Carbon dioxide is an essential ingredient for life on earth. The earth breaths in and out as the processes of life ebb and flow. Society has been disrupting this balance for a couple centuries now, and around 1950, when we started to measure its concentration in the atmosphere, we discovered this proverbial smoking gun.
Prior to then, the data in the graph was extracted from the ice in Greenland, by some very clever scientists. Read about this here.
The image is from a Nasa post and you can see the full article here:
The following Wikipedia page is an excellent summary of the scientists and their findings that have paved the way for our current day understanding of climate change and the science behind it.
Referencing the Wikipedia page, in 1824, Joseph Fourier discovered that our atmosphere was responsible for keeping all of us warm and that human activity could in fact modify our level of comfort.