If you live in western or northern Europe it is already obvious that the climate is getting warmer. Summers we previously thought were heat waves are now normal

If you live in western or northern Europe it is already obvious that the climate is getting warmer. Summers we previously thought were heat waves are now normal and winters with -5C to -10C days which were previously normal are now extremely cold. The change in the last 15 years has been very notable. But evidence? How can you prove that a smoker's lung cancer was caused by them smoking their entire life?