What were some events that were harmful to the environment in the past but we weren't aware of that until nowadays?
In the early bonanza days of the westward movement and settlement in virgin territory like Montana, California and the Pacific Northwest, every conceivable method of taking resources from the land was used, with almost no consideration for the environment. Gold mining corporations blasted away entire mountains to get at the precious metal, while ancient old growth forests like the Redwoods were clearcut for easy profit. These are two examples of where environmental and economic shortsightedness are costing us today in quality of life and habitat.
I would say that practically every major invention of the last few hundred years has been harmful to the environment in ways that people did not anticipate. But perhaps the most harmful has been the invention of the internal combustion engine.
So many of our environmental problems come from the automobile. Much of global warming, most air pollution, oil spills, etc are directly or indirectly caused by cars.
I'm not saying cars are a net bad, but I am saying that we certainly did not know how much the would harm the environment when they were invented.
Most of the causes and effect of the environmental pollution have been known for many centuries, and some isolated efforts have also been made to control pollution in limited ways. Wider recognition of pollution as a major problem which needed serious scientific study and integrated effort developed only towards end of 19th century. Among all major source or causes of pollution the last one to be recognized was the green house effect and the global warming. Perhaps Svante Arrhenius (1859-1927), a Swedish scientist, that was the first to identify in 1896, atmospheric carbon dioxide concentrations and temperature. He found that the average surface temperature of the earth is about 15 degrees C because of the infrared absorption capacity of water vapor and carbon dioxide. More details on history of greenhouse effect and global warming are available on the website referred below.
Because I am 55 years old I can remember when people would throw their trash out of the windows on the road. The streets were filled with litter and the highways were really awful. I recall the roadway that was most popular was called Route 66. I was looking out the window and I saw trash blowing across the road. Later they came out with an anti-litter campaign and began to try and straighten up what man had damaged.
Another issue I recall was chemical dumping into rivers and waterways. The Savannah River at one time was nothing but sludge. There were rivers all over the states like that and the government had to go in and establish guide lines and safe water standards.
Another issue was the air. Once upon a time San Bernardino, California had clean air to breathe and there was no such thing as a smog alert. The air is now so thick on humid hot days that you can cut a knife through it. Yet, it is actually better than it was fro awhile thanks to emission controls on cars.