So returning to the brick wall - scientists are doing research everywhere contributing to the climate change theory, adding new facts brick by brick. They look the directly measured CO2 levels in the atmosphere and see an alarming trend. The trend has never reproduced previously according to research (both magnitude AND acceleration rate simultaneously, so "cycles" don't explain). They measure ocean temperature. They dig into Antarctic ice and check elements in older layer. And so on, all around the planet. When they encounter something that is hard to quantify, like say global atmosphere temperature over time they estimate. They also check if the existing data fits into an existing theory, and unsurprisingly it does. Unfortunately so far everything fits in the global climate change theory.