If lung cancer is caused by smoking, are deaths caused by lung cancer lesser than 20 years ago?

In ratio to the population yes. But you don't need to be a smoker to get lung cancer. There will always be lung cancer until researchers find a cure.

There is no such thing as an ABSOLUTELY CONCLUSIVE conclusion in science. This is particulary so when the evidence is largely epidemiological, as in the case of smoking. But keep in mind that there is no absolutely conclusive evidence that gravity causes things to fall down, either. There comes a point where the evidence for something is so overwhelming that it's almost certainly true. The evidence in the case of smoking includes studies that show that lung cancer rates track the incidence of smoking, and a laboratory studies in which animals forced to smoke or painted with tobacco tars develop cancer while control animals do not. It is *possible* that the animal studies don't apply to people and that all the people who smoke are getting cancer for some reason that doesn't have to do with smoking, but the chance is so miniscule that it isn't worth bothering with.

No, smoking history is a major risk factor that can lead to lung cancer, but it is not the only cause of the cellular abnormalities that form cancer cells. Then there are the second hand smoke risks. Those born after 1970 have still been exposed to that. Many people develop lung cancer with no history of smoking or major exposure. I find it hard to believe that only 10% of current adults smoke, could be true but I think that number is much higher still.