JC (please delete the previous post)
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 year successive years as below (for hadcrut3)
Period > Trend (deg C / decade)
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