By examining ice (precisely, bubbles inside the ice) collected from Green Land in the North and Antarctica in the South, scientists can calculate 1) the CO2 level and 2) the temperature of the air in the atmosphere dated back million years ago. Isn’t it awesome? Just by analyzing an ice sample, you can know so much about history. What they found is that CO2 has been dramatically risen since the industrial revolution.
**Extra note on CO2 level:
You can see the oscillation of CO2 level during the year because in the summer plants suck up CO2 while in the winter dead leaves release CO2.
(“The zig-zaggy motion of the line is due to the level falling each year in the summer when plants are sucking up CO2 and rising up again during the winter when the leaves are dead.” http://waitbutwhy.com/2015/06/how-tesla-will-change-your-life.html)
Conclusion: Humanity is populating the Earth so badly, and thanks to advances in science and technology (specially ice drilling technique), we can understand that destruction precisely.