Fossil Fuels
Burning fossil fuels, such as oil, natural gas and coal for energy is one of the most harmful things human beings are doing to the planet. Energy from fossil fuels sparked the Industrial Revolution and enabled most of the products and advanced technologies used today. The downside is that carbon emissions from their use are slowly changing the climate of the Earth, already leading to some disastrous consequences. Beyond the burning of fossil fuels, their extraction is also a highly destructive process. In the case of coal mining, it can involve the removal of entire mountain tops.
Automobiles
Automobiles are detrimental to the Earth because they are a primary contributor to greenhouse gas emissions. In addition, their existence has changed the way humans settle and use land. City planning has adjusted to the expectation of their perpetual use, leading to urban sprawl. This sprawl, which surrounds the older, denser cores of cities, has destroyed whole ecosystems and turned the countryside into swaths of parking lots and large retail stores.
Industrial Agriculture
It is arguable that industrial-scale agriculture ranks with fossil fuel consumption as one of the most disastrous human activities. Industrial agriculture consumes massive amounts of fossil fuels in machine operation, fertilizer and pesticide production and transportation. The scale of industrial agriculture requires a lot of land, so it leads to deforestation and habitat destruction. Water to irrigate fields, especially in arid regions, dries riverbeds and underground aquifers. Run-off from large farms and animal feed lots poison rivers with animal waste, toxic chemicals and nitrogen. Over-farming land strips topsoil of its nutrients, causing soil erosion and high salt concentrations. Lastly, since the scale requires efficiency and uniformity, fewer kinds of crops are grown, causing an overall reduction in biodiversity.
Industrial Production
Though it provides us with most of the products of modern society, industrial production is also destructive to the Earth. It consumes massive amounts of fossil fuels in factories and transportation. The raw materials for products are often gathered by mining, itself very destructive. Many products are made with toxic chemicals, some of which are released into the environment in the manufacturing process, or later as waste. Additionally, many manufacturing processes release toxic gases into the environment, and use unsustainable levels of freshwater. Post-consumer waste is huge problem with industrial-scale production, with poor design leading to broken or obsolete products that sit in landfills, or are burned.