Does the climate change on it's own? Of course. The axial tilt of the Earth has a huge impact on our weather- that is literally what determines winter. The output of the sun has a impact on our weather. But those have cycles and are tracked.
Now, we've been pulling ice cores out of the arctic and antarctice since the 50s. We know from these ice cores what the atmopshere was like going back 1000s of years ago. And we know that starting in the 1800s the amount of carbon in the atmosphere started going up at a crazy pace.
We know from chemical analasys that this new carbon comes from burning fossil fuels. It's not a guess. It's not a "well, we think..." it's the exact carbon you get when you burn fossil fuels.
Carbon is a greenhouse gas and it absorbs heat and retains it. Observation and tracking shows us that the carbon is absorbing the heat and driving the temperature gains.
But how do we know that it's bad?
Great question! Science has this answer too. The end-Triassic mass extinction (ETME) was a mass extinction event that occurred around 201 million years ago. It was when 76% of the life on planet earth died.
What caused that extinction? The breakup of the supercontinent Pangea by a string of volcanoes known as CAMP (Central Atlantic Magmatic Province). The volcanoes erupted and belched all sorts of carbon into the atmosphere.
How do we know that? Sample cores from rocks!
We know that for close to 600,000 years that volcanoes pumped carbon and sulfer into the atmosphere, splitting Pangea into what would be the continents today. We also know that within 1,000 years of the final major eruptions in present-day Morocco, the ETME started.
So, we know that excess carbon in the air traps heat. We know that if there's enough carbon in the air, it will change the temperatures on the planet and drive an extinction event.
We know that the carbon in the air now is a result of buring fossil fuels. We know that the levels are approaching the levels of the ETME.
To deny this is to deny the reality that we live in.