It has been a combination of bad use of resources, bad timing, and bad control of how the population uses the resources. I would say that from thr 1940's and on, the advent of technology and new consumer items has more than tripled the amount of garbage that the average family produces. In the 80's there were still talks about recycling as a very optional thing until it became a "must" in the 90's. It is amazing how little we know about our little planet, however. We can go back in time and see all the poisonus materials used for the purpose of building things, cleaning things, and even to make things more decorative. On top of that, you have the burning of trash, dumping trash in the oceans, the oil leaks, the petrochemical industries puffing poison into the air and water of the cities they ocupy, and the abuse of nature for the purpose of construction sites.
We are a race that has extended their welcome in the planet and has kind of abused its hospitality. We need to shape up before it all gets taken away somehow.
Humans have found numerous ways to change and or harm the environment. We have made an effort to begin undoing some of the harm caused, some of it however may not be able to be reversed.
I think you can only clearly say that certain kinds of pollution constitute harm. When, for example, our fertilizer runoff goes into the Gulf of Mexico and creates a huge dead spot in the Gulf, that is harm. The oil spill is harm as well. So I would say that we have harmed the Earth by creating pollution that has killed off all life in some areas.
However, I would point out that saying that global warming "harms" the Earth is not that accurate (in my opinion). Global warming changes the Earth, yes. But does change = harm? That's not always the case. Warming will change the earth, but so have other things. So when you answer this, I think you should be clear on your terms -- what is harm and what is simply change?
How haven't they harmed the Earth? Humans, in the end, are animals, and they consume resources and occupy space. The difference between us and other animals is our ability to think and reason, so we have harmed the Earth with technology, drilling, mining, settlement, overharvesting, overfishing, large scale carbon emissions and pollution of all sorts, particularly water pollution and nuclear waste.
I think it is just now we are starting to realize the cumulative effects of humans' impact on Earth, especially in terms of species extinction and Global Warming. Human population growth is a major factor in the impact as well, and at some point, the number of humans on the planet will exceed its carrying capacity.
Humans have harmed the Earth in numerous ways. Global warming is the first thing that comes to mind when thinking about harm that has been done to the Earth. Global warming means that air and ocean temperature is increasing. Global warming occurs when there is a high concentration, or elevated levels, of greenhouse gases. Greenhouse gases exist primarily because of human activity. For example, we burn a lot of fossil fuels. When these fossils fuels burn, harmful gases are released into the atmosphere. They form a sort of "blanket" that keeps warm air in.
There are numerous ways that humans can help put a stop to global warming. One way that we are doing this is by choosing to use electric cars instead of cars that burn fuel. Recycling is also another way to help.
If you look at the big picture, then humans haven't harmed the Earth. The Earth is very old and has been through many different phases and changes. Mankind probably couldn't do very much permanent harm to the Earth.
Of course, we are changing the environment so that it will soon become very difficult for us to live here. But the Earth and 'life on earth' won't really be troubled. Life on Earth has survived serveral extinction events far more serious than our dirty lifestyles can create.
Mass extinction events are 'normal'. Life goes on. We can't really hurt the planet, but we can really, really damage our preferred environment and damage ourselves.