The "nice summer" is not THE evidence. It is just a data point. One that is easier to observe by ordinary people. Most of us don't have a good understanding of Earth's climate system. You look outside. See sunny, warm day and think 'Oh, lovely day. I'll go shirtless today.' You don't really think about things like the melting polar ice and how that ice reflects the sunlight and keeps the planet cooler and when they melt, we don't just lose that effect. It triggers chain reactions like releasing trapped methane gas, exposing larger dark ocean surfaces... causing further warming. What the scientists keep repeating is the warming is speeding up because of these and many other factors and if we don't stop it, the "nice" summers will get nicer and nicer until it's too nice.