With the help of a comment on my last post, thanks for that, I finally figured out what the cooling period between 1940 -1975 may have caused.
It is quite good explained in the blog (http://manmadeclimatechange.blogspot.de/2015/10/correlation-between-co2-and-temperature.html) and in the paper from Thomson et al 2008. I will give you a short summary.
If you look at the following graph, you will see that the global temperature decline is mainly because of a temperature decline in the seawater temperature. In particular the temperature drop in 1945. The explanation for that is, according to the Thomson et al paper, a change in the methods of collecting temperature data. But not only in the change of the methods, but more in the problem of mixing different methods. Especially after WWII the nations, who were collecting data, changed from mainly UK to mainly US. Through this change it is now very difficult to evaluate the right temperature. This confusion could have caused the abrupt decline in temperature in 1945.
Figure 3: Thomson et al 2008
To conclude, the argument used in the video loses meaningfulness.