The main reason that scientists think humans caused warming since 1950 is that none of the natural processes that influence Earth’s climate have changed enough during that time period to explain the warming.

Also, when looking at historical graphs of the relationship between CO2 concentration and temperature, it is obvious that when the amount of carbon dioxide increases the temperature increases; because humans have added CO2 into the atmosphere at increasing rates since aprox. 1950, one must assume we are responsible for this warming trend.