The earth exists within protective layers of gases which deflect most of the harmful ultra-violet rays from the sun, and keep the atmosphere at an inhabitable temperature by insulating the planet by trapping heat.
Humans have been adding to this blanket since the agricultural revolution, where a steady population increase, (which continues to increase today) has added to the gases, largely through increased methane (CH4) production from livestock. More recently activities involve the combustion of fossil fuels creating carbon dioxide (CO2) and Ozone (O3), more nitrous oxide (N2O) has also been produced by denitrifying bacteria breaking down our waste. Man-made chlorofluorocarbons found in aerosol cans and refrigeration devises can be up to 7000 more effective as a greenhouse gas than carbon-dioxide, because even in small amounts they are more effective at absorbing infra-red radiation, and can remain in the atmosphere for up to 60 years. The additional gases have increased the blanketing effect of the greenhouse gases, and some have destroyed areas of the protective ozone layer.
There are many reasons why these changes are considered to be bad, such as global warming, where the increased gases create a stronger greenhouse effect, which warms up the earth. This warming also adds to the greenhouse gases in a cycle by increasing the evaporation from the sea, creating water vapour. Global warming is melting the polar icecaps, which makes the seas rise and will eventually cause a dramatic decrease in land and habitats. The because ozone layer contains holes and is less effective, it does not prevent as much ultra-violet light from reaching the earth. U.V light is harmful to all forms of life because it causes mutations within the cells, which causes cancer, and usually death to the organism.
Although human activities have increased the amount of greenhouse gases and destroyed parts of the ozone layer, it could be said that the process of global warming is an inevitable part of life on earth, for example even without the added human influences life on earth does naturally create these gases, and a random increase in any species population would have similar effects over a period of time. There are also natural global weather cycles which switch between a rise in temperature and a decrease ice-ages', caused by the nature in which the Earth travels around the sun. It could be argued that the increased greenhouse gases caused by human influences are actually protecting life on earth by heating it to prevent a repeated ice-age, which in the past has caused mass extinctions.
In conclusion specific man-made gases such as chlorofluorocarbons have a lasting effect on the earth's atmosphere, and it is widely agreed that contributing to greenhouse gases has negative effects in the short run. However research still continues and it is unknown what challenges future generations may have to face concerning how they handle climate change and what effects it will have upon them.