The majority of Northern Hemisphere warming over the last century is man made, and due to data tampering by government scientists.
In 1975, the National Academy of Sciences published the graph below, showing a dramatic cooling in the Northern Hemisphere from the 1930s until 1970, with the 1970 temperature about the same as 1910.
NASA has since erased most of that post-1930s cooling, and made 1910 much cooler than 1970.
The next graph overlays the current GISS graph (green) on the 1975 National Academy of Sciences graph. You can see how they massively cooled the period from 1890 to 1970, with the most dramatic data tampering being during the hot years of the 1920s and 1930s.
The graph below shows the impressive hockey stick of data tampering done by GISS relative to the original National Academy of Sciences temperatures.
After the Northern Hemisphere tampering was done, the new fake data became the basis for US data tampering, ignoring Briffa’s trees, and ultimately the hockey stick.
Well behaved skeptics are expected to agree that global warming is real, but I believe it is all contrived – other than the warming from 1910 to 1940 and then again from 1978 to 1998, after the ice age scare. Most likely the temperature graph would appear cyclical if not for the data corruption.