Hi Francisco, you are absolutely right that the ice on the mountains in Norway — and many other mountain ranges around the world — built up over time. We know that the Earth’s climate has changed many times over billions of years, for various reasons, and it has been very warm at some points! Geological records and atmospheric deposits locked in very deep ice in places like Greenland and Antarctica tell us this: https://climate.nasa.gov/evidence/

However, we have also been able to study atmospheric changes and anthropogenic (human-caused) emissions of CO2 and other greenhouse gases over the past century or so. By comparing records of the past to today we can see how human emissions are having a direct effect on the climate we are experiencing now. One of the key giveaways is just how rapidly things are changing. This five-minute video offers a very good explanation: https://www.youtube.com/watch?v=H5kejSYPD7U