Mining and industry can release toxic metals and other wastes into ground water, and they release metals into the atmosphere as fine particles. They produce acidic gases, such as sulphur dioxide and nitrogen oxides and green house gases, notably carbon dioxide. Human activities, using fossil fuels, notably power generation and transport, are the biggest net emitters of green house gases since they extract the carbon from no renewable sources.
Farming practices result in the release of nitrates and phosphates from fertilizers and animal waste into ground water and rivers, adding to sewage already released into rivers and seas and causing blooms of algae which subsequently deplete the oxygen in water. Although crops absorb carbon dioxide as they grow, they do not store it to the extent that a forest does, and rice cultivation and cattle rising produce another green house gas, methane. Clearance of natural vegetation releases carbon dioxide into the air and soil minerals into water and increases the likelihood of soil erosion.
Natural systems absorb some, but not all, of the excesses of human activity. Alkaline soils can neutralize acidic gases washed out of the air; forests, grasslands and plankton can absorb carbon dioxide; some waste decomposes and some nutrients are recycled. But few human activities are sustainable in the long term.