What are some consequences of human activities on the environment?

There are many consequences of human activities on the
environment.  Most of the ones that we notice are
negative.

Some consequences of human activities are very
easily seen.  When there are too many cars and too many factories in a city, we can
easily see the pollution in the air.  When waste from factories and such is pumped into
rivers, we can see that the water is dirty and that it stinks and has very little life
in it.

Other kinds of consequences are less easily
noticed.  The most important of these is global warming.  This is something that cannot
be perceived by the average person, but it is also the consequence that might
potentially have the greatest impact on life on the planet.