002
1893-1923 0.000
1894-1924 -0.002
1895-1925 -0.001
1896-1926 0.006
1897-1927 0.031
1898-1928 0.049
1899-1929 0.058
1900-1930 0.065
1901-1931 0.088
1902-1932 0.110
1903-1933 0.121
1904-1934 0.118
1905-1935 0.120
1906-1936 0.126
1907-1937 0.138
1908-1938 0.146
1909-1939 0.151
1910-1940 0.153
1911-1941 0.154
1912-1942 0.155
1913-1943 0.152
1914-1944 0.148
1915-1945 0.161
1916-1946 0.170
1917-1947 0.153
1918-1948 0.132
1919-1949 0.116
1920-1950 0.102
1921-1951 0.083
1922-1952 0.075
1923-1953 0.065
1924-1954 0.058
1925-1955 0.036
1926-1956 0.019
1927-1957 0.003
1928-1958 -0.001
1929-1959 -0.002
1930-1960 -0.014
1931-1961 -0.018
1932-1962 -0.014
1933-1963 -0.012
1934-1964 -0.017
1935-1965 -0.032
1936-1966 -0.045
1937-1967 -0.051
1938-1968 -0.049
1939-1969 -0.044
1940-1970 -0.031
1941-1971 -0.019
1942-1972 -0.011
1943-1973 0.001
1944-1974 0.020
1945-1975 0.030
1946-1976 0.037
1947-1977 0.027
1948-1978 0.033
1949-1979 0.033
1950-1980 0.040
1951-1981 0.040
1952-1982 0.050
1953-1983 0.058
1954-1984 0.079
1955-1985 0.073
1956-1986 0.063
1957-1987 0.052
1958-1988 0.066
1959-1989 0.083
1960-1990 0.091
1961-1991 0.105
1962-1992 0.119
1963-1993 0.124
1964-1994 0.133
1965-1995 0.126
1966-1996 0.129
1967-1997 0.125
1968-1998 0.134
1969-1999 0.151
1970-2000 0.160
1971-2001 0.163
1972-2002 0.165
1973-2003 0.175
1974-2004 0.192
1975-2005 0.188
1976-2006 0.186
1977-2007 0.173
1978-2008 0.171
1979-2009 0.159
1980-2010 0.160
1981-2011 0.162