The US annual mean temperatures, according to data from the "National Climatic Data Center" varied roughly as follows:-
1897 - 1925 51 to 53 degrees 1926 - 1950 52.5 to 54 degrees 1951 - 1975 52 to 53 degrees 1975 - 2000 52 to 54 degrees
This breakdown into 25 year periods was the cycle I observed on the graph contained in the link below. There was the odd year outside the ranges above but the majority fell within. If global warming is 90% attributed to human activity, can someone explain the variations over the last century in the USA.
